Adjustable Fixed Grid Bias
 

I am looking for a way to use fixed battery bias on a grid, but to make it adjustable via a pot of some sort. Seems like it should be simple enough to do, but I have to say I'm stumped. Any suggestions?

You can put the pot. across the battery as an adjustable voltage divider, just as is done with a rectified supply. The problem is that the battery will go dead. :(

Mixed cathode and battery bias. Put the pot in the cathode lead.

You could wire a high value pot in series with the battery (probably a few meg ohms) or use a low noise regulator. The only drawback is they tend to come in microscopic packages, like a .040" square :eek:

http://www.national.com/pf/LP/LP5900.html

Charge the battery (with a drop charge regulator) when the amplifier is switched off, disconnect the charger when it is on. L200 can be used for this purpose:

http://www.st.com/stonline/products/...re/ds/1318.pdf

Hi Doug,
How about variable plate voltage as long as you stay within the allowable dissipation limits this might work well enough and prevent drain on your bias battery with the potential risk for mischief that entails.. :D I've never tried it so I can't really say how well it would work, but for limited operating point tweaking it should work ok. (Anyone using a plate load ccs with fixed bias is implicitly doing the same thing any time they swap tubes.)