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New High-Efficiency 5-KW AM Transmitter
"Unique Class C Amplifier Operates with 90% Efficiency"

By I. R. Skarbek
Broadcast Transmitter Engineering
RCA Broadcast News #107, March 1960

RCA BTA-5T AM Transmitter

Fig -1. Easy Access to all tubes is made through two interlocked front doors of the BTA-5T transmitter. The PA, modulator, and silicon rectifiers are in the cabinet on the right, and the exciter, driver, and control equipment is on the left. The attractively styled cabinets are available with red or gray doors.

This new 5-kw AM transmitter, type BTA-5T, incorporates the only worthwhile development in class "C" power amplifier design in 20 years. The newly-designed PA circuit operates with a plate efficiency of 90 percent. This represents an improvement of 20 percent over normal class "C" operation. As a direct result power savings of approximately 15,000 kilowatt hours per year can be realized.

Basically, the new transmitter uses the design proved in the BTA-5R/5R1. (see footnote 1).

However, using the recently-developed high-efficiency circuit, only one PA tube is needed. Other circuit innovations (such as silicon rectifiers and improved protection) have been made to improve performance and to extend operating life.

Principle of Operation

The high-efficiency plate-modulated power amplifier uses a single tube to deliver the nominal 5 kw with 5.5 kw power output capability at 90 to 92 percent plate power conversion. Referring to the simplified schematic, the circuit arrangement is very similar to a conventional class "C" amplifier except for presence of two resonators L1, C1, and L2, C2. In fact, the new high-efficiency stage behaves so much like the conventional class "C" stage that with, the resonator shorted, or mis-tuned, the PA tube returns from the high-efficiency to the conventional class "C" operation. This characteristic, as will be shown later, is very useful in the initial tune-up of the transmitter. For the moment it will be helpful in making a detailed comparison.

Rear of BTA-5T

Fig - 2. This is the rear of the BTA-5T. Complete access to all components is easily made through the removable interlocked panels.

In both systems the angle of the tube current conduction is restricted to that portion of the cycle wherein the instantaneous plate current is high and the instantaneous plate voltage low, corresponding to a low anode dissipation at a relatively high-power output. In the class "C" operation, however, the waveform is sinusoidal and is substantially rounded off. Therefore, a large portion of the power is lost in the anode, resulting in an average efficiency of about 70 percent. The new system provides corrective means for maintaining a flat waveform near the peak, resulting in 90 percent average plate efficiency.

The waveshaping is done by two LC parallel resonant circuits, one located in the plate and the other in the cathode circuit of the power amplifier tube. Both resonators are adjusted to resonate at the third harmonic of the carrier frequency.

When the amplifier tube is driven, the harmonic component of the grid input power sets up and maintains circulating current within each resonator. Since the resonator is designed to store high KVA, the total voltage supply at the plate is composed of the usual d-c plate supply and the superimposed oscillatory potential equal to the voltage build-up across the resonator. This oscillatory voltage, being at the third harmonic, vectorially adds twice to, and subtracts once from the fundamental, producing a flat top waveform (see 7A). When the cathode resonator is adjusted to resonate at the third harmonic, the instantaneous grid-to-cathode potential modifies the cathode emission to approximate a rectangular pulse (see Fig. 7B).

Improved Efficiency

In the BTA-5T transmitter, adjustment of the plate resonator improves the efficiency by 6 to 8 percent. Subsequent adjustment of the cathode resonator improves the efficiency by an average total of 20 percent above conventional class "C" operation.

Improved Tube Performance

The energy stored within the resonator modifies the instantaneous current voltage waveform of the conventional class "C" amplifier to reduce its amplitude and broaden the top. For the same power output this means not only reduction in the plate dissipation, but also considerable reduction in peak-plate to plate-peak grid current and operation at a much lower cathode emission (see Table I). As the result of the high-efficiency operation, the BTA5T transmitter employs only one PA tube, an RCA type 5762. It is worthy to note that plate dissipation in the new system is comparable to the filament power consumption of the old.

BTA Exciter
Fig - 3. The exciter and IPA stages are shown here. Note the three crystal sockets on the exciter chassis, and the easy to reach tubes.

