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New 10-KW FM Transmitter

"Only two tubes beyond exciter reduces operating costs, and produces a Compact High-Power FM Transmitter"

By A. H. Bott
Broadcast Transmitter Engineering
RCA Broadcast News #107, March 1960

Fig 1

FIG 1: Two Compact cabinets house the 10kw BTF-10C, Operating controls and meters are mounted on the front panels.

Designed for multiplex and conventional operation at a full 10 kilowatt output, the BTF10C FM Transmitter represents the latest in transmitter design. Only two tubes are used beyond the exciter, the IPA and PA, for greater circuit simplicity. The compact two cabinet transmitter has ample space provided for mounting either a BTX-1A Subcarrier Generator or the BTR-11A/20A Remote Control Equipment. When the BTF-10C is used with the BFA series of FM antennas ERP's up to 120 kilowatt can easily be obtained.

Compact Exciter

The standard BTE-10B FM Exciter is used in the new BTF-10C Transmitter as it is in every other recent RCA FM Transmitter. The exciter is completely self-contained including its own power supply. Its direct FM system assures full fidelity operation for both conventional and multiplex operation. The BTE-10B, when combined with necessary subcarrier generators, is ideal for multiplexing, and its crosstalk level is very low.

The tilt-out unit is completely accessible, and it has a built-in multimeter and oscilloscope to permit rapid tune-up. Output of the BTE-10B Exciter is approximately 10 watts at the final frequency which provides adequate drive for the IPA stage.

Fig 2

FIG 2: With doors open access to all stages is easy. The large cubicle on the left contains the PA stage, rectifiers, and the blower. On the right, the exciter and IPA stages are accessible through a non-interlocked door. The BTE-10B exciter is mounted at the bottom of the right cabinet and the BTX-1A subcarrier generator has been mounted in the upper portion.

Tetrode IPA Stage

The exciter drives a single 7034 IPA stage which produces 250 watts of driving power. Located in a shielded compartment above the exciter, the IPA stage is completely accessible, and tuning does not require removal of the cover. Input and output networks of this stage are conventional pi-networks with variable capacitors as the matching components. Tuning from 88 to 108 mc is achieved by varving the inductance in the pi-network, which is accomplished by insertion of a silver-plated brass slug into the center of the coil (see Figs. 5 and 6). This IPA stages does not require neutralization for operation throughout the FM band.

Plate voltage for the IPA is obtained from the center tap of the PA power supply. A variable screen voltage supply is used on both PA and IPA stages to control power output. The IPA is protected by overload relays in the cathode circuit. Air interlocks are also provided to remove plate and screen voltage if cooling air flow should stop.


FIG 3: This is a simplified block diagram of the BTF-10C. Note that only two single ended RF stages follow the exciter.

Simplified PA

The input of the PA is a modified pi network in which the input capacity of the tube is shunted by an inductive line to reduce the effective input capacity of the stage (see Fig. 4). This inductance is also used to vary input loading. A capacity in parallel with the coil varies the inductive component of the circuit.

Plate loading and tuning are achieved by variation of two inductive line components in a pi-network arrangement. Tube capacity is shunted by the variable inductor (see Fig. 7). The pi-network has been inverted for mechanical simplicity and this results in grounding one end of the inductance. This eliminates the problem of insulating the variable component from ground; however, the output line must be parallel to the inductance to bring it to ground potential. This is done by extending the output line down one side of the inductive line.


FIG 4: Here the PA cavity is shown with the 4CX5000A yube in place. The rectifiers can be seen at the right.

PA Tuning

Initial tuning is made by approximate setting of all variable components according to a tuning curve. Final tuning is made under reduced plate and screen voltage for circuit protection. The PA stage is neutralized by variation of the inductance in series with the screen supply. Tuning across the 88 to 108 mc FM band required changing of only one frequency determining part in the PA input circuit.

