RCA ET-3656A

GENERAL DESCRIPTION

A New High-Frequency Broadcast Transmitter

photo RCA ET-3656A

Sociologists have an intensive interest in international broadcasting because of the effect such wide intercourse will have in bringing together the diverse races of the earth. Students of economics, history, and language are similarly intrigued by its possibilities. But their interests are purely acedemic-concern only distant and indirect results. Broadcasters have a more immediate and tangible interest - the commercial application of international broadcasting. True, the presently available channels are restricted to experimental use - but this may not always be the case. Many manufacturers are building thousands of short-wave adapters as well as combination sets. The army of short-wave listeners being formed may soon start demanding more and better programs - something that can only be furnished by a service which is self-sustaining. The networks and several of the larger independent stations are preparing for this by operating short-wave relay stations on regular schedules. Unfortunately, the operation of a short-wave auxiliary is, for most stations, not yet economically justifiable. Nevertheless all broadcast stations are anxiously interested in the future of short-wave broadcasting and hence most readers of BROADCAST NEWS will be much interested in the new transmitter which engineers of the RCA Victor Company, Inc. have designed for W2XE, Columbia Short-Wave Relay station.

Design

In the past, most short-wave broadcast transmitters have been "hay-wire" in design and "composite" in contruction. Two reasons for this are obvious. First, the work was mostly experimental and frequent changes in equipment were made. Second, the programs carried were non-commercial and reliability was thus less important. But developments of the last few years have altered this situation. Engineers of the Columbia Broadcasting System in planning the new W2XE installation decided that the equipment, at least, of a short-wave transmitting station was no longer "experimental" and that therefore a permanent installation was justified. Moreover, since this service might some time be commercialized, reliability and quality equal to that of standard broadcast transmitters was necessary.

Rigid Specifications

As a matter of fact the specifications of this transmitter were even stiffer than those of a broadcast transmitter, since operation was required at different frequencies according to the time of day. Experience has led to the use of the 15-30 meter band during daylight hours and the 30-50 meter band at night. By designing this transmitter for operation in the band of 15-50 meters continuous 24-hour operation was made possible. Space is provided in the heater box for six crystals - a switch on the front of the panel allowing selection of the one to be used. All tuning controls are furnished with micro-vernier dials so that they may be quickly and accurately set at previously determined points. A flexible antenna coupling arrangement makes possible use of the same antenna at all frequencies. As a result of these features this transmitter may be used at six different frequencies during the day, and the shift from one frequency to another made with almost no loss of time.

Radio Frequency Circuits

The radio frequency circuits of this transmitter are best understood by reference to the schematic circuit diagram. The six crystals mentioned above are place in the grid circuit of a UX-210 oscillator. Low plate voltage on this tube limits the radio frequency current through the crystal. The oscillator stage is folloed by a UX-860 buffer stage. The use of this buffer stage allows the oscillator to be very lightly loaded, thus improving its regulation. Moreover, by thus isolating the oscillator stage, reaction from the following stages is practically eliminated. The buffer stage is followed by two UX-860 doubler stages. The fourth amplifier stage utilizes two UX-860's in parallel. This stage, which also acts as a doubler at higher frequencies, operates as a straight amplifier in the ... amplifier stage consists of two UV-861's oin parallel. This stage (as well as the preceding amplifier stages) operates as a "Class C" amplifier. Modulation of the last stage is economically advantageous by the use of a push-pull modulating system in which the modulators operate as "Class B" audio amplifiers.

Simplicity

The simplicity of the radio-frequency amplifier circuits of this transmitter is particularly note worthy. The tubes used in these circuits (type UX-860 and type UV-861 Radiotrons) were developed specifically for use in transmitters designed to operate in the medium high-frequency range. Since they are all of the screen-grid type the plate to grid feedback coupling is negligible and no neutralization is required. This elimination of critical neutralizing adjustments makes quick frequency change feasible
The power amplifier tank circuit is capacity coupled to an antenna tuning system which permits of great flexibility. By means of two change- ... either "voltage-fed" or "current-fed" for an antenna reactance which is either inductive or capacitive. Also it may, if desired, be fed through a transmission line.

Audio Frequency Circuits

The transmitter is designed to accomplish 100% modulation of the carrier when the audio signal voltage is fed to it at -6 db. level. Two speech amplifier stages precede the modulator stage. The first of these utilizes two UX-245's in a push-pull circuit, the second, of two UV-845's in a similar circuit. The second speech amplifier stage drives the modulator stage which consists of two UV-849's operating as "Class B" audio amplifiers in a push-pull modulating stage. This modulator stage, operating through a modulation transformer, modulates the plate and screen-grid voltages of the power amplifier. The overall audio characteristic of the transmitter varies less then 2-db. from 30 to 10,000 cycles - thus making it the equivalent of standard broadcast transmitters.

photo RCA ET-3656A

TUBE COMPLEMENT
RF stages		AF stages and modulator		Rectifiers
Number	Type	Number	Type	Number	Type
2	UV-861	2	UV-849
5	UV-860	2	UV-845
		2	UV-245

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	BOL	BOLIVIA	USA	USA