Tetrode Manual

Tetrode Boards: AN-1 Issue 1.21, May 2006 1998-2006 IFWtech Limited

All the hard work done!

• Screen supply

• Grid bias

• Tube protection

• TX/RX sequencing

• ALC output

Versatile!

• For one or two tetrodes, including: 4CX250–350–400 (all types), 4CX800/ GU-74B, 4CX1000, 4CX1500B, 4CX1600A, 4CX1600U/GS-23B, YL1050/52/56, GU-73B, GU-78B, GU-84B... and more.

• 'Universal' DC grounding – use with grounded cathode, grounded screen or grounded control grid.

• Ideal for your new amplifier – or as an upgrade for your existing tetrode PA.

THE TETRODE BOARDS Control and Protection

for your Tetrode RF Power Amplifier

TM


2

WARNING

These notes are intended for users who have sufficient experience to work safely with high-voltage circuits.

Use at your own risk! We cannot accept responsibility for any damage or injury.

DANGER - AC mains voltage and high DC voltages!

The names of tag connections on the boards are shown underlined, as in “the G2-REG OUT tag”. On the PC boards and the Interconnections diagram, some labels had to be shortened to save space, e.g. G2-REG OUT is labeled G2REG on the board and the Interconnections diagram.

CAUTION

DO NOT use the Tetrode Boards with a screen supply derived from the anode high voltage through a dropper resistor – it will cause serious component damage!

Always use a separate transformer winding for the screen supply.

REVISION NOTES

AN-1 Issue No G2-CONTROL board Issue No

REC-G1-ALC board Issue No

Changes (where significant)

1.0, April 1998 3B 3B

Intermediate changes 1.1–1.17 See earlier versions

1.18, 1.19 Not issued

1.20, Nov 2004 3B 3D Major revisions, to give more help on the wider variety of tetrodes that are now in use.

1.21, May 2006 3B 3D R106 changed to 470Ω. Many changes to Farnell stock codes.

Any trademarks mentioned in this manual that are not the property of IFWtech Ltd are acknowledged to be the property of their respective owners.


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CONTENTS

1. Features......................................................................................................... 4

2. Introduction................................................................................................... 4

2.1 What You Get....................................................................................... 4 2.2 What You’ll Need.................................................................................. 5 2.3 Choosing Configuration Options .......................................................... 5

3. Tetrode Grounding Connections ................................................................ 6

3.1 Grid Driven, DC-grounded Cathode ..................................................... 6 3.2 Cathode Driven, DC-grounded Screen Grid......................................... 7 3.3 Cathode Driven, DC-grounded Control Grid ........................................ 7 3.4 Screen-grid Components ..................................................................... 8

4. Screen-grid Supply Configuration .............................................................. 9

4.1 Examples of Tubes............................................................................... 9 4.2 Off-board Component Calculations.................................................... 11 4.3 On-board Component Changes ......................................................... 15 4.4 Further Information............................................................................. 15

5. Control-grid and Relay Supply Configuration ......................................... 16

5.1 Control-grid Supply............................................................................. 16 5.2 Relay Supply....................................................................................... 16

6. Basic Inter-board Connections ................................................................. 17

7. Power and Control Options....................................................................... 19

7.1 TX/RX Changeover Sequencing ........................................................ 19 7.2 Coax Relay Voltage............................................................................ 21 7.3 HV Supply Control .............................................................................. 21 7.4 G1 Switching ...................................................................................... 21 7.5 Automatic Level Control (ALC)........................................................... 22 7.6 Additional Fault Monitoring ................................................................. 22

8. Building the Kit ........................................................................................... 23

8.1 Mounting the Boards .......................................................................... 23 8.2 Assembling the Boards ...................................................................... 23

9. Initial Power-up........................................................................................... 26

9.1 Procedure........................................................................................... 26 9.2 Problems? .......................................................................................... 27 9.3 Screen Supply Adjustments ............................................................... 29 9.4 Screen-current Trip ............................................................................ 30 9.5 Control-grid Protection and ALC......................................................... 31 9.6 Warm-up Timer .................................................................................. 32

10. Power-up Your Amplifier............................................................................ 33

10.1 Final Checks....................................................................................... 33 10.2 RF Testing.......................................................................................... 33 10.3 Final ALC Adjustment......................................................................... 33 10.4 False Alarms ...................................................................................... 34 10.5 That’s All !........................................................................................... 35

11. Updates and Product Support................................................................... 36

Schematics................................................................................................. 37–42

Parts List..................................................................................................... 43–48


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1. Features

• ‘Universal’ control unit for almost any amateur-size Tetrode Power Amplifier and its High Voltage supply.

• Carefully designed to help your tetrodes deliver a high-quality signal.

• Suitable for either one or two tubes.

• Two compact PC boards (both 5in x 4in – boards can be stacked).

• Regulated and adjustable screen-grid supply.

• Regulated and adjustable control-grid supply.

• Sequenced relay switching with transceiver RF drive inhibit.

• Screen-grid currents monitored for sensitive fault detection. No risky grid fuses!

• Fault conditions disable PTT and HV supply for safety – just press RESET to recover.

• Comprehensive metering.

• User-configurable for special requirements, with help from these detailed Instructions.

For a general introduction to these circuits and the ideas behind them, see Power and Protection for Modern Tetrodes by Ian White, G3SEK, in QEX for October 1997 (PDF version downloadable from the Tetrode Boards website – see Section 11).

2. Introduction

The full Tetrode Boards kit includes all the components for the PC boards, and some of the hard-to-find accessories.

To give you the best possible value for money, we do not supply expensive ‘off-board’ components such as meters and large heatsinks. You can probably find these components much more cheaply as surplus, or out of the junk-box.

2.1 What You Get

The full Tetrode Boards kit includes:

1. Two PC boards, tinned, ready-drilled, and with printed component locations

2. All the on-board components – premium quality for reliability

3. Power MOSFET (Q2) and mounting washers

4. Push-on tags for the on-board connectors

5. Two extra VDRs for mounting directly at the tube sockets

6. Complete schematics and these Instructions.


5

2.2 What You’ll Need

This is a summary of the additional components that you’ll need. Most of these are easier to buy surplus than at new prices, so we didn’t include them in the kit.

1. Mains transformer(s) to supply screen grid, control grid, relays and heaters. For the 4CX250/350/400 family, the transformer ratings should typically be:

• Screen grid: minimum AC voltage depends on requirements for regulated screen voltage and current. Typical ratings are at least 0–330V AC, at least 100mA, but may need to be higher for large tubes – check Section 4.2.3 before ordering a transformer.

CAUTION

If the AC voltage of the screen supply transformer is too low, the voltage regulator will not be able to function correctly.

• Control grid: 0–105V AC (anywhere in the range from 100 to 150V AC) at 50–100mA.

• Relays etc: 15–0–15V AC 1A min for 12V coax relays, higher voltage for 24V relays. Note: a center-tapped winding is essential here.

• Heaters: the appropriate AC voltage and current for the tube(s). Allow for voltage drops in the heater wiring, and adjust for exactly the correct on-load voltage at the tube pins.

2. RESET switch: SPST momentary push-button (low-voltage).

3. ALARM LED: ordinary 20mA red LED.

4. Heatsink for Q2: see notes on page 48.

5. Mounting pillars and hardware for the two PC boards.

6. M1: typically 0–50mA or 0–100mA moving-coil meter for screen grid current. The G2-CONTROL board has provision for an optional meter shunt resistor R17.

7. M2: 0–10mA moving-coil meter for control grid current. The REC-G1-ALC board has provision for an optional meter shunt resistor R108.

8. RF choke wound on a 100Ω resistor, for mounting near the tube (see Section 3.4).

9. Screen-cathode bleeder resistor Rs (see Section 3.4 for values and ratings).

10. Power resistors R12 and R14 (see Sections 4.2.4 and 4.2.5 for values and ratings).

You will also need some temporary resistors for the setting-up procedures.

2.3 Choosing Configuration Options

Every power amplifier is different, so there are many possible options for voltages, metering, TX/RX control etc.

CAUTION

Please read ALL of Sections 3, 4, 5, 6 and 7 BEFORE you switch on the soldering iron!


6

3. Tetrode Grounding Connections

CAUTION

Do not start to build the kit until you have read all of Sections 3, 4, 5, 6 and 7, and have decided on the options you need to install.

You can use the Tetrode Boards with any configuration of DC and RF grounding of the tube(s).

Remember that RF grounding and DC (chassis) grounding are different! For example, the screen grid of a tetrode is always RF-grounded, but in most configurations the screen is not DC-grounded to the chassis.

This section explains the three practical combinations of DC and RF grounding, and shows you exactly how to connect the Tetrode Boards. Note that some configurations require additional components, shown in the schematics below as Rs, Cs, RFC, Rd, VDR – see Section 3.4 for more details of these components.

Two-tube Amplifiers

The Tetrode Boards can handle either one or two tubes, though only one tetrode (V1) is shown

in these examples. If yours is a two-tube amplifier, use the connection points marked →→→→V2 on the schematics below.

Also see the note on page 10 about Matched Pairs of Tubes.

3.1 Grid Driven, DC-grounded Cathode

RF drive is to the control grid, and the screen grid is bypassed by Cs (usually built into the tube socket). The tube cathode is grounded to chassis (maybe through a small RF feedback resistor at X).

Connect both of the CATHODE tags on the G2-CONTROL board and on the REC-G1-ALC board to chassis ground as shown above. Do not connect the G1 OUT and G2-REG OUT rails to chassis!

Note that the G1 meter is at control-grid potential below chassis ground, and the G2 meter is at VG2 potential above chassis ground. Your anode-current meter in the B-minus rail will be at chassis ground potential.

Rs

V1RFC1 SEE TEXT

VDR1

R1SEE TEXT

R2 100R 1WINPUT CIRCUIT RF

G1 BIAS

C1

G2

CATHODE

G1

V2 IF V2 IS USED, REPEAT RFC1, R2, VDR1

Rs

Rd 100Ω 1W

RFC

VDR Cs

To V2 Repeat RFC, Rd, VDR and Cs at each tube


7

3.2 Cathode Driven, DC-grounded Screen Grid

RF drive is to the cathode, the screen grid is DC-grounded (so there is no screen bypass capacitor Cs). The control grid is bypassed to chassis ground by Ca. The RF bypass capacitor for the input circuit is shown as Cb. Ca and Cb should both be 10-100nF; VHF/UHF amplifiers may need additional capacitance here.

