12-02-2021, 06:39 PM
In the Philco 86, the plate voltage is around 180 or more, so the -40 V bias applies. In comparison, at the same 180V plate voltage the 112A only requires -13.5 V.
You really can't measure the bias on the 71A's without them installed because the grid bias is developed mainly by the voltage drop across the power supply bias resistor. If the output tubes are not conducting, there will be little voltage drop and resulting low bias. The output 71A's provide the largest current drain on the supply and therefore the largest contribution to the bias voltage developed.
After checking all the wiring, I would install all the tubes and slowly bring up the AC with a Variac. Carefully monitor the voltages as you proceed.
You really can't measure the bias on the 71A's without them installed because the grid bias is developed mainly by the voltage drop across the power supply bias resistor. If the output tubes are not conducting, there will be little voltage drop and resulting low bias. The output 71A's provide the largest current drain on the supply and therefore the largest contribution to the bias voltage developed.
After checking all the wiring, I would install all the tubes and slowly bring up the AC with a Variac. Carefully monitor the voltages as you proceed.
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