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

Typical Operating Conditions

Wingfoot 813 Amplifier Pages
 Main Page and Front and Side Views  Tank Coil Information
 Interior and Back Views  Typical Operating Conditions
 Circuit Description and Schematic Diagram  Power Supply Photos
 813 Tube Information  Power Supply Circuit Description and Schematic Diagram


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 by setting the bias for a zero signal plate current of about 10mA produces the best results. 10mA keeps the zero signal input to the amplifier at about 25W and also provides a good bleeder load for the power supply. At an output of 200W, the regulation of the power supply from zero signal to 200W out is about 7.6%.

Maximum Power Output of 260 Watts:
Operating with the bias set for a zero signal plate current of 10mA, 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 Outputs of 250 Watts and 200 Watts:
Although the amplifier can produce 260 watts of output, the amplifier must be carefully tuned to achieve this maximum. Cutting the drive back to an output of 250 watts is a little easier on the amplifier, and keeps the plate dissipation well under the ICAS maximum of 125 watts. In most of my operating, I keep the output cut even further back to 200W, since this is the legal limit on 30m, where I often use the amplifier. This decrease of 1.1 dB from the maximum of 260 watts substantially reduces all operating currents and results in a cooler and thus happier amplifier.

Drive Requirements and Input Impedance:
With the 1:4 input transformer, the 813 amplifier can be driven with either tube or solid state transmitters. At 200W output the amplifier requires 16.5W of drive from the exciter on a frequency of 7.020 MHz. This represents a power gain of 10.8 dB. At 250W output the amplifier requires 26W of drive for a power gain of 9.8 dB.

Summary of Typical Operating Conditions:
The first table below summarizes the operation of the amplifier at an output of 250 watts on the 40m band. The second table summarizes the operation of the amplifier at a very conservative output of 200W. The third table indicates typical zero signal conditions.

Typical Operating Conditions With 250 Watts Output
 Parameter:  Value:
 Test Frequency  7.018 MHz
 Power Output  250 Watts
 Drive Power  26 Watts
 Power Gain  9.8 dB
 Plate Current  160 mA
 Grid Current  26 mA
 Screen Current  7.5 mA
 Plate Voltage  2241 Volts
 Plate Input  359 Watts
 Plate Dissipation  109 Watts
 Plate Efficiency  70%

Typical Operating Conditions With 200 Watts Output
 Parameter:  Value:
 Test Frequency  7.020 MHz
 Power Output  200 Watts
 Drive Power  16.5 Watts
 Power Gain  10.8 dB
 Plate Current  130 mA
 Grid Current  21 mA
 Screen Current  6 mA
 Plate Voltage  2290 Volts
 Plate Input  298 Watts
 Plate Dissipation  98 Watts
 Plate Efficiency  67%

Typical Zero Signal Conditions
 Parameter:  Value:
 Zero Signal Plate Current  10 mA
 Zero Signal Plate Voltage  2464 Volts
 Zero Signal Plate Input  24.6 Watts


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


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