Connect the G2-REG OUT tag on the G2-CONTROL board to chassis ground as shown above. Do not connect the CATHODE or G1 OUT rails to chassis!

Note that the G2 meter is close to chassis ground potential, but the G1 meter is at (VG2 + VG1) potential below chassis ground. Also your anode-current meter in the B-minus rail will be at VG2 potential below chassis ground.

3.3 Cathode Driven, DC-grounded Control Grid

RF drive is to the cathode, the control grid is DC-grounded and the screen grid is bypassed by Cs (usually built into the tube socket). The RF bypass capacitor for the input circuit is shown as Cb, and should both be 10-100nF; VHF/UHF amplifiers may need additional capacitance here.

V1

VDR1R1SEE TEXT

RF

G2

CATHODE

G1

V2

V2

IF V2 IS USED, REPEAT VDR1, C1

INPUT CIRCUIT

C210n

C1

B-MINUS

VDR

Ca

Cb

Rs

If V2 is used, repeat Ca at each tube

V1

R1SEE TEXT

RF

G2

CATHODE

G1

V2

INPUT CIRCUIT

C210n

B-MINUS

VDR1

R2 100R 1W

C1

RFC1 SEE TEXT

IF V2 IS USED, REPEAT RFC1, R2, VDR1, C1

V2

VDR

Rs

Rd 100Ω 1W

Cs

RFC

To V2 Repeat RFC, Rd, VDR and Cs at each tube

Cb


8

Connect the G1 OUT tag on the REC-G1-ALC board to chassis ground as shown above. Do not connect the CATHODE or G2-REG OUT rails to chassis!

Note that the G1 meter is close to chassis ground potential, but the G2 meter is at (VG2 – VG1) potential above chassis ground. Also your anode-current meter in the B-minus rail will be at VG1 potential below chassis ground.

3.4 Screen-grid Components

This section describes the components that are common to all of the schematics above.

Screen-cathode bleeder resistor Rs

Rs serves two purposes:

1. To prevent the screen grid from ‘floating’ when the screen switching relay K1 is changing over.

2. To provide a bleed current which makes the screen current meter read up-scale by about 10mA. This allows negative screen currents of up to 10mA to be indicated in an ordinary left-hand-zero meter. A 0–50mA meter will read screen current from –10mA to +40mA.

To obtain a bleed current of about 10mA, R1 = VG2 / 10 kΩ, e.g. 35kΩ for a 350V screen supply,

40kΩ for 400V etc. The value doesn’t have to be exact because you can ‘zero’ the meter to any major scale mark using the mechanical adjustment. Section 4.2.1 gives some recommended values and component ratings.

VDR

This VDR is identical to the two VDRs on the G2-CONTROL board. It is the first line of defence to protect the screen grid, the screen bypass capacitor and the power supply in the event of a flashover. Connect the VDR directly from the screen tag on the tube socket to the nearest cathode tag, with short leads to minimize inductance.

There are already two VDRs on the G2-CONTROL board, but an extra VDR will be needed right here at the screen grid of the tube. Because a two-tube amplifier needs a separate VDR at each tube, two extra VDRs are provided with the kit (making four in total). If you are only using a single tube, connect both VDRs in parallel for extra protection.

Cs - RFC - Rd

Cs is the screen bypass capacitor, and is usually built into the tube socket.

RFC and Rd prevent spurious resonances between Cs and the LF bypass capacitor C9 on the G2-CONTROL board, which could lead to the screen grid becoming ‘un-bypassed’ at HF. Rd is a

100Ω 1W carbon or metal-oxide resistor (not wire-wound) and RFC is about 40 turns of 24-gauge enameled wire, scramble-wound over the body of R2.

If you use two tubes, you need a separate Cs-RFC-Rd network at each tube.

If the screen grid is directly DC-grounded, as described in Section 3.2, then Cs, RFC and Rd are not necessary.

V1RFC1 SEE TEXT

VDR1

R1SEE TEXT

R2 100R 1W

C1

G2

CATHODE

VDR Cs

Rs

Rd 100Ω 1W

RFC


9

4. Screen-grid Supply Configuration

CAUTION


The Tetrode Boards can be used with many different tubes that have a wide range of screen voltage and current requirements. For correct operation, you will need to select certain component values, depending on the following factors:

• Type of tube

– regulated voltage required

– maximum positive screen current (without losing voltage regulation) per tube

– maximum negative screen current (without losing voltage regulation) per tube

– screen trip current, per tube

• Number of tubes – simply multiply the current requirements by the number of tubes

• Mains transformer

– AC voltage

– current capability and/or winding resistances.

4.1 Examples of Tubes

The following are examples of the wide range of tubes that have been used with the Tetrode Boards, with some typical operating conditions and suggested settings for the screen current trip.

Typical operating conditions (SSB, Class AB1) *

Tube

Screen voltage (V)

Peak screen current (mA)

Suggested current trip level

(mA)

GS-15B 350 1 8

4CX250B 350 5 15

4CX250R / 8930 375–400 5 15

4CX350A 375–400 –3 15

4CX400A 375–400 ≤ 20 ≥ 20

4CX800 / GU-74B 350 30 40

4CX1000A 325 ≤ 35 ≥ 35

4CX1500B 225 –15 20

4CX1600A / GU-91B

350 240

48 21

55 30

continued...


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Typical operating conditions (SSB, Class AB1) *

Tube

Screen voltage (V)

Peak screen current (mA)

Suggested current trip level

(mA)

4CX1600U / GS-23B

520 ≤ 23 ≥ 23

GU-73B

GU-78B

GU-84B 360–380 40 60

YL1050/1052/1056 500 26 35

* These values are taken where possible from manufacturers’ data sheets, or selected from information

developed by amateur users. Data given here are in no way warranted by IFWtech Ltd.

The screen current of a tetrode is a very sensitive indicator for a wide range of fault conditions, including:

• Incorrect plate-circuit tuning

• Loading too light, or too heavy

• Too much RF drive

• Loss or major change in anode, screen or control grid voltage

• RF and DC arcs, flashovers and other ‘glitches’

• Blower failure, resulting in overheating of the tube(s).

All of these faults will result in too much screen current, either positive or negative. Continuous electronic monitoring of the screen current is thus one of the most important features of the Tetrode Boards.

The suggested current trip levels are normally about 20–25% above the ‘typical’ peak screen current recommended by the manufacturer. This is generally high enough to avoid false alarms during normal operation, but still low enough to give adequate protection to the screen grid.

However, for some tubes the manufacturer’s recommended ‘typical’ screen current (at the recommended screen voltage) equates to the maximum allowable power dissipation. Where this limit applies, the ‘typical’ screen current is given as a ‘≤’ value in the table above, and the suggested trip current is given as a ‘≥’ value. The screen trip in the Tetrode Boards is very fast-acting if anything goes wrong, so in practice it may be OK to set the screen trip current to 20% above the manufacturer’s maximum recommended current.

Matched Pairs of Tubes

Two-tube amplifiers require tubes that are well matched in terms of DC and RF characteristics. New tubes from the same manufacturer should start out well-matched, but they must then experience the same operating history.

Tubes from different manufacturers (even new) may not be well-enough matched for use in a two-tube amplifier. Unless you are very lucky, used tubes with different operating histories will have very different characteristics and should not be paired together.


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4.2 Off-board Component Calculations

This section gives simple worksheets to calculate the voltages, currents and off-board component values for the screen regulator. Here is a very simplified schematic:

Each of the worksheets below contains an example column based on 2 x 4CX800/GU74B tubes, and a blank column where you can enter the values for your particular amplifier. (If your calculated values are dramatically different from the example values at any stage, you need to check your arithmetic!)

1

4.2.1 Screen bleeder resistor, Rs

Rs is the resistor connected between the screen supply and cathode potential, close to the tube(s). The purpose of Rs is to ensure there is always a DC return path for the screen, even while the relay contacts of K1 are changing over.

Rs is calculated to bleed about 10mA of current, but this value is not critical (and need not be changed if using two tubes). Suggested values are:

Screen voltage range Rs

Up to 250V 22kΩ 5W

250–400V 33kΩ 7W or 10W, or 2 x 15kΩ 5W in series

400–550V 2 x 22kΩ 5W in series

> 550V Above the voltage limit of the Tetrode Boards

If in doubt, use the next recommended lower value of resistance here.

1 There is also an Excel spreadsheet on the Tetrode Boards website. The spreadsheet approaches the

problem in a different way, but the results are equivalent.

R14

R12

Q2

+

METER

RS

+30V

VDR

K1

Rs

Unregulated input voltage

Standing current

Regulated screen voltage

Screen current – +

Bleed current

R12/Q2 current


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4.2.2 Calculate the Trip Current and the Standing Current

The ‘standing current’ is the current that flows through R14 all the time the amplifier is in the TX condition (K1 energized as shown above).

Fill out this worksheet:

Calculation steps Your amplifier

Example: 2 x 4XC800/GU-74B

A Suggested current trip level, mA (from table above, based on 20–25% above manufacturer’s recommended peak screen current)

40mA

B Multiply A by 1.1 40 x 1.1 = 44mA

C Multiply B by number of tubes 44 x 2 = 88mA

D Round C upward to the next multiple of 10mA 88 rounds up to 90mA

This is the trip current

E Add 20mA to D 90 + 20 = 110mA

This is the standing current through R14

4.2.3 Calculate the transformer AC voltage requirement

Fill out this worksheet:



A Recommended regulated screen voltage (from table above, or tube manufacturer’s data)

350V

B Add 50V to A 350 + 50 = 400V

C Required standing current through R14 (= E from Section 4.2.1 worksheet above)

110mA

D Multiply C by 0.75, and add to A

(110 x 0.75) + 350 = 432.5V

E Choose whichever is larger, B or D D is the larger = 432.5V

F Round E upwards to the next multiple of 10 432.5 rounds up to 440V

This is the minimum unregulated input voltage that will meet your requirements

Continued...

