Dual Adj. Fixed Bias

[chunkitup]

This is my first time using a dual bias. My tubes are not matched and I want to try this out. Please check out my scheme and let me know if everything looks alright. Thanks alot.

[RWood]

You don't say what your voltage source is, but I assume it is high.

Looks okay to me.

What I've done is, instead of locking into the 56K value, I make temporary use of a 100K pot and dial in the resistance that gives me the most useful range on the bias adjust pot. Then measure that result and replicate it with fixed resistors of the same value. For instance , you could determine the bias point that you don't want the amp to exceed, and use fixed resistors to ground to ensure that the bias pot cannot be adjusted any hotter than that point.

RWood

[chunkitup]

You don't say what your voltage source is, but I assume it is high.

Looks okay to me.

What I've done is, instead of locking into the 56K value, I make temporary use of a 100K pot and dial in the resistance that gives me the most useful range on the bias adjust pot. Then measure that result and replicate it with fixed resistors of the same value. For instance , you could determine the bias point that you don't want the amp to exceed, and use fixed resistors to ground to ensure that the bias pot cannot be adjusted any hotter than that point.

RWood

Yea the supply voltage is the HV tap. That is a good idea about not locking into the 56k resistor. I have always done that but then I have always used 6l6's. If I wanted to use KT88's or EL34's the range would be different.

[defaced]

The bias circuits will interact with each other. You need to lay them out in parallel and with their own caps. See this thread: http://music-electronics-forum.com/t5415/

Also, don't let the wipers hand out in the wind. If the pot goes open, you lose bias, and your tube goes to hell. Tie it either to the "top" of the pot so it will get max negative voltage instead of going into runaway.