OT: amp bias pot

[doug deeper]

can anyone direct me to some info on instaling a bias pot on a tube amplifier for the purpose of guitar rockage?
hahahah
just the first part would be fine.

[aron]

Try these links:

http://www.hoffmanamps.com/charts/bias_conversions.htm

http://www.hoffmanamps.com/charts/Biascircuits.htm

[doug deeper]

cool thanks!
i dont know if this really applys to my amp but its a step in the right direction!
i think this is for modding the weird not right bias pot to a correct one...
i need to replace the bias resistor with a pot...i think?
i dont know tubes!

[R.G.]

Vacuum tubes are depletion mode devices. In normal human words, that means that they are fully on when you do nothing to them, and that you have to apply a back bias to get them to turn off.

There are two ways to do this. You can tie the cathode to ground and apply a negative voltage to the grid, or you can put a resistor between cathode and ground and ground the grid. The cathode conduction through the resistor elevates the cathode above the grid voltage and so the grid is then more negative than the cathode. The two methods are called fixed bias and cathode bias respectively. Fixed bias amps have a separate negative bias supply. Cathode biased amps don't need one.

To mess with the bias on a cathode biased amp, you vary the cathode resistor. I'm assuming that you do not have one of these by the language of your question.

To bias a fixed bias amp, you vary the negative voltage applied to the grid. This usually takes the form of one or more resistors set up as a voltage divider between the negative voltage bias supply and ground. If the resistors are fixed resistors, you have to solder and unsolder them to change values. If one of them is a pot, you can rebias by truning the pot.

If you can supply us with a schematic of just the bias supply and output tube grid section of the amp by tracint it out, one of us can tell you which resistors to replace with a pot.

[doug deeper]

ahhh info.
yeah i understand biasing (thanks though  )
i just dont know tubes, as in what pin is what,
and where the resistor should be in relation.
(im sure i can find it...just cant find the correct info)
im working on this 60's univox bass head (not sure of the model)
its  2 6L6 affair.
i guess i should do some hunting.