G Divide F by 1.2 440 / 1.2 = 367V AC

This is the minimum transformer voltage (RMS AC) estimated to meet your

requirements


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CAUTION

The transformer voltage is an ESTIMATE based on typical transformers. The suitability of any particular transformer cannot be guaranteed until the practical testing stage.

If in doubt, choose a transformer giving a higher RMS AC voltage than estimated here.

Notes on transformer AC voltage

Maybe the minimum AC voltage is higher than you expected? Remember that the unregulated DC input contains 100/120Hz AC ripple, so the instantaneous minimum voltage can easily be 30–50V lower than the average voltage that you measure with a multimeter. If the instantaneous minimum voltage is too low, the voltage regulator will ‘drop out’ during the negative part of the ripple cycles, and the ‘regulated DC’ screen voltage will have negative spikes at 100/120Hz. An oscilloscope will show this very clearly.

In the worksheet above, the factor of 1.2 on line G is an allowance for the difference between the average DC voltage and instantaneous value at the minimum of the 100/120Hz ripple cycle. However, the actual minimum voltage will depend on the transformer winding resistances, so the suitability of your particular transformer cannot be guaranteed in advance.

The only way to be certain is to check the screen voltage using an oscilloscope, and make sure it is a clean, constant DC voltage with no negative 100/120Hz spikes. If you see small spikes, you may be able to adjust R12 to remove them – see below.

4.2.4 Calculate R14

The purpose of R14 is to deliver the required standing current (= E from Section 4.2.1 worksheet above) into the shunt regulator circuit. We do not know what the actual value of your unregulated input voltage will be, so this calculation is only a first estimate. The exact value of R14 will be adjusted on test (see later, in Section 9.3.1).

To calculate R14, fill out this worksheet:



A Unregulated input voltage (= F from Section 4.2.3 worksheet above)

440V

B Recommended regulated screen voltage (= A from Section 4.2.3 worksheet above)

350V

C Voltage drop across R14 = A – B 440 - 350 = 90V

D Required standing current through R14 (= E from Section 4.2.1 worksheet above)

110mA

E R14 = C x 1000 / D Ω 90 x 1000 / 110 =

818ΩΩΩΩ

This is the estimated value for R14

F WR14 = C x D / 1000 W 90 x 110 / 1000 = 9.9W

This is the minimum power rating for R14 – use a higher-rated component for better reliability


14

Notes on R14

The recommended component for R14 is an Ohmite adjustable wirewound resistor. The adjustment feature allows simple setup (Section 9.3.1) without having to swap resistors.

Ohmite part number D50K1K0 (1kΩ 50W max) will be suitable for R14 in almost all cases (see

Components List for ordering information). This resistor can be adjusted to any value below 1kΩ. The power dissipation is up to 50W, proportional to the length that is actually being used. The resistor is air-cooled, and with good ventilation it will run reliably at a moderate temperature.

4.2.5 Calculate R12

The purpose of R12 is to remove some of the heat load from the power MOSFET Q2.

To calculate R12, fill out this worksheet:

Calculation steps Your tube(s)


A Recommended regulated screen voltage (= A from Section 4.2.3 worksheet above)

350V

B Required standing current through R14 (= E from Section 4.2.1 worksheet above)

110mA

C Maximum value of R12 = (A - 80) x 1000 / (B + 20)

(350 - 80) x 1000 / (110 + 20) =

2076ΩΩΩΩ

This is the maximum value for R12 –

D Round this resistance value down to the nearest standard value

2000ΩΩΩΩ

E WR12 = (B + 20)2 x R12 / 10

6 (110 + 20)

2 x 2000 /

106 =

33.8W

This is the minimum power rating for R12 – use a higher-rated component for better reliability

Notes on R12

R12 can be either a large wirewound resistor mounted with good ventilation, or a 50/100W metal-clad resistor mounted on a large and well-cooled heatsink. If you choose the second option, the heatsink for R12 must be separate from the heatsink for Q2!


15

4.3 On-board Component Changes

Very few on-board component changes are needed to configure the G2-CONTROL board for different tubes.

4.3.1 Screen voltage

You should normally use the screen voltages suggested in the table on page 9.

The following table shows the on-board changes that may need to be made for screen voltages from 210V up to the maximum limit of 550V. (‘OK’ means you should use the standard values in the component list.)

Voltage range VDR1, 2 * LK3 R9 R10

210–260V (4CX1500B, 225V)

V250LA40B (marked 250L40B)

shorted 47kΩ 100kΩ 1W metal film

275–350V OK (V320LA40B) shorted 33kΩ OK (150kΩ)

320–420V OK (V320LA40B) shorted OK (27kΩ) OK (150kΩ)

410–560V V420LA40B (marked 420L40)

150kΩ 1W (use the resistor listed as R10)

OK (27kΩ) 220kΩ 2W metal film

* Also use the same components for the two VDRs that are connected at the screen(s). A total of four VDRs will be needed.

In recent issues of the Tetrode Boards kit, Q2 has been changed from an IRF840 (500V) to a higher-voltage MOSFET, STP4NB100 (1000V). See the Components List for further details.

4.3.2 Screen current sensing

Resistor R15 sets the working range of the screen-current trip circuit. Choose a value for R15 so that your trip current (D from the Section worksheet) falls about in the middle of its working range. R15 should be a 1W power metal oxide resistor.

Trip current range R15

10mA (GS-15B) 220Ω

30–60mA 82Ω

50–100mA 47Ω

70–140mA 33Ω

100–200mA 22Ω

4.3.3 Control-grid voltage

See Section 5.1.

4.4 Further Information

There is more detail about screen voltages and currents in Application Note 3, available from the Tetrode Boards website:

• http://www.ifwtech.co.uk/g3sek/boards/tetrode/tetrode3.htm – click Downloads.


16

5. Control-grid and Relay Supply Configuration

CAUTION


5.1 Control-grid Supply

The control grid bias is generated from a negative supply that delivers approximately 10mA via R105. The fixed shunt voltage regulator consists of zener diode D114 and transistor Q101, and the grid bias potentiometer RV102 gives an adjustment range of about 5V (from the nominal zener voltage to about 5V more negative).

For different grid bias voltage requirements, change the 33V zener diode D114 (1N5364B) to another voltage in the 1N53xxB series. For example, 4CX250s with a screen voltage of +360V typically require about –55 to –60V of grid bias, so change D114 to a 1N5370B (56V).

Depending on your transformer voltage, you may need to change R105 (and possibly also R103) to deliver approximately 10mA DC into the bias regulator (approximately 5V measured across RV102).

Also see Section 7.4 for information about optional control-grid bias switching.

5.2 Relay Supply

The bridge rectifier BR101 provides both positive and negative DC voltages, so you must use a center-tapped winding (or two equal windings correctly phased). The input tags are labeled 18-0-18V AC; the center-tap connects to the center tag.

18–0–18V AC is a ‘universal’ input voltage that will usually work with both 12V and 24V relays.

For 12V relays only, 15–0–15V AC is recommended, though transformer voltages down to about 12–0–12V AC may work with some relays.

Do not use transformer voltages lower than 12–0–12V; such low AC input voltages will not give a regulated +12V DC supply from U101. Do not use transformer voltages higher than 25–0–25V AC; they will exceed the voltage ratings of C105 and U101.


17

6. Basic Inter-board Connections

CAUTION


This section deals with the basic inter-board connections that all configurations will need.

The Interconnections diagram on page 37 shows all of these basic interconnections, and also some of the Power and Control options (Section 7).

Basic Interconnections

1. Wire the REC-G1-ALC board and the G2-CONTROL board together as shown in the Interconnections diagram (page 37).

2. Wire the CHASSIS GROUND points on the two boards to a secure chassis ground. Do not rely on board mounting pillars for chassis ground connections.

3. Always wire the CATHODE tags on the two boards together. Also re-check Section 3 to confirm you are using the correct DC-grounding options.

Now connect the following off-board parts:

4. Mains transformer

The tags on the REC-G1-ALC board are marked for typical input voltages. Connect the transformer windings to the following tags on the REC-G1-ALC board:

– Screen supply: 330V AC

– Control grid supply: 105V AC

– Relay supply: 18-0-18V AC.

Do use these transformer windings for any other purposes.

5. G2 current meter M1

The full-scale deflection of the meter should be a round number, above the maximum trip current suggested in the table on page 9. Typically the meter movement will need to be either 0–50mA full-scale for small amplifiers, or 0–100mA full-scale for larger amplifiers.

Observe correct meter polarity as marked on the G2-CONTROL board. R17 is an optional meter shunt for calibration.

A typical 0–50mA meter will indicate screen current from approximately –10mA to +40mA (see Section 9.3.3 for a detailed explanation, and page 29 for a typical meter scale).

6. G1 current meter M2

For class-AB1 operation, this meter must normally be 0–10mA full-scale.

Observe correct meter polarity as marked on the REC-G1-ALC board. R108 is an optional meter shunt for calibration.

7. RV102

This is the G1 bias control potentiometer, usually mounted on the rear panel.

8. ALARM LED and RESET switch

Mount these components on the front panel. Observe correct LED polarity.


18

9. Q2

Mount Q2 on its own heatsink. There are a number of options, depending on the screen voltage, the regulator standing current (Section 4.2.2) and the MOSFET you have selected for (see further notes on page 48):

• An area of chassis that is cooled directly by cold air from the PA blower

• A finned heatsink of outside dimensions at least 4in x 3in x 1in, in a well-ventilated location with the fins vertical

• A heatsink with direct fan cooling – even a small fan helps a lot.

Use the insulated mounting hardware provided. The special thermally conductive washer requires no grease, but make sure there are no burrs on the heatsink – they can easily cut through the washer. Connect the body of the heatsink to chassis ground for safety.

Take care to connect the MOSFET correctly to the GATE, DRAIN and SOURCE tags as marked on the G2-CONTROL board, and do not expose the MOSFET to electrostatic voltages.

CAUTION

If the total standing current (Section 4.2.2) is greater than about 100mA, you may require two MOSFETs connected in parallel with equalizing resistors.

If this may apply to your amplifier, e-mail [email protected] for further details.

