Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on July 16, 2013, 08:47:32 am

Title: Bias pot questions.
Post by: hesamadman on July 16, 2013, 08:47:32 am

Im building a 2204.

I bought a BIAS pot from mojotone (i think) ive been buy parts from all over the place but I think this came from there. Well, this is an on board bias pot. I dont know if I like the idea of a chassis mount bias pot. Anyway. The leads on this is set up a little different. They are in a triangle configuration sort of. 2 are side by side and the thgird is kind of at the tip of the triangle configuration. Im not real sure which lead is which and which wire goes where. Can someone guide me in this?

Title: Re: Bias pot questions.
Post by: sluckey on July 16, 2013, 09:00:32 am

The two that are side by side are probably the outer legs of the pot and the other one is the wiper. Use your ohm meter to verify.

For the 2204, connect the wiper to one end of the pot.

Title: Re: Bias pot questions.
Post by: hesamadman on July 16, 2013, 09:12:17 am

Quote from: sluckey on July 16, 2013, 09:00:32 am

The two that are side by side are probably the outer legs of the pot and the other one is the wiper. Use your ohm meter to verify.

For the 2204, connect the wiper to one end of the pot.

connect either end to the wiper? and then connect that to ground correct?

Title: Re: Bias pot questions.
Post by: eleventeen on July 16, 2013, 10:06:58 am

IMHO, there's a good argument to be made that you want to connect the pot as a rheostat (eg; wiper connected to one end, converting it to a 2-terminal device, and then connect the wiper+end, the dual connection, to the supply side, the more negative side. We don't especially care about the direction of rotation, since whenever you have your tweaker in the slot of the pot, you will be looking at a meter or some sort of monitoring device and will know right away if you are turning the wrong way.

This way, if the wiper fails, the bias voltage snaps to the more negative side and shuts down (or really chokes off) the output tubes. If the wiper fails and the pot is oriented the other way, the bias voltage would snap to the ground (or "groundier") side and go positive...That would jack the output tubes into overcurrent. Remember, we are talking about negative voltages, so "ground" is much more positive than both the nominal bias voltage and whatever "raw" supply voltage you are feeding into the voltage divider network (usually something like resistor-pot-resistor)that lets you adjust the bias.

Title: Re: Bias pot questions.
Post by: Willabe on July 16, 2013, 10:32:41 am

Quote from: eleventeen on July 16, 2013, 10:06:58 am

IMHO, there's a good argument to be made that you want to