Table 1 - Efficiency Comparisons
Output Amplifier (Carrier) Output Powerkw5.0 5.0
Number of tubes (RCA 5762) 21
Inst. Plate Current (calculated) *Amps5.63.5
Inst. Grid CurrentAmps2.71.8
Peak Cathode EmissionAmps8.35.3
Plate Dissipationkw1.5.57
Filament Powerkw.74.37
Plate Efficiency%7690
Modulator & Power Ampl (100% Mod)kw2.25.85
Modulator Plate Dissipationkw2.01.7
Overall Inputkw13.311.2
Overall Efficiency%56.467
Transmitter Power Input (.95 P.F.) Carrierkw11.610.0
Average Programkw12.711.0
Input Power/Yr Savingkwh - 15,000
* The high efficiency calculation is based on a rectangular waveform, where the peak efficiency is equal to the average efficiency, i.e. Vp/Eb. The average power value is proportional to the angle of tube conduction, so the plate input, plate output, and the grid dissipation is: Ip EpK; Ip VpK and Ig(Vg-Eg) K respectively (where K is the duty cycle or 1/3 for 120-degrees of tube conduction). The actual value given in Table 1 takes into account the correction factor determined experimentally.

Fig - 4
Fig - 4. The author is shown adjusting thenew PA stage. Only one 5762 tube is used for high efficiency operation. The coil, shown on the right, is the third harmonic coil used in the plate circuit of the PA to increase efficiency by shaping the plate waveform.

Improved Stability and Tuning

The third important feature in favor of the new transmitter is: simplicity of the initial tune-up and stability of operation. Upon mistuning the harmonic resonator, the PA tube returns from the high-efficiency to the conventional class "C" operation. Except for the loss in the efficiency of the PA, the circuit neutralization, tank tuning, the stage loading remains the same for both type of operation.

Figure 5
Fig - 5. This is a simplified schematic of the PA stage showing the location of the third harmonic coil.

Applying the above procedure in reverse, the initial tune-up consists of the resonator adjustment to obtain the maximum power output as indicated by the line or the antenna current ammeter. The plate resonator chiefly contributes toward the power output, while the cathode resonator in addition to increasing power output to some extent increases the power input.

With the system properly in operation, tuning of the output tank is similar to tuning of the conventional class "C" amplifier, except the tuning is broader. On either side of the tuning-dip the plate current rise is more gradual, the power output slightly rising on one side and falling on the other side. This self-adjusting property would provide additional stability in the case of accidental mistuning or mistuning due to a reactive load.

Fig 7A
Fig - 7A. This is the harmonic addition of the plate voltage and the oscillatory voltage from the third harmonic L-C circuit. The resultant plate voltage waveform, (below), resembles a square wave, and it is obvious the average level of plate voltage is higher.

Fig - 7B
Fig - 7B. With the cathode resonator operating at the third harmonic the grid to cathode voltage modifies the cathode emission to produce a rectangular pulse of grid current. Again, the higher average level of the signal has been increased, and this results in higher operating efficiency.

Overall Performance

Both the conventional and the high efficiency system were compared using essentially the same transmitter circuit with identical components. Each offered similar performance insofar as modulation capability, audio distortion, carrier shift and noise level are concerned (see Fig. 8). Tests were carried out over the broadcast frequency range, using a number of tubes of different socket life. Therefore, the high-efficiency class "C" operation data listed in Table I may be considered as typical for this power level and frequency of operation.2

Fig - 9

Fig - 9. Typical response and distortion curves for the BTA-5T. The high efficiency PA stage actually helps to improve the overall performance of the transmitter.

Longer Tube Life

In order to determine what effects the high efficiency circuit would have upon 5762 tube life, tubes with known expectancy life characteristics were obtained and placed for service in a transmitter operating at maximum conditions with greater than average modulation applied for extended periods of time. The tubes were then retested, opened, the parts measured, and examined for evidence of deterioration. It was found that the life expectancy of all the tubes examined to be as good or greater than the expected life of tubes operating under conventional class "C" service.

Compact and Convenient

The entire transmitter, except the plate transformer, is housed in two attractively styled cabinets (see Fig. 1). All meters, indicators, control switches and tuning controls are located on the front panels. Vertical center chassis are fastened between the end panels to form a basic "H" cross section. The front doors give immediate access to tubes, feedback ladders and overload relays, which are mounted on the vertical chassis (see Fig. 2). Remaining components are mounted on the rear of these chassis, behind removable rear panels, while the large power components are mounted on the base of the cabinet. This type of construction offers excellent accessibility, while retaining the compactness of the transmitter (see Fig. 9).

