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"Single tube design produces an inexpensive unit for low power applications with provisions for expanding to higher power"

By J. A. Aurand
Broadcast Transmitter Engineering
RCA Broadcast News #107, March 1960

Fig -1-The new RCA BTF-1D FM transmitter was designed for both conventional and multiplex operation. It uses the standard BTF-10B Exciter, (see note 1), as its modulating and frequency control element. For simplicity of operation, it was desired to reduce the additional RF circuitry following the exciter to a minimum. The ideal solution was to use only one tube. By choosing the 4CX1000A, a modern high-gain air cooled tetrode, a power gain of 200 including all circuit losses is achieved. This made it possible to eliminate intermediate stages and to resistance load the 4CX1000A input circuit for added stability. Further simplification is achieved in the control circuits and power supplies by using silicon diodes.

Fig-1. (left)
A single cabinet houses the complete BTF-1D transmitter. The 1 kW PA stage is mounted near the top of the cabinet with the BTE-10B Driver/Exciter below. Meters and operating controls are mounted on the right end panel.

Fig 2

Fig 2. (right)
The author is shown demonstrating the accessibility of the BTE-10B exciter which tilts out for servicing.

Easily Accessible

Controls and meters are mounted on the vertical panel on the right side of the cabinet. Amplifier tuning controls, a multimeter, and the exciter controls are accessible through a non-interlocked front door (see Fig. 1). The exciter is mounted so it may be tilted out giving complete accessibility to all components (see Fig. 2). All other transmitter components are accessible from the back through an interlocked door.

Simplified Circuit

The output of the exciter is approximately 10 watts at the carrier frequency and is fed by 125-ohm coaxial cable to the input of the 4CX1000A amplifier tube (see Fig. 3). The amplifier input circuit is a simple parallel resonant circuit, tuned by a variable inductance with resistance swamping for stability of operation (see Fig. 4). This stage is neutralized by varying inductance in series with the screen. The output circuit is a modified pi network, having a variable inductance across the tube capacity-which is used to adjust the loading. All capacitors in the final stage are of the fixed ceramic type. A small blower on the back of the RF compartment provides sufficient filtered air for cooling the 4CX1000A for operation up to 7500 feet. The filament transformer is of the regulator type and keeps filament voltage regulation within one percent.

Easy Neutralization

Fig 3
Fig 3. (above)
This is a simplified block diagram of the BTF-1D. Note the simplicity of the entire transmitter including the silicon rectifier power supplies.

Fig 4
Fig 4. (above)
This is a schematic of the PA stage showing parallel resonant input circuit and modified pi-network output. Neutralization adjustments are made with the variable screen inductor.

Fig 5

Fig 5. (left)
The single 4CX1000A PA stage is shown here. The input and output tuning coils are shown on each side of the tube. The shield cover is easily removed to provide easy access to the stage, especially for tube replacement

The 4CX1000A tube is designed to operate at maximum power without driving the grid into the positive region. A parallel resistor and diode combination in series with the grid bias lead prevents positive grid current during tuning but permits values of negative grid current without changing the fixed bias during normal operation (see Fig. 6). For tuning, this circuit provides the transmitter with means of adjusting the input circuit and neutralizing the 4CX1000A. Without plate or screen voltage, the drive from the exciter develops a voltage across the grid resistor, which is used for tuning of the input circuit and the exciter output.

Fig 6
Fig 6. (left)
A parrallel resistor and diode combination, in series with the grid bias lead prevents positive grid current during tuning; however, it does not permit negative grid current during normal operation without adjusting the fixed bias.
The feedthrough power can also be measured by using a diode probe, which is supplied. Neutralization is completed by adjusting the screen inductance for a null on the multimeter. Operating bias is obtained from a resistor in the negative lead of the high-voltage power supply. The cathode can then be connected direct to ground and cathode bypasses eliminated.

Silicon Rectifiers

Fig 7

Fig 7. (left)
In this rear view the silicon diode recitfiers are shown at the bottom of the cabinet next to the plate transformer. Two legs of 18 series diodes are used, and each is shunted by a 27,000 ohm resistor for equal voltage distribution.

The high-voltage power supply uses a bridge circuit of silicon diode rectifiers (see Fig. 7). Each leg consists of 18 silicon diodes in series, each diode shunted by 27,000 ohms to insure equal voltage distribution. Printed circuit construction is used for each leg, making a unit with a peak inverse voltage rating of 7,200 volts. This supplies more than ample safety factor when used in a 2,700 volt supply. The screen silicon supply uses four diodes to supply the screen voltage for the 4CX1000A stage. A variable transformer in the primary of this supply provides control of the screen voltage, which is used to control the power output of the transmitter.

Breaker Protection

Overload protection for the transmitter is supplied by a line circuit breaker and two overload relays in the ground leads of the two power supplies. These overload relays also protect the 4CX1000A tube, since it constitutes the total load for both supplies. The blower is protected by a thermal overload, and the tube by an air interlock. Only two fuses are used, both are in the crystal heater circuit.

Remote Control Provisions

The BTF-1D FM transmitter incorporates connections for remote control and remote meter reading, when combined with a remote control system such as the BTR-11B. To control transmitter power level remotely, an accessory motor drive may be connected to the screen supply control. Furnished with the RCA harmonic filter, the BTF-1D meets current FCC and industry requirements with regard to spurious emission, cabinet, and harmonic radiation. Hence, this new unit is an excellent low-power, low-cost transmitter for both conventional and multiplex FM operation.


  1. "New FM Transmitter and Multiplex Equipment", Broadcast News, Vol 102, October 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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