10. R12, R13, R14

R12 and R14 are large power resistors, and generate a lot of heat in the TX condition. If you use metal-clad resistors, they must be mounted on a large heatsink using thermally conductive grease (mounting on the chassis will usually not be good enough, and will cause overheating). You can use a heatsink about the same size as the one for Q2 (4in x 3in x 1in) –but do not make Q2 share the same heatsink!.

In the RX or Standby condition the power dissipation is much lower than on TX. No current flows through R14, and only a small ‘keep-alive’ current flows through R13 and R12.

If the RX/Standby voltage across R13 is more than about 150V (unusual), use two identical

10kΩ 3W resistors in series.

11. PTT

This is the ground-to-transmit connection from the transceiver. Check that your transceiver’s PTT output is capable of switching 12V at 140mA to chassis ground.

You can connect a SPST switch in series with the PTT line to disable the amplifier on Standby.


19

7. Power and Control Options

The Tetrode Boards offer a wide range of user options. You will need to configure these options while you are assembling and interconnecting the boards.

CAUTION


7.1 TX/RX Changeover Sequencing

7.1.1 General options

This section gives you an overview of the options for TX/RX changeover sequencing.

Sequencing Requirements

The two DPCO relays K2 and K3 give you several options for TX/RX changeover sequencing. When you press and release the PTT, K2 operates quickly but K3 operates slowly. This combination of fast and slow changeovers can generate all the necessary TX/RX sequencing by inter-linking appropriate contacts.

RX →→→→ TX TX →→→→ RX

Fast Slow Fast Slow

Screen relay K1 • •

Coaxial relays • •

RF drive (optional TX inhibit)

• •

Grid bias switching (optional)

• •

When the PTT line is grounded, the current is about 140mA. When the PTT line is un-grounded, the open-circuit voltage is regulated at +12V.


20

7.1.2 Coax relay control

You have two options:

• Energized on transmit – the usual way.

• Energized on receive – less commonly used, but has the advantage of ‘failing safe’ and protecting your preamplifier when the system is not in use.

To use one of these options, you must wire the connections to the two relays K2 and K3 as shown below.

COAX RELAYS ENERGIZED ON TX COAX RELAYS ENERGIZED ON RX

7.1.3 TX inhibit

A common problem with TX/RX sequencing is that the transceiver starts to generate RF drive before the PA is ready for it. The Tetrode Boards include a TX Inhibit feature that holds the transceiver’s EXT ALC INPUT line fully negative, preventing RF drive until the correct time in the changeover sequence. You can use this feature if your transceiver has an external ALC INPUT connection, and if the transceiver’s ALC recovery is fast enough to allow an acceptably quick changeover.

2

The TX Inhibit feature can only be used if you are also using ALC – see Sections 7.5 and 9.5. Also, you cannot use both TX Inhibit and the G1 switching feature (Section 7.4) because they use the same changeover contacts on K2 and K3.

K2 - FAST

K3- SLOW

RL IN

4

1

NONC

G2-CONTROL BOARD

(TRACK SIDE)

REC-G1-ALC BOARD

TO 'INH IN' TERMINAL ON

2 The Yaesu FT-990 and FT-1000 series have an alternative way to inhibit the transmitter until the

amplifier is ready. The transmitter will only operate when pin 8 of the BAND DATA socket is grounded. To use this feature, wire the ‘N.O’ contacts of K2 and K3 in series, and use them to switch pin 8 to chassis ground.

RL IN

NONC

1N4001

G2-CONTROL BOARD(TRACK SIDE)

RL IN

NONC

1N4001

4

1

4

1


21

7.2 Coax Relay Voltage

The operating voltage for the external coaxial relays is connected to RL IN on the G2-CONTROL board.

If your relays work quickly and reliably with 12V DC, link +12V OUT on the REC-G1-ALC board directly to RL IN on the G2-CONTROL board. The maximum total current from the +12V HV ENABLE and +12V OUT tags is 1.5A, limited by the 7812 voltage regulator IC which also provides short-circuit protection.

For 24V coax relays, link RL-UNREG on the REC-G1-ALC board to RL IN on the G2-CONTROL board. The unregulated voltage is 24–25V, which should be enough to operate relays up to 24V DC.

For other relay operating voltages, you must organize your own power supply. CAUTION – MAXIMUM VOLTAGE 50V AC/DC.

7.3 HV Supply Control

The +12V HV ENABLE tag on the G2-CONTROL board is the safety interlock to your HV (B+) supply. This tag provides +12V DC to a mains power relay in the HV supply. The control voltage is available after the warm-up timer has cycled. If the trip circuit operates for any reason, the HV control voltage is removed in less than 5 milliseconds.

HV control is an important safety feature. We strongly recommend that you use it!

To use the +12V HV ENABLE feature you must install a 12V DC-operated relay to interrupt the mains supply to the HV transformer. Make sure that the relay is capable of handling and breaking the maximum overload current of the transformer – with a large safety margin.

The maximum total current available from the +12V HV ENABLE and +12V OUT tags is 1.5A. The current is limited by the 7812 voltage regulator IC which also provides short-circuit protection.

7.4 G1 Switching

The Tetrode Boards offer you the option to switch the control grid to a more negative voltage on receive. However, for most tetrodes, switching the screen grid to cathode using K1 is enough to ensure zero anode current in the RX condition.

If you know that you will not require G1 switching, make a wire link on the REC-G1-ALC board in place of R107, and ignore the rest of this section.

If you do require G1 switching, R107 (4.7kΩ 2W) is supplied with the kit. Also connect the G1 SWITCH tag on the REC-G1-ALC board to K2 and K3 on the G2-CONTROL board as shown below, and connect the switched line to the CATHODE tag.

G2-CONTROL BOARD

(TRACK SIDE)

RL IN

NONC

G1 SWITCH

CATHODE

G1 SWITCH


22

If you are using G1 switching, you may also need to alter R107 from the value provided in the kit

(4.7kΩ 2W), but do not exceed the tube manufacturer’s maximum grid-cathode voltage in the RX condition.

Note that you cannot use both G1 switching and the TX Inhibit feature (Section 7.1.3) because they use the same changeover contacts on K2 and K3.

7.5 Automatic Level Control (ALC)

The Tetrode Boards provide an automatic level control (ALC) output to feed back to your transceiver. ALC is recommended to prevent overdriving your linear amplifier.

ALC is derived by sensing the control-grid current. The ALC circuit on the Tetrode Boards is normally configured for class-AB1 operation. That should mean zero grid current under all conditions of drive! In reality, ALC control will begin at a few hundred micro-amps of control-grid current.

To use ALC, simply connect the ALC OUT tag on the REC-G1-ALC board to the EXTERNAL ALC input of your transceiver. The ALC OUT tag provides an industry-standard negative voltage, adjustable to suit your transceiver.

Even if you choose not to use ALC, the Tetrode Boards will still be monitoring your control-grid current to protect the tube. The circuit will disable the PA and light the ALARM LED if you run more than a few milliamps of grid current – reduce the RF drive level and press the RESET button to continue.

7.6 Additional Fault Monitoring

The AUX TRIP IN tag on the G2-CONTROL board can be used to prevent PA operation if this tag is grounded. For example you could use a blower airflow switch (open-circuit when air is flowing).


23

8. Building the Kit

CAUTION


The standard Tetrode Boards kit contains extra resistors that you can select according to the screen grid voltage and current required. Sections 4 and 5 explained how to select these values for specific tubes. (At the end of assembly, you will therefore have a few resistors left over.)

If you are providing your own components, use the component list at the rear of this manual (check the Tetrode Boards website for any updates) and follow these instructions as applicable.

8.1 Mounting the Boards

Use the bare boards as templates to mark the chassis fixing holes (hole centers 4.5in x 3.5in).

Fix the two boards to the chassis on 0.5-in (12mm) pillars. Take care with insulation around the isolated mounting holes – high-voltage tracks pass nearby. The REC-G1-ALC board generates significant heat, so it must be mounted above the chassis to allow the heat to rise freely.

If you intend to stack the two boards, the REC-G1-ALC board must be mounted on top to dissipate heat. Use 1.5-in (35–40mm) pillars. To give access to RV1 and RV2 on the G2-CONTROL board for setting-up, drill out the two holes on the REC-G1-ALC board marked ‘RV1’ and ‘RV2’.

CAUTION

Do not drill the holes in the REC-G1-ALC board any larger than 0.25 in (6mm) diameter. To adjust RV1 and RV2, use a slim INSULATED trimming tool, to avoid shock hazards or

short-circuits between the two boards.

8.2 Assembling the Boards

1. Insert the connector tags into the boards first.

Lay the board on a flat sheet of expanded polystyrene, component-side up. To insert a connector tag, hold it with long-nosed pliers and tap it gently into place with a very small hammer. When all the tags have been inserted, solder them to the PC pads underneath the board.

2. Wire the above-board links using insulated solid wire.

The G2-CONTROL board has two links (LK1 and LK2) above the board. For screen supply voltages up to 400V, also wire the link LK3 in line with R8. For higher voltages, use a second resistor in this position (see Section 4.2).

The REC-G1-ALC board has two wire links (LK1 and LK2).

3. If necessary, install suitable meter shunt resistors: R17 on the G2-CONTROL board; and R108 on the REC-G1-ALC board.

4. Identify the components in the kit – see below.


24

COMPONENT MARKINGS

Check the markings on each component BEFORE you solder it in place... make that a DOUBLE-check!

Facts of modern life

♦ Components are getting smaller.

♦ Markings are often microscopic – and often don’t agree exactly with the full part number.

♦ There are now two different resistor color codes.

♦ The whole world is changing over to solders that contain no lead.

Sorry, that’s just the way things are... we hope the following notes will help. Resistors

Some resistors are marked with the familiar three-band value code, e.g. 10kΩ is brown-black-orange... BUT... Many resistors in the kit are marked with the newer four-band value code: 1st digit, 2nd digit,

3rd digit (always black), number of zeroes. In this coding, a 10kΩ resistor is brown-black-black-RED – so take care! If in doubt, measure the resistors with a multimeter.

