A New Air-Cooled 5-Kilowatt Broadcast Transmitter*F. W. FISCHERt, ASSOCIATE, I.R.E.

Summary--This papers gives details in design and constructionof a ne-,v 5-kilowatt transmitter for use in the broadcast band of 550 to1600 kilocyclhs. By the utse of illustrations the arrangement of compo-nents in eatch cubicle is shtown and the text refers to the various majordletails. One method of adapting a single blower to the cooling needs ofan cntire transmitter is described. The supervisory, sequence inter-locked, control systemi is outlined and the placetnent of indicator lights,sw-itches, and relays used in conjunction wilth this control systeni areoztlined.

lVith the use of a sirnplified schematic several specialized adapta-tions of new audio circzuits are shown and discussed. These circuitsredulce distortion to very low values and a flat frequency response isobtainted over wide linuits. Distortiont values at various percentages ofmiodulation (ire plotted in a resporise curec of a typical 5-kilowatt trans-mitt(cr.

A necons of reducinsg radio-frequency output power with split-sec-ond puts I-button control of a mnagnetic switch is also touched upon.

hIle siii;zarY at the clOSC of the paper outlines various performancechara,cteristics s lIi (is poeer consunption (it several modulation levels,audio resposlz, iadio frcquency stability, and power output.

TI .I-,5-kilow-att transmitter conisidered hierewithT is dlesignsedl for use in the standard broadcast

hsandl of z'5() to 1600 kilocycles. Design emphasishas litbee pisia (lcd ii the incorporation of a low-audio-distortion charactceristic and a flat frequeincy response.For de pewLthilit high-efficiency reccox uniits have

blower is fed into this tiansmitter subbase from whichit is distributedl to all cubicles.

Fig,. 2 Frooit view of axciter.

1 i Fro,t icew of tryanSmitti r.

becin used to replace all lon-vx-oltage tube rectifiers.All tubes iUII well below their maxinmuni ratings. Theentire transnsitter is fully air-cooled by a single blower.Adequate iiel riing is provided in all circuits.The equiprnm it is designed to be reasonablv compact

andcI still inaintaini good accessilbility to components.'lIme transinlittr proper is compossed of three ctubicles

wx hich, fIioui left to iight, containi time exciter, pow er-amplifier-, ant(i iiioclulator circuits. All three cubicles aremountecl oji a single subbase wlhiclh serves as an initer-Ullit X-nillg tYouglh anid air diuct. Thle air from a single

* Decimial classification: R355.21. Original manuscript receivedby the Institute, Junle 26, 1941. Presented, Summer Convention,Detroit, Michi<.an, June 93, 1941.

t NN7e xiiti I I.U CtliC aI d Maniifa.ictiriiL Comiipan.y, lialti-more, Nlary la I ig. 3-Front view of power amplifier.

Proceedings of the I.R.E.t February, 1942

Fischer: Air-Cooled 5-Kw Broadcast Transmitter

On the exciter and modulator are located groups ofnine lights. These lights are used in conjunction withthe overload supervisory control circuits. The over-

load supervisory control consists of overcurrent andunder- and overvoltage relays in all circuits. When oneof these relays operates, an indication is given in the re-spective circuit by a pilot light which remains lighteduntil released manually.The exciter is divided into two sections. The audio

stages, consisting of two 1603 tubes, two 807 tubes,

Fig. 6-Fundamental schematic of audio circuits.

Fig. 4-Front view of modulator.

and four 828 tubes, are arranged from bottom to topin the left half. In the right half are located the twoWestinghouse Type HH crystal oscillators, the 807radio-frequency buffer stage, and in the upper com-partment the 805 radio-frequency driver stage.

Behind the center door are located the exciter OFF-ON switches, the low-power stage indicating instru-ments, and all bias-voltage and tuning controls. All

Fig. 5-Fundamental schematic.

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Proceedings of the I.R.E.

circuits are interlocked so that proper sequence start-ing is fully realized.

Behind the lower dooi in the power amplifier arelocated the control relavs associated witlh the power

0-

400 1000 500 7500FREQUENCY CYCLES PEY SECOND

Fig 7 Distortio0 CUrCes.

amplifier and the modulator. On the power-amplifierpanel the mailn-rectifier control switches and the recti-fier regulator conitrol buttons are located. The grid-an(d plate-toiling controls are brought out on this

Fig. 8 zRear of excite(r

ptanel and push buttons for reducing transmitter out-

put power ft-oni 5 to 1 kilowatts are located here also.P'ower reduction is accomplished bv reducing finalamplifier plate voltage through interlocked contactors.

