Thanks Phil...... I have more tubes on the way should have them mid-week. The tubes I have bias fine in the other amp? I put another resistor in parallel to the 6.8k... The bias is now adjustable to a sane level which should add some tube life.. Lowest on the bias pot went from 45mA to 27mA so I was able to dial in around the 33-35mA that seems to do well in this circuit..Phil_S wrote:Lack of restraint noted...
Your bias voltage is -40/-41. The schematic I'm looking at says -40V on it. IMHO, there is nothing wrong with your circuit. Unless I'm missing something (and that's possible), I think you have a pair of power tubes that just want to do what you are observing. I take it that you don't have a spare pair of power tubes to verify that it's the tubes? Then the cure for this is to modify the bias circuit to give you a little more tweakable voltage.
You need to go more negative, maybe to -45V (not really sure). Increase the voltage supply by tacking in another 6.8K in parallel and see what that does, or decrease the bleeder resistor by adding a like value in parallel on that side of it. Do it and see what happens!
Maybe I'm insane or more OCD'ish but it still bothers me to not know what's causing this... If I didn't have the other amp with the same circuit which biased fine I would probably not think twice about this one...