Trimpots These have a two-digit marking: 1st digit is value, 2nd is number of zeroes:

500Ω 52

1kΩ 13

10kΩ 14

Ceramic capacitors The 10nF capacitors are marked 103 (read the code as “1, 0 and 3 more zeroes”, i.e.

10,000pF). The 0.1µF (100nF) capacitors are marked 104. The 4.7nF (4700pF) capacitors are marked 472 or 4n7.7

Diodes Check the small glass diodes carefully using a magnifier. All the 1N4148 diodes will usually be banded together. Some of the zener diodes have the voltage in the part number – the BZX79C12 diodes are 12V zeners, and the BZX79C15 diodes are 15V.

Transistors and ICs Install all the small transistors and the TO-220 devices according to the outlines printed on the board. Q2 is mounted separately on its own large heatsink, following the G-D-S connections printed on the board. (Note – the rectangular outline printed on the board is for an optional 3-pole connector.) Take extra care to install all of the DIL sockets with the index notch at the correct end.

Lead-free parts In future, all parts will be supplied with lead-free plating – the boards are now silver-plated! For reliable soldering, we strongly recommend you continue to use regular tin/lead solder. (In Europe, this is still legal for home constructors.)

Heatsinks You must provide the large off-board heatsink for Q2, as stated in Section 2.2. You must also provide nuts and screws to fix the TO-220 transistor tabs to all of the heatsinks. For Q2 there is a plastic bush to insulate the bolt from the transistor tab, and also a special insulating, heat-conducting washer– do not use heatsink compound with this washer. For the three small heatsinks on the boards, use heatsink compound with a nut and bolt. No insulation is required.


25

5. Assemble the components to the two boards. Observe polarity of diodes, IC sockets and ICs. Use a fine-tip soldering iron – watch out for missed pads and solder bridges.

6. Wire the two high-voltage links LK4 and LK5 under K1 on the G2-CONTROL board (view below is from under-side of board). Use Teflon insulated wire or sleeving.

7. For on-board TX/RX wiring options, follow the instructions in Section 7 and wire the necessary links in the area beneath K2 and K3 on the G2-CONTROL board.

8. When you have finished all wiring, remove flux residues, solder balls etc. from the under-side of both finished boards, using denatured alcohol or isopropyl alcohol and an old toothbrush. Rinse well and allow to dry.

9. Check both boards very carefully for missed connections, dry joints or solder bridges. Use a magnifier!


26

9. Initial Power-up

CAUTION

Do not connect the Tetrode Boards to the amplifier yet.

Do not insert tubes into the amplifier until you reach Section 10 – Power-up Your Amplifier.

If you find any problems, look in Section 9.2 for help.

9.1 Procedure

Follow these instructions carefully. Check-off each step as you go.

1. Remove any socketed ICs.

2. Disconnect the following tags at the REC-G1-ALC board: G2-UNREG, +12V OUT, CCW and RL-UNREG (if used).

3. Apply mains power to the transformer and check that the following DC voltages appear on the REC-G1-ALC board:

• G2-UNREG to CATHODE: +450V (approx, depends on transformer voltage)

• CCW to CATHODE: –150V (approx, depends on transformer voltage)

• RL-UNREG to chassis +25V (approx, depends on transformer voltage)

• +12V OUT to chassis: +12.0V

• Pin 4 of U102 (LM324) socket to chassis: +12.0V

• Pin 11 of U102 (LM324) socket to chassis: –12.0V (approx).

CAUTION

If there are any problems here, fix them before you go any further.

4. Switch off and disconnect from the mains.

5. Replace the +12V OUT connector and the RL-UNREG connector (if used). Connect the coax relays to pin 4 near K2 and K3. Connect the LED and RESET switch (the PTT test below will not work without the LED connected).

Apply power to the mains transformer. If you have configured the G2-CONTROL board to have the coax relays energized on RX (Section 7.1.2) they should change over when you apply power.

Ground the PTT line: all relays should change over, with a sequenced ‘ker-lick’ from K2 and K3. Un-ground the PTT line: all relays should change back, again with a sequenced ‘ker-lick’ from K2 and K3.

Switch off and disconnect from the mains. Then replace all the other connectors on the REC-G1-ALC board.

6. Remove the tube(s) and the HV connector from the PA, and connect the Tetrode Boards to the PA.


27

CAUTION

Before this step, you must have installed the screen-grid components in the PA, as described in Section 3.4.

In particular, you MUST have installed the screen-cathode bleeder resistor Rs!

7. Adjust R14 to the value that you calculated in Section 4.2.4. Set all trimmer potentiometers and RV102 to mid-range. Insert U2 (748) on the G2-CONTROL board (observe polarity). Do not insert the opto-couplers U3 and U103 yet.

8. Apply mains power to the transformer(s) and check that the correct voltages appear at all terminals of the tube socket(s) in the RX condition:

• Nominal heater voltage (check again later, with the tubes inserted)

• G2 at same potential as the tube cathode and the CATHODE tag (because in the RX condition G2-REG OUT is connected to CATHODE by K1)

• Approximately correct negative G1 voltage in RX condition (depending on your choice for G1 switching – see Section 7.4).

• The ALARM LED should light dimly, but not brightly.

9. Ground the PTT line to switch to the TX condition, and check the G2 voltage again:

• G2 voltage should now be present, and should be approximately the correct value with respect to the tube cathode and the CATHODE tag.

• Check that RV1 can vary this voltage around the required value. In case of problems, see Section 9.2.1.

• Move the positive test probe to the G2 METER + tag (or the meter itself). The voltage should be exactly the same as at the G2 tag on the tube socket. Un-ground the PTT line and check that the voltage does not change significantly in the RX condition. This checks the ‘keep-alive’ function that reduces power consumption in the RX condition.

10. Ground the PTT line and check the G1 voltage in the TX condition:

• Check for approximately correct negative G1 voltage in the TX condition.

• Check that RV102 can vary this voltage over a range of about 5V.


9.2 Problems?

The most likely source of all problems is wiring errors – either between boards or on the boards themselves.

9.2.1 Screen supply troubleshooting

Problem Possible Causes and Solutions

Zero voltage at screen grid

• Not in TX mode, i.e. PTT not grounded. (In RX mode, zero screen voltage is correct !)

• Unregulated supply not reaching G2-UNREG IN tag – check wiring continuity.

• K1 not switching, or under-board links to K1 missing.

• Check wiring continuity from G2-UNREG IN through to G2 METER + , G2 METER – and K1, and on to G2-REG OUT .


28


Output voltage high (unregulated – RV1 has little or no effect)

• Q2 installed ‘backwards’ with gate and source connections interchanged – check polarity (Q2 often survives this error).

• No shunt path through R14, Q2 and Q1 to CATHODE rail – check wiring continuity.

• Q2 gate failure – handle Q2 with care and observe anti-static precautions!

• Current leakage in D6 is preventing the MOSFET Q2 from turning on. (If D6 is a 4.7V zener, as supplied with earlier kits, change this component to a 12V zener – see the Components List.)

• Voltage on cathode of D7 (band) should be +82V with respect to CATHODE rail.

• Voltage on collector (tab) of Q1 should be +30V with respect to CATHODE rail.

• If Q1 collector voltage is +30V, then the reference voltage at pin 2 of U2 should be +23V with respect to CATHODE rail.

• When the circuit is regulating correctly, the voltages at pin 2 and pin 3 of U2 should both be almost exactly 23V.

Output voltage low (unregulated – RV1 has little or no effect)

• Output from U2 not reaching Q2 gate.

• If the voltage is correctly regulated for a few seconds but then drops out of control, the standing current is probably too high for a single MOSFET (e-mail for details of modification).

9.2.2 Control grid supply troubleshooting

All voltages in this section are measured with respect to the CATHODE tag on the REC-G1-ALC board, and are negative.


Zero output voltage • Unregulated supply not reaching RV102.

Output voltage high (unregulated – RV102 has little or no effect)

• If you are not using G1 switching, check that R107 has been replaced by a permanent wire link.

• No path through Q101 to G1 SWITCH and CATHODE rail – check continuity.

• When G1 SWITCH is linked to CATHODE, voltage on collector (tab) of Q101 should be 0; voltage on emitter of Q101 (= RV102 CW tag) should be -33V.

• If you are using G1 switching (Section 5.4), and the PTT is not grounded, a high negative G1 voltage on RX may be correct !

Output voltage low (unregulated – RV102 has little or no effect)

• Check Q101, D114 and associated components for short-circuits.


29

9.2.3 ALARM LED Lights at switch-on

This can happen occasionally, due to switching surges. False alarms are more likely in wiring layouts that have long ground connections. Simply press the RESET button and continue as normal.

If problems persist, see Section 10.4 for possible solutions.

9.2.4 TX/RX sequencing

To investigate problems with TX/RX sequencing, you can slow down the make/break operation of K3 by connecting a higher-value capacitor in parallel with C25.

If you have successfully arrived here, everything is basically working.

You are ready for the next stage of adjustments, in preparation for power-up and RF testing.

9.3 Screen Supply Adjustments

Now is the time to set the screen standing current, voltage and trip current to the design values that you selected in Section 4.2.

9.3.1 Screen standing current


2. Connect a well-insulated current meter in series with R14, apply mains power to the screen transformer and adjust R14 for the correct standing current as calculated in Section 4.2.2.

WARNING – ELECTRIC SHOCK HAZARD ON R14! Before making any adjustment to R14, always switch off, disconnect from the mains

and short-circuit the screen supply.

CAUTION

To avoid damage to the exposed windings of R14 (if you are using the recommended Ohmite resistor), ALWAYS slacken the screw clip until it rattles and will move freely.

NEVER attempt to slide the clip while the circuit is live!

9.3.2 Screen voltage

If the screen supply has passed all the tests described earlier in this section, you should be able to adjust RV1 on the G2-CONTROL board to the exact G2-REG OUT voltage required (measured with respect to CATHODE – remember to ground PTT).

9.3.3 Screen meter

The exact screen voltage will also affect the current passing through the screen-cathode bleeder resistor Rs (Section 3.4) and this in turn will affect the reading on the screen current meter. The bleed current through Rs will make the meter read approximately 10mA up-scale from zero.