Complete Protection

The control circuit provides a starting sequence which prevents application of plate voltage before filaments are warmed up and the blower is operating. It also provides overload protection and off -frequency shut-down. Warm-up time of the exciter and filaments is controlled by a 45-second time delay relay. The overload circuit returns the transmitter to the air on the first two overloads after a clearing time of two seconds. On the third overload resetting is manual. The cathode circuits of the IPA and PA as well as the screen supplies and rectifiers are protected by overload relays. For remote control of the transmitter the necessary control and metering leads are made available at terminals located in the rear of the cabinet.

FIG 5: This is the IPA stage RF compartment. Both input and output controls are shown.

Fig 6

FIG 6: Simplified schematic of the 7034 IPA stage. The slug tuned input and output pi-networks provide frequency variation across the FM band without neutralization.

Power Supplies

This high voltage supply is a 3-phase full-wave circuit using six 8008 mercury vapor rectifier tubes. A single-section inductive input filter produces 6200 volts at approximately 2.3 amperes to supply the plate of the 4CX5000A tube. The center tap voltage of the transformer supplies the 7034 plate through a double section RC filter, which filters and reduces the voltage to approximately 1800 volts. The screen voltage supply uses germanium rectifiers in a bridge circuit to feed both stages. The primary of this transformer is supplied from a variable transformer to control its output voltage.

Fig 7

FIG 7: This is a simplified schematic of the PA. The inverted pi-network output uses two variable inductors to control loading and tuning. The input pi-network is shunted by an inductive line to reduce input capacity.

Direct Air Cooling

Three-phase power is applied to the transmitter at the line breaker located on the right-hand front panel of the 10 kilowatt cabinet. Power is then fed to three other breakers: the first supplies power to the filaments, exciter and control circuit; the second, to the high-voltage supply; and the third, to the screen supply.

The filament on switch applies power to the blower, and also to the filaments, and exciter provided the blower is operating. The filament line passes through a buck-boost circuit so that it may be adjusted to the exact voltage for which the primary filament transformer taps are set. The 45second time-delay relay is also energized which prevents application of the high voltage before the time-delay contacts close and all door interlocks are closed. This is indicated by the lighting of the ready light on the front panel.

The transmitter on switch applies the plate and screen voltage to the two tetrode stages. This is indicated by the lighting of the transmitter on light. A motor-controlled variable transformer in the screen-supply line adjusts the screen voltage by means of the power adjust switch.

The cooling system blower is located in the bottom of the 10 kilowatt cabinet, feeding air directly into the 4CX5OOOA stage. The air for the 7034 stage is tapped off the side of the 4CX5OOOA box by means of flexible tubing feeding through the side of the cabinet. Air interlocks are provided in both stages to remove plate and screen voltage if the cooling air should stop.

Fig 8

FIG 8: A third cabinet can be added to the BTF-10C to house another subcarrier generator, input and monitoring equipment or any other associated equipment. With a third accessory cabinet the BTF-10C remains very compact, yet it offers greater operating convenience.

Complete FM System

Coupled with RCA's new broadband FM antennas (see footnote 2) the BTF-10C Transmitter can provide effective radiated powers from 8 to 120 kilowatt. Supplied with RCA's standard harmonic filter, the entire chain meets all current FCC and industry standards, including the new requirements on spurious emissions, cabinet radiation, and harmonic radiation.


1 - "New 5KW FM Transmitter", Broadcast News, Vol 103, March 1959

2 - "New Broadcband FM Antennas", Broadcast News, Vol 101, August 1958

These pages were scanned, and copies from the original articles by Fred Vobbe. The purpose of this page is to present broadcasting's great history as it was applied to engineering, equipment, and the development of the industry. If you have any pictures, catalogs, flyers, or information which you would be willing to share, please contact Fred Vobbe, W8HDU at 706 MacKenzie Drive, Lima OH 45805-1835. You can also reach me via my personal E-mail account at w8hdu@hf-antenna.com.

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