In the lower front portion of the modulator, the six872-A rectifier tubes arc located. The modulator panelhas the bias-rectifier switch and individual bias con-

trols for the 89t- " tubes.A n(ix\ tvpe of inveise audio feedback is used in this

transmitter. It consists first of two loops; one fromplate circuits of the 828 driver stage to cathode of the807 stagc; and one from the plate ciircuit of the 891-Rmodulator tube to the cathode of the 1603 tube. Theuse of the internal 828 to 807 loop approaches thedesired condition of making the four stages of ampli-fication appear like two stages from the standpoint ofphase shift over the frequency bands covered.

Fig. 9 -Rear of pox\ or amplifier.

Second, the feedback circuit is brought to a practicalapplication by treating the audio stages as two sepa-rate amplifier channcls in parallel from the input ofthe 1603's to the plates of the modulators. This isaccomplished by eliminating all common grid, cathode,and plate impedances. Treating the audio circuits inthis fashion makes possible the use of feedback loopswhich have even-order harmonic components as well asodd harmonic components. A convxentional push-pullamplifier prevents to a great extent the proper feed-back of even-order harmonic distortion.

Third, the inverse-feedback circuit of the innier loopis designed to have a time constant of infinity at thelower frequencies by the elimination of any capacitivereactance in series with the feedback circuit. This hasthe feature of actually giving "feedback" at the lowerfrequencies rather than a dropping off of the feedbackvoltage and a consequent increase of gain through theamplifier.

Fotirth, a capacitor. reactor, and resitor combination

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Fischer: Air-Cooled 5-Kw Broadcast Transmitter

with values selected to give a falling gain charac-teristic at both the very high and very low ends ofthe audio band covered are included. The function ofthis combination has the effect of eliminating motor-boating and oscillating caused by a phase displacementdue to design limitations of the modulation trans-former and reactor.The modulation transformer and audio reactor are

The power-amplifier cubicle houses all radio-fre-quency circuits associated with the 892-R radio-fre-quency power amplifier. The plate tank capacitor forthe 892-R is of the compressed-gas type. The inductiveneutralizing coil for the 892-R tube is located cen-trally in the cubicle and suspended from the grid-coilshield can.The modulator cubicle, as the name implies, houses

the 891-R modulator tubes. The air-cooled metal biasrectifier for these tubes is located on the floor of thecubicle. The over-all loop feedback resistors are sus-pended from the ceiling of this cubicle.The power and filter frame houses, in addition to

various control contactors, the audio coupling ca-

Fig. 11-Power and filter franme.

Fig. 10-Rear of modulator.

made to appear as a constant impedance load to theplates of the modulator. This is done by utilizing theircapacitances in combination with an inductance tosimulate a "low-pass" ir-section filter.By using the circuits as described above, audio dis-

tortion from 50 to 7500 cycles per second is reduced toless than 3 per cent and the frequency response iswithin + 1 decibel from 30 to 10,000 cycles at all per-

centages of modulation up to 100 per cent. As a mat-ter of fact, in factory setups over-all distortion iswithout difficulty reduced to considerably less than2 per cent from 50 to 7500 cycles per second. Hum andextraneous noise level are at least 60 decibels below 100per cent sine-wave modulation, unweighted.

Inside of the exciter cubicle are located the radio-frequency circuits for the first three stages, the exciterstarting contactors, and the air-cooled metal rectifiers.The radio-frequency lead to the power amplifier andthe audio leads to the 891-R modulator grids leave thecubicle through the top.

pacitor, the voltage regulator for the main-rectifieralternating-current plate input, and the filter capac-itors and reactor for the main rectifier.

In summarizing the performance of this air-cooled5-kilowatt transmitter, the following are given:

(a) 5 kilowatts of radio-frequency power are de-livered into a 70- to 300-ohm load.

(b) 16.5 kilowatts from a 3-phase, 60-cycle, 230-voltline are required at 0 percentage modulation.

(c) 18 kilowatts are required during average modu-lation.

(d) Audio response is flat within + 1 decibel from30 to 10,000 cycles at all percentages up to 100per cent.

(e) Audio distortion from 50 to 7500 cycles is lessthan 3 per cent root-mean-square at 95 per centmodulation.

(f) Rapid reduction from an output power of 5 kilo-watts to 1 kilowatt is obtained without retuningor other adjustments.

(g) Frequency stability is maintained well within± 10 cycles.

(h) The entire transmitter is fully air-cooled.

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