Use the meter’s mechanical adjusting screw to place the needle exactly on the 10mA scale mark. This indicates zero screen current (the extra 10mA is going through the bleeder resistor). In effect, a typical 0–50mA meter is now reading screen current from –10mA to +40mA.

Optionally, you can re-scale the meter to look something like this (drawn using AutoSketch).


30

9.4 Screen-current Trip

This section explains how to set the screen-current trip to the value specified from Section 4.1.

First, fill out this worksheet to calculate the resistance value RL that will be needed to load the screen supply up to the trip current level:

Calculation steps Your tube(s)


A Required screen voltage (A from Section 4.2.3 worksheet)

350V

B Required current trip level for a single tube (A from Section 4.2.2 worksheet)

40mA

C Multiply B by number of tubes 40 x 2 = 80mA

D RL = A x 1000 / C 350 x 1000 / 80 =

4375ΩΩΩΩ

E WRL = A x C / 1000 350 x 80 / 1000 = 28.0W

F Select appropriate resistors to make up RL 3.3kΩ 25W in series

with 1.0kΩ 10W; or Ohmite D50K5K*

* The value of RL only needs to be as accurate as you wish to set the trip current – a few percent is accurate enough.

Setup Instructions

1. Switch off and disconnect from the mains. Disconnect the Tetrode Boards from the amplifier.

2. Turn RV2 on the G2-CONTROL board fully clockwise.

3. Insert the opto-coupler U3 (observe polarity).

4. Apply power, and ground PTT. The ALARM LED should light dimly as usual – it should not light brightly.

When PTT is grounded, confirm that the screen current meter comes up to the new zero mark (see above).

Switch off and disconnect from the mains.

5. Connect the load resistor RL between the G2-REG OUT tag and the CATHODE tag on the G2-CONTROL board.

Apply power, and ground PTT. The screen current meter should now read the correct trip current level.

Confirm that the regulated DC voltage does not change when this load is applied. If you have a suitable oscilloscope, also check that this voltage remains clean and constant under maximum load. If you see negative spikes at 100/120Hz, these are caused by AC ripple on the unregulated input. You may be able to adjust R12 slightly to remove the spikes; but if this is not possible, the transformer AC voltage is too low – you will need to review Section 4.2.3.


31

6. With the load resistance applied, rotate RV2 very slowly counter-clockwise until the ALARM LED lights brightly and the relays drop out. If you overshoot, release the PTT, turn RV2 back a little and press the RESET button. The LED should go dim again (it is normal that the LED does not go out completely).

7. Switch off and disconnect from the mains. Remove the temporary load resistor and restore all connections to normal.

The screen current trip will now protect the amplifier during your further tests.

9.5 Control-grid Protection and ALC

Many modern tetrodes are intended for class-AB1 operation, which should involve no control-grid current at any time. The control grids of such tubes often have a very small power dissipation and require protection against excessive grid current.

The Tetrode Boards will protect the control grid of tube against excessive grid current. The circuit will disable the PA and light the ALARM LED if you drive the tube into more than a few milliamps of grid current. (If this happens, reduce the RF drive level, and press the RESET button to continue operating.)

Although class-AB1 amplifiers should operate with zero grid current at all times, tube manufacturers state that small amounts of grid current are acceptable for peak signal sensing purposes. The Tetrode Boards provide an automatic level control (ALC) output that is derived by sensing a few hundred microamps of peak control-grid current.

(Some ceramic tetrodes also display negative control-grid current at medium levels of RF drive. This appears to be normal for those tubes. At higher levels of drive, the grid current will turn around and come upward through zero.)

Setup Instructions

1. Insert the LM324 op-amp U102 and the opto-coupler U103 (observe polarity). Turn RV103 fully counter-clockwise.

2. Disconnect both of the 105V AC tags from the REC-G1-ALC board. Temporarily connect the G1 OUT tag to chassis ground (see hookup diagram below).

3. Disconnect the wire going to RV102 slider from the RV102 SLIDER tag on the REC-G1-ALC board.

4. If you are using the TX Inhibit feature (Section 7.1.3), disconnect the lead to the INH IN tag on the REC-G1-ALC board.

5. Apply power. With no grid current flowing, the voltages at the ALC OUT tag and the G1 TRIP OUT tag should both be zero.

6. Remove power. Temporarily connect the RV102 SLIDER tag via a 22kΩ 0.5W resistor to the +12V OUT tag. (Do not disconnect the 12V feed from +12V OUT to the G2-CONTROL board – it will be needed for step 9.)


32

RV

10

2 S

LID

ER

G1

ME

TE

R +

G1

ME

TE

R -

G1

OU

T

+1

2V

OU

T

10

5V

AC

RV102

G1 BIAS

REC-G1-ALC BOARD

+

M2

G1 CURRENT

DISCONNECT

DISCONNECT 22K

G1

TR

IP O

UT

G2_CTRL BOARD

TEMPORARY GROUND

7. Apply power. The grid current meter should read 0.5mA. This current also flows through the grid opto-isolator U103 and activates the protection and ALC circuits.

8. Confirm that RV103 is fully counter-clockwise. Then confirm that, at some point within RV101’s total range of travel, the voltage at the ALC OUT tag will change quickly between 0V and almost –12V.

Trouble-shooting for step 8:

• Problem: the ALC OUT tag is permanently at –11 to –12V, and RV101 has no significant effect. Solution: short-circuit R122 (under the board) and repeat step 8. You should now be able to vary the voltage correctly.

• Problem: the ALC OUT tag is permanently close to 0V, and RV101 has no significant effect. Solutions: first check that you have the correct opto-isolator in U103 (the MCT5211,

not the 4N36). If that is not the problem, change R122 to 1.0kΩ and repeat step 8. You should now be able to vary the voltage correctly.

9. Switch off power, and turn RV101 fully clockwise. Replace the 22kΩ test resistor with 3.3kΩ. Apply power, and observe that the grid current meter now reads about 3mA. Turn RV101 slowly counter-clockwise until the ALARM LED lights. Turn RV101 a little clockwise, and press the RESET button to cancel the current trip. Now turn RV101 very slowly counter-clockwise again, to find the exact trip point – if in doubt, repeat this step until you are sure.

10. Switch off and disconnect from the mains. Remove all temporary hookups and return all connections to normal.

The above adjustments will ensure that full ALC feedback will be developed at peak grid currents of only a few hundred microamps, and the amplifier will trip at 3mA to protect the tube(s). See Section 10.3 for final ALC setup instructions.

9.6 Warm-up Timer

1. If you are using an ‘instant-on’ tetrode with a directly heated filament, you do not need the warm-up timer at all. You can skip this section, and you can also leave out C15, D11, R19, R20 and U5 from the G2-CONTROL board.

2. Switch off and disconnect from the mains, and insert U5 (LM555CN) in the G2-CONTROL board (observe polarity).

3. Switch on the power – the ALARM LED will light brightly, and you will find that the PTT has no effect.

4. After about 3 minutes the ALARM LED will go from bright to dim (the LED will not go completely dark – this is normal). Now +12V appears at the +12V HV ENABLE tag and you can use the PTT.


33

10. Power-up Your Amplifier

10.1 Final Checks

1. Switch off and disconnect from the mains. Insert the tube(s) in the amplifier.

Connect the heater and G1 supplies but DO NOT connect the HV or G2 supplies.

2. Switch on the power and check the heater voltage, right at the tube pins.

Hint: stretch transparent plastic film wrap over the bottom of the PA chassis, so that you can reach all the socket terminals with a sharp-pointed voltmeter probe while still applying cooling air to the tube(s).

3. Check that approximately the correct G1 voltage is actually reaching the tube pins, in both RX and TX conditions.

4. Switch off and disconnect from the mains. Disconnect the heater supply and connect the G2 supply. Apply power and check that the correct G2 voltage is actually reaching the tube pins in the RX and TX conditions.

5. Switch off and disconnect from the mains. Connect the heater and HV supplies. Apply power and wait for the warm-up period to complete. If you have used the +12V HV ENABLE option (which is strongly recommended – see Section 7.3) the HV will not be enabled until the end of the warm-up period.

6. When the warm-up period completes, the ALARM LED will go dim and HV will be applied. The tube anode current should still be zero until you key the PTT.

7. Ground the PTT line but do not apply RF drive. Adjust the control grid bias using RV102 to obtain the correct zero-signal anode current.

Congratulations –the Tetrode Boards are completely checked out and ready for use!

10.2 RF Testing

RF testing of power amplifiers is outside the scope of this manual... but whichever way you do it, the Tetrode Boards will protect the tube(s).

You should disable ALC feedback until you have finished testing the amplifier and established correct RF drive levels. To disable ALC, turn RV103 fully clockwise.

10.3 Final ALC Adjustment

ALC adjustment should be the very last step in commissioning the amplifier.

Section 9.5 configured the ALC circuit for class-AB1 operation. That should mean zero grid current under all conditions of drive! In reality, full ALC control will be available at a few hundred microamps of control-grid current. You may just see the G1 meter move, but nothing more.

ALC is intended to deal with transient modulation peaks only. If the ALC meter in your transceiver is flickering all the time you are speaking, reduce the RF drive to the amplifier until the meter only flickers on occasional speech peaks. With correct adjustment of tuning and loading, this should ensure a clean signal.

There are two alternative ALC adjustment procedures, depending whether or not you are using the TX Inhibit feature (Section 7.1.3).


34

If you ARE using TX Inhibit

1. Disconnect the PTT line from the transceiver to the G2-CONTROL board. Ground the PTT tag at the G2-CONTROL board, so that the amplifier comes up to the TX condition, independently of the transceiver’s PTT.

2. Key the PTT on the transceiver and apply full SSB modulation. If RV103 on the REC-G1-ALC board is turned fully clockwise, you should see full RF output from the amplifier.

3. Turn RV103 counter-clockwise until the RF output from the amplifier is reduced to a very low level under all conditions of drive. Do not turn RV103 beyond this point.

This sets the correct ALC level for TX Inhibit operation, and hopefully should provide good ALC control under normal modulation conditions too. However, not all transceivers are suitable for both normal ALC operation and TX Inhibit.

If you are NOT using TX Inhibit

Adjust RV103 on the REC-G1-ALC board for smooth ALC operation with your transceiver. The ALC meter on your transceiver should only flicker on occasional speech peaks. If it moves all the time you are speaking, reduce the RF drive to the amplifier.

