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In Reply to: RE: Using fixed bias on a 6J5G posted by DAK on November 01, 2016 at 17:28:41
Most will use a battery in this position to provide negative bias. The problem with using a power supply derived fixed bias on a driver tube is that there is a lot of gain so any noise gets amplified, very small changes in the voltage 0.1 v will have an impact on bias versus the wider tolerance for output tubes. This can lead to a signal compression. You could use a small volt reg, but that will have an impact.
It is possible, but you will need a cap on the grid of the tube so your negative bias voltage does not go upstream. Usually the cap offsets any advantage of fixed bias when resistors or diodes are pretty capable.
Follow Ups:
I have not tried that yet. It seems like you would be changing batteries regularly. Dak
John Broskie of Tube Cad wrote a pretty good article outlining battery bias:
Since there is almost zero current to support the bias they last a long time. Need a low leakage cap on the input.
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