[Niteshade]

I tried biasing the filaments of a problem preamp yesterday and it worked!!! I had written before of how it didn't work and messed up a tube. Well- I did it wrong. Presently, the 6SN7's filaments have a +35 volt bias on them.

[jeffdavison]

I never make mistakes...... once I thought I did...... but I was wrong

[Niteshade]

There's all kinds of nifty little circuits out there that can easily eliminate pesky issues. Star grounding is good for some noise problems too. This is a little off the subject of noise: I was looking at a Hammond Type F 2A3 push-pull parallel amplifier that has an interesting bias circuit. It appears to develop a negative bias current by deriving the voltage difference between two field coils (one is 5K and the other is 1/4K.) The 5K coil is in series with the HV transformer centertap and goes directly to ground. The 250 ohm coil goes between and the negative side of the bias supply. Well- it's very interesting. You'd have to see the schematic.