Proved Design

A simplified schematic of the BTA-ST transmitter is shown in Fig. 6. The exciter unit, the PA modulator driver stages and the high-level linear modulator remain essentially the same as in BTA-5R/5RI transmitter. The power supplies are silicon rectifier type. New instantaneous circuit breakers are employed in the control circuits. The plate-modulated power amplifier is of course improved by virtue of the new high-efficiency technique and the resultant power and tube savings.

Two Tuning Controls

The BTA-5T transmitter has only two front panel tuning controls with one local-remote power control. The driver stage is tuned by means of a slug-tuned coil, and the PA by means of a variable vacuum capacitor. Remaining circuit adjustments, necessary only at installation, consist of tap changing in accordance with the calibration chart. The PA plate resonator is adjusted by means of a front panel screwdriver slot actuating a variable vacuum capacitor. Capacitor of the cathode resonator is adjusted in similar manner. Both capacitors are adjusted for peak of the line ammeter. The variable vacuum PA neutralizing capacitor, employed in conjunction with a broadband neutralizing transformer is also preset at the initial installation.

Semiconductor Power Supply

Fig - 10
Fig - 10. The silicon high voltage rectifiers, shown here, consist of two legs each containing thirty diode units. The rectifier unit was designed with a 200 percent safety factor for increased stability.

The BTA-5T uses silicon-type rectifiers throughout. This type of rectifier offers excellent reliability in normal operation and even more so in a remote-control application. The transmitter will operate within ambient temperatures from -20 to +45 degrees and up to 7500 feet above sea level.

The proven reliability high-voltage rectifier (see Fig. 10) is arc-back protected for trouble free operation, requiring neither warm-up time nor thermostatic cooling control. It carries an over-current safety factor of 200 percent, or in other words, it is capable of continuous short circuit operation. The peak inverse voltage rating is 180 percent, allowing 30 percent above the starting transient and the silicon peak inverse voltage safety factor.

The bias and low-voltage rectifiers are sealed silicon units permitting a more reliable operation.

Complete Overload Protection

To increase reliability, improvements were made in the control and protective circuitry of the BTA-5T transmitter. Primary lines are protected by circuit breakers with instantaneous and thermal overload trip protection. The 3-phase blower motor is protected by a contactor with thermal cutoff in each phase; the relay switching is sequential so that the filament not come on unless the blower is operating. Starting surges in the plate transformer, high-voltage reactor, and the filter capacitor are eliminated by the use of step-start and damping circuit.

Protected Cooling System

Fig - 11
Fig - 11. The new high efficiency PA permits the use of a slow speed blower. This unit is mounted on rubber shock mounts in the base of the PA cubicle.

In keeping with modern trends, the transmitter is air cooled. Added refinements such as a delay relay have been built-in, to keep the blower system in operation for one minute after the transmitter has been shut down. This continues the supply of air to extend tube life. As the result of the high efficiency, the air pressure has been reduced, permitting use of a slow speed blower unit, resulting in quiet operation. (see Fig. 11).

Functional and Economical

Many years of design experience are reflected throughout. Functional styling predominates affording convenience of operation, furthermore, the user has a choice of red, or gray doors to enhance the station decor.

Emphasis, however, has been placed on the reliability and the dependable performance of the transmitter. This will result in many years of trouble-free operation. Power savings achieved by the new high-efficiency circuits provide for economical operation without sacrifice of performance.

* * *


  1. High efficiency circuits in the new transmitter offers an average power input saving of 15,000 kilowatt hours/year at a continuous program operation.

  2. Very low plate dissipation in the output stages reduces the heat dissipation within the transmitter, and also permits use of a quiet slow-speed blower.

  3. Expensive power amplifier tubes reduced from 2 to 1. Fewer tubes throughout-a total of twelve-which also saves on tube replacement cost.

  4. All semiconductor type, hermetically sealed silicon rectifiers are of proven reliability.

  5. Complete overload protection is provided for all circuits. All line breakers carry an instantaneous over-current protection, while main breakers retain the instantaneous and thermal protection combined. The remaining circuits are protected by fast acting overload relays with provision for external indicators.

  6. Built-in provision for the remote control, conversion to Conelrad, power cutback and a carrier off monitor is retained with improvements.

  7. Meets new FCC Spurious Emission requirements.

  8. The new style cabinets offer excellent accessibility to all components and allows a great saving in the floor space.


/1 "New 5000-Watt AM Transmitter", by J. Novik and I. R. Skarbek, Broadcast News, Vol. 103, March, 1959.

/2 Data on operation at other power levels is given by V. J. Tyler, Marconi Review, No. 130, Vol. XXI.


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 gnbc@wcoil.com.

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