If you choose not to use ALC

The Tetrode Boards will still be monitoring your control-grid current to protect your tube(s).

If you drive the tube into more than a few milliamps of grid current, the trip circuit will disable the PA and light the ALARM LED. Press the RESET button and reduce the RF drive level.

10.4 False Alarms

You should not experience ‘false alarms’ and trip-outs when you press the PTT. If you do, it may be due to voltage spikes on the control lines, or a spike of full RF power when your transmitter is first keyed... or there may be a genuine intermittent problem such as sparking in the tube(s).

Trouble-shooting

Apply the following tests in the sequence shown:

1. Remove the tubes and test again by repeatedly keying the PTT. Re-check the screen current trip setting that was described in Section 9.4.

2. Test again with tubes that are known to be problem-free.

3. Try shorter and thicker wires for the CHASSIS_GROUND straps on the two boards. Route the CHASSIS_GROUND connections from both boards to a single point on the chassis.

4. Remove the G1_TRIP_IN connection to the G2-CONTROL board. If the false trips no longer happen, there may be a spike on this line caused by inadequate grounding. Alternatively, your transceiver may be producing a spike of full RF output when the key is first pressed.

If you cannot improve the grounding any more (step 2) then change R126 to 10K and insert a 10K resistor on the G2-CONTROL board in place of LK2.

5. If steps 3 or 4 do not work – perhaps because the problem is in your transceiver – increase

C14 to 0.47–1.0µF. This has the disadvantage of increasing the reaction time of the trip sensing (but the main delay is still in the HV control relay).

6. If you are using DC-grounded screen, with the cathode and B-minus line floating below ground, you may see a screen-current trip on TX/RX switching. This is because the switching makes the B-minus line change potential, and thus charges and discharges the RF bypass capacitors at the cathode. These pulses of charge/discharge current are sensed as ‘screen current’ and may cause a false trip. The solutions are to check that the bypass capacitors are not larger than needed for RF performance, and to equalize the total capacitances from anode to chassis and from cathode to chassis. It may also help to increase C14 to 0.47–

1.0µF.


35

10.5 That’s All !

You should not need to make any of the above adjustments again, except perhaps to readjust RV103 (ALC output level) if you change transceivers.

The Tetrode Boards will continue to provide protection and stable operating voltages for your amplifier under all operating and fault conditions.

Normal operation

Normally, the trip circuit will never operate, and the ALARM LED will stay dim.

When the screen-current trip operates, the ALARM LED lights and the PA is placed in Standby mode with the screen voltage removed. HV will also be removed if you have used the +12V HV ENABLE option, which is strongly recommended – see Section 5.3.

When the trip has operated, the tube heaters are still powered, so you only have to press the RESET button to return to normal operation. Before you press the PTT, wait a few moments for the HV to come up.

If there is still a fault, the trip will operate again as soon as you press the PTT.


36

11. Updates and Product Support

Updates and further Application Notes will be provided on the web:

• http://www.ifwtech.co.uk/g3sek/boards

For advice on details not covered by these notes, you can e-mail G3SEK direct:

• [email protected]

If you purchased the Tetrode Boards from Tom’s Tubes in the USA, please e-mail [email protected] to be included on the list for any future e-mail updates.

Interconnections


37

Labels for terminals on the boards are shorter versions of the ones you see here.

CATHODE

R14R14

G2-UNREG IN

G2-REG OUT

RL I

N

PT

T

1 2 3 4

J1

G1 T

RIP

IN

AU

X T

RIP

IN

+12V

HV

EN

AB

LE

+12V

IN

R12R12GATE

TO Q2 - DRAIN

SOURCE

G2 METER +

G2 METER -

R12

R14+

M1

G2 CURRENT

RV

101 C

CW

G1 S

WIT

CH

RV

101 C

W

RV

101 S

LID

ER

G1 M

ET

ER

+

G1 M

ET

ER

-

G1 O

UT

G1 T

RIP

OU

T

+12V

OU

T

18-0

-18V

AC

ALC OUT

CHASSIS GROUND

INH

IN

330V AC

G2-UNREG OUT

CATHODE

105V

AC

RV102

G1 BIAS

TO HV SUPPLY ALC

PTT

G2

G1

CATHODE

TO TUBE(S)

GROUND CATHODE / G1 / G2, AS REQUIRED

24V 12V COAX RELAY OPTIONS

TRANSCEIVER

CH

AS

SIS

SE

E N

OT

ES

RESETALARM

SEE NOTES

SEE NOTES

SEE NOTES

SEE NOTES

SEE NOTES

REC-G1-ALC BOARD

G2-CONTROL BOARD

RL-UNREG

+

M2

G1 CURRENT

TX/RX OPTIONS

G2-CONTROL Schematic – Sheet 1


38

VI3

VO2

AD

J1

U1LM317L

D1

BZX79C15

D2(15V 0.5W)

Q1TIP122

R1 270R

R24k7

C210u 35V

C110u

R439k

C54n7

R52k7

R3470R

R10150k 1W typ

R15 82R 1W typ

D7BZT03C82

VDR1V320LA40B

C1010n 1kV

C11470n

31

4 2

BR1

R16 1k5

C633p

C7100n

D51N4148

D41N4148

R8

RV110k

C84n7 1kV

C9100n

U3A4N36

VDR2V320LA40B

D91N4001

D101N4001

R11 82k

C3100n

3

27

46

158

U2748

C4100n

1kV

470k1W

G2-UNREG IN

(COMMON NEGATIVE)

1

2

(U3B on sheet 2)

+30V rail

+23V reference

R927k typ

R17Meter Shunt

D31N4148

Q2STP5NB100 typ

R1310K 2W typ K1A

D

S

G

K1B

R6 22k

- Ig2

+ Ig2

METER, 50mA

50W

RX

RX

Q1 REQUIRES A SMALL HEATSINKQ2 REQUIRES A LARGE HEATSINKR12 AND R14 HAVE HIGH HEAT DISSIPATION

4k7 = 4.7k = 4700 ohms etc4n7 = 4.7nF = 4700 pF etc.

EUROPEAN COMPONENT CODES

WARNINGTHE SCREEN VOLTAGE INPUT AND OUTPUT

ARE UNGROUNDED!USER MUST PROVIDE GROUNDING

AS REQUIRED.

CATHODE

G2-REG OUT

(82V 3W)

(12V 0.5W)

(4.7V 5W)D8 1N5337B

LK3

Issue 3C 040416 (c) IFWtech Ltd

TETRODE BOARDS

G2-CONTROL BOARD: SCREEN REGULATOR

G D S

R7 22k

D6BZX79C12

R14 - SEE TEXT

1k

typ

R12 - SEE TEXT 50-100W typ

Values marked "Typ" depend onscreen voltage and current required -see text and Components List

G2-CONTROL Schematic – Sheet 2


39

U3B4N36

VI1

VO3

GN

D2

U4 78L05

C1210u

RV2

500R

R2310k

R2210k

R2710k

R2610k

D131N4001

R2410k

R2510k

C13100n

5

4

D111N4148

R20100k

R191M

R21470R 1W

LK1

D121N4001

GND1

TR2

OUT3

RST4

VCC8

DSC7

THR6

CNT5

U5 LM555CN

Q4

2N4403

Q5

MPS2222A

12 K1 12V

AUX TRIP IN

G1 TRIP IN

Q32N5061 C15

100u

1 2 3 4

J1

LEDALARM RESET

SW1

C16100n

R2882R 1W

D161N4001

D171N4001

C172200uF 16V

D151N4001

D141N4001

(TO HV PSU)

12 K3 6V

+12V HV ENABLE

12 K2 12V

PTT (GROUND)

K2B

K3A

K3B

K2A

+12V REG IN"FAST"

"SLOW"

USER-CONFIGURABLE RELAY SEQUENCING

LK2

C14100n

R18220R

Q6

IRF9520

Issue 3C 020317 (C) G3SEK

TETRODE BOARDS

G2-CONTROL BOARD: CONTROL CIRCUIT



R2910k

FOR EXTERNAL COAX RELAYSRL-IN : SUPPLY VOLTAGE INPUT

TO

CO

AX

RE

LA

YS

ETC

SEE APPLICATION NOTESFOR WIRING OPTIONS

REC-G1-ALC Schematic


40

R101

100K 2W

R102100K 2W

C101100u 385V

C102100u 385V

D1141N5364B (33V)

Q101TIP122

C103220u 200V

R105

3k3 2W

R1094K7 2W

R1031K5 2W

BR101

2W04

D110-113

1N4007

D101-108

1N4007

IN1

OUT3

GN

D2

U101 7812

INH IN

+12V OUT

G1 OUT

(REAR PANEL)

D1201N4001

C1054700u 35V

D1191N4001

C106100n

D116 1N4001

R108 Meter Shunt

R106

10K 1W

330V AC

105V AC

G1 METER 0-10mA

A

K C

B

E

A

K

E

B

C

1

2

6

5

4

U103MCT5211

D123

1N4148

RV1011K

U102A

LM324

R1192K7

C116

1u0

R120

1M0

U102D

LM324

R116 10K

R115 10K

C11433p

C11233p

R111 10KD121

1N4148

U102CLM324

C111100n

C113100n

D1221N4148

D1171N4001

C118

100n

U102B

LM324

R124 10K

R12322K R125 470K

R121 10KC119

33p

R126 100K

D1241N4148

C121100n

G1 CURRENTTRIP (TO Q3)

R104100K 2W

+12V

C11733p

411

1N5370B (56V)

U102

LK1

R107

4K7 2W

R118 470K

R122470R

C120100n

C115 100n

RV103

10K

G2-UNREG

CATHODE

G1 SWITCH

C104100n 100V

D115 1N4001

18-0-18V AC

C107100u 35V

R110 470R 1W

ALC OUT

+12V

C108100n 35V

C109100n

1

2

3

5

6

7

8

9

10

12

13

14

D118

BZX79C12

LK2

C110100n

R112 10K

R113100K

-12V

RV102

500R 1W

R114 100R

+

_

R117470K

RL UNREG (+24V)

Board Rev 3D 000831 (C) G3SEK

TETRODE BOARDS

REC-G1-ALC BOARD



R12822K

R12733K

(or link)

470R

G2-CONTROL Board Layout

Actual size is 5in x 4in


41

REC-G1-ALC Board Layout

Actual size is 5in x 4in


42

If you are stacking the two boards, drill 0.25in (6mm) max.

