The AA8V Wingfoot 813 Amplifier
High Technology Of The 1950's In The 2000's
by Greg Latta, AA8V

Typical Operating Conditions

Zero Operating Bias:
The operating bias on the amplifier can be set to any value from 0 volts to the maximum available from the bias supply. However, experimentation has shown that operating the tube with zero bias produces the best results. The bias supply is then used only to cut off the tube during standby periods.

Maximum Power Output of 260 Watts:
Operating with zero bias, and driven by the Wingfoot VFO exciter, the 813 amplifier can produce an output of 260 watts on 40m as measured with a Bird wattmeter. This is consistent with the 813 data sheets.

Conservative Output of 200 Watts:
Although the amplifier can produce 260 watts of output, the amplifier must be carefully tuned to achieve this maximum. 260 watts of output also exceeds the legal limit on the 30m band, which is 200 watts. It turns out that cutting the drive back and limiting the output to 200 watts on all bands (a decrease of 1.1 dB) substantially reduces all operating currents and results in a cooler and thus happier amplifier. For this reason I usually run the amplifier at a conservative output of 200 watts, rather than 260 watts.

Drive Requirements and Input Impedance:
Running 200 watts of output on 40m, the amplifier requires only 5 watts of drive, as measured with a Bird wattmeter. The power gain of the amplifier is then 16 dB, a very respectable gain. The input impedance of the amplifier under these conditions is approximately 300 ohms, which translates into an apparent 6 to 1 SWR for the driver. This is no problem for the Wingfoot Exciter, which has more than enough power to drive the amplifier. However, a transmitter with only 5 watts of output may not be able to drive the amplifier to 200 watts of output unless a matching network ("antenna tuner") is used between the driver and the amplifier input.

Summary of Typical Operating Conditions:
The first table below summarizes the operation of the amplifier at an output of 200 watts on the 40m band. Operation on 30m, 20m, and 15m yields similar values. The second table shows typical zero signal (idle) conditions. The third table shows typical standby conditions, when cutoff bias is applied to the tube.

Click here for pictures and information on the matching Wingfoot VFO 2E26 Exciter

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