Friday, December 5, 2008

Adjustable Bias for an Ampeg V4B

Ampeg V4 V4B adjustable fixed bias mod

This is a shot from an Ampeg V4B that's currently in for repairs. It blew the flyback protection diodes and then shorted a power tube. I thought it would be better off not biased quite as cold as these V series amps are stock.

The change is pretty simple - I replaced R49 (a 75k resistor) with a 47K resistor in series with a 20 turn 25K cerment pot. The wiper of the resistor is connected to the supply diode and one leg is attached to the 47K resistor. The other leg of the pot is unused. A dab of silicone under the pot holds it in place and keeps adjustments from stressing the leads or the solder joints. It gets the bias voltage down as low as about -40 volts. Stock its about -60 volts. The nice thing about this arrangement is that if the pot were to fail with an open circuit on the wiper the bias voltage would actually go up and the tubes would be protected.

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