See warning note!

The Tetrode Boards

Components List

Boards: G2-CONTROL issue 3B, REC-G1-ALC issue 3D

Manual: Issue 1.21, May 2006 43 1998-2006 IFWtech Limited

SUPPLIERS

There are many sources for most of these components. The Farnell # columns shows order codes from Farnell Electronics (http://www.farnellinone.co.uk). Farnell have associate

companies in many countries, including Farnell Chicago in the USA (1 800 718 1977– note that US order codes may differ).

In the USA, Mouser Electronics (http://www.mouser.com) is probably the best single source; Digi-Key (http://www.digi-key.com) is also a good source for most parts.

Resistors and capacitors may be subject to minimum order quantities. Small quantities can often be bought more cheaply from other dealers, e.g. Maplin in the UK.

Note that you must also supply some off-board parts – see this list and also Section 2.2.

‘TYPICAL’ VALUES

Some component values depend on the output voltages and currents required. These values are marked ‘typ’ in the list below and in the schematics –

see the cross-references for further details.

The Tetrode Boards

Components List



Capacitors

Total C # Value Volts Comments Farnell #

5 C6, C112, C114, C117, C119

33pF 100 Ceramic, 0.1" radial leads 113-8847

1 C5 4n7 (4700pF) 63 Ceramic, 0.1" radial leads 113-8879

1 C8 4n7 (4700pF) 1000 HV ceramic, Murata 952-7249

1 C10 10n (0.01uF) 1000 HV ceramic, Murata 952-7222

16 C3, C4, C7, C13, C14, C16, C106, C108, C109, C110, C111, C113, C115, C118, C120, C121

100n (0.1uF) 63/50 Multilayer ceramic, 0.2" radial leads

750-992

1 C104 100n (0.1uF) 100 Polyester, BC (formerly Philips) 368

304-013

1 C9 100n (0.1uF) 1000 Polyester, BC/Philips 375 1.1" radial leads

659-733

1 C11 470n (0.47uF) 100 Polyester, BC/Philips 368 0.6" radial leads

304-050

1 C116 1.0uF 50 Electrolytic, 0.1" radial 945-1358

3 C1, C2, C12 10uF 35 Electrolytic, 0.1" radial leads 945-1242

1 C15 100uF 16 Electrolytic, 0.1" radial leads 945-1080

1 C107 100uF 50 Electrolytic, 0.2" radial 945-1412

2 C101, C102 100uF 385 Electrolytic, Panasonic TSUP

652-295

1 C103 220uF 200 Electrolytic, Panasonic TSUP

383-6927

1 C17 2200uF 16 Electrolytic, 0.2" radial leads 945-1137

1 C105 4700uF 35 Electrolytic, Panasonic TSUP

652-088

The Tetrode Boards

Components List



Resistors

‘R’ in resistor values means Ω, e.g. 15R = 15Ω, 0R33 = 0.33Ω, 3K3 = 3.3kΩ, 1M0 = 1.0MΩ etc.

1W, 2W and 3W resistors are metal film power types, e.g. BC/Philips PR02 and PR03.

Total R # Value

("R"= ΩΩΩΩ)

W Comments Farnell #

1 R15 47R typ 1 See Section 4.3.2 for alternative values

947-4625

1 R28 82R 1 337-729

1 R114 100R 0.25 543-147

1 R18 220R 0.25 543-226

1 R1 270R 0.25 934-1633

3 R3, R106, R122

470R 0.25 934-1943

2 R21, R110 470R 1 337-810

1 R12 1K typ 50 Optimum value will vary – see Section 4.2.5.

1K0 50W metal-clad supplied in kit, but may not be optimum for all applications.

Calculate the correct value before ordering.

1 R14 1K 50 Wire-wound adjustable resistor – see Section 4.2.4.

Mouser: 588- D50K1K0

Digi-Key: D50K1K0-ND or AVT50-1.0K-ND

1 R16 1K5 0.25 543-421

1 R103 1K5 2 550-190

2 R5, R119 2K7 0.25 543-482

1 R105 3K3 2 550-218

1 R2 4K7 0.25 934-1951

2 R107, R109 4K7 2 550-220

13 R22, R23, R24, R25, R26, R27, R29, R111, R112, R115, R116, R121, R124

10K 0.25 934-1110

1 R13 10K 3 If the RX/Standby voltage across R13 is more than about 150V (unusual), use two identical 10K 3W resistors in series.

327-8335

4 R6, R7, R123, R128

22K 0.25 543-706

The Tetrode Boards

Components List



Total R # Value

("R"= ΩΩΩΩ)

W Comments Farnell #

1 R9 27K typ 0.25 See Section 4.3.1 for alternative values.

934-1650

1 R127 33K 0.25 934-1757

1 R4 39K 0.25 934-1862

1 R11 82K 0.25 543-846

3 R20, R113, R126

100K 0.25 934-1129

3 R101, R102, R104

100K 2 550-309

1 R10 150K typ 1 See Section 4.3.1 for alternative values

338-102

3 R117, R118, R125

470K 0.25 544-024

1 R8 470K 0.75 337-493

2 R19, R120 1M0 0.25 544-103

0 R17 Meter shunt

Not provided in kit

0 R108 Meter shunt

Not provided in kit

RV # Value Comments Farnell #

2 RV1, RV103 10K Bourns 3306P series 108-239

1 RV2 500R Bourns 3306P series 108-235

1 RV101 1K Bourns 3306P series 108-236

1 RV102 500R 1W

Allen-Bradley W1 series, rear panel mounted

351-246

The Tetrode Boards

Components List



Semiconductors etc

Total Part # Type A / PIV / W Comments Farnell #

2 BR1, BR101 AM152 (2W02) 2A 200V 938-1449

2 D1, D2 BZX79C15 15V 0.4W 369-512

8 D3, D4, D5, D11, D121, D122, D123, D124

1N4148 984-3680

2 D6, D118 BZX79C12 12V 0.4W 369-494

1 D7 BZT03C82 82V 3W 939-8490

1 D8 1N5337B 4.7V 5W 955-7946

13 D9, D10, D12, D13, D14, D15, D16, D17, D115, D116, D117, D119, D120

1N4001 Or any higher-numbered 1N400x

956-4993

12 D101-108, D110-113 (D109 not used)

1N4007 1A 1000V 956-5051

2 D114 (kit contains two alternatives)

1N5364B

1N5368B

33V 5W

47V 5W

See Section 4.3.3

955-8217

955-8250

2 K1, 2 8A 2PCO 12VDC relay, Schrack or Potter & Brumfield type RTE24012

117-5020

1 K3 8A 2PCO 6V DC relay, Schrack or Potter & Brumfield type RTE24006

117-5019

2 Q1, Q101 TIP122 100V NPN Darlington 980-4021

1 Q2 IRF840, STP5NB100 etc

Depends on screen voltage and current – see notes on page 48.

1 Q3 2N5061 1V 350µA gate 363-121

1 Q4 2N4403 40V PNP, hFE 100 min

E-B-C pinout 920-7473

1 Q5 MPS2222A 40V NPN, hFE 100 min

E-B-C pinout 955-6842

1 Q6 IRF9530N 100V 6A,

Rds(ON) 0.6Ω

P-MOSFET, G-D-S pinout

864-8603

1 U1 LM317LZ Any LM317 in TO-92 package 948-8545

1 U2 UA748CN Any 748 equivalent (but must be a 748)

302-4660

1 U3 4N36 Current transfer ratio 100% @ 10mA

102-1094

1 U103 MCT5211 Current transfer ratio 110% @ 1mA

326-616

1 U4 LM78L05ACZ Any 78L05 in TO-92 package 948-9444

1 U5 LM555CN Any CMOS 555 (but must be CMOS)

948-8243

1 U101 MC7812CT Any 7812 (12V 1A, TO-220 pkg) 966-6109

1 U102 LM324N Any LM324 in DIL package 975-5926

The Tetrode Boards

Components List



Total Part # Type A / PIV / W Comments Farnell #

4 VDR1, 2

V320LA40B typ See Section 4.3.1 for alternatives

Install the 2 extra VDRs at the tube sockets. For a 1-tube amp, connect in parallel

241-465

Other parts

Total Part # Type Comments Farnell #

2 6 DIL socket 428-5610

2 8 DIL socket 428-5566

1 14 DIL socket 428-5578

1 For Q2 TO-220 mounting kit (‘dry’ silicone insulating washer and plastic bush)

User provides nut & bolt

522-636

3 TO-220 heatsinks Vertical mounting with lugs User provides nuts and bolts

170-070

60+ Connector blades PCB mounting

2.8 x 0.8mm 25 for each board

342-3815

60+ Blade sockets 2.8 x 0.8mm 25 for each board

209-314

Options for Q2

There are several options for Q2, depending on the screen voltage and current required.

• Voltages up to about 400V, screen currents up to about 100mA: IRF840 is best value.

• Higher voltages and/or higher currents: STP4NB100, STP5NB100 (TO-220 package); or STW8NB100, IRFPF40, IRFPG30, IRFPG40 (TO247/TO3P package).

The kit is now supplied with the STP5NB100 (Farnell 323-9342) which is good for most applications.

When screen voltages and currents are high, the FET requires a specially good heatsink – an area of chassis cooled directly by the PA blower is ideal, and even a small heatsink fan will help a lot.

If the total standing current (Section 4.2.2) is greater than about 100mA, you may require two MOSFETs connected in parallel with equalizing resistors as shown below. The transistors must be mounted at least 40mm (1.5in) apart on the heatsink.

S

G

D

NEMOSx2STP5NB100

47

47

4.7 1W4.7 1W

S

G

D

NEMOSx2STP5NB100

47

47

4.7 1W4.7 1W

The Tetrode Boards

The Tetrode Boards

1998-2006 IFWtech Limited

Documents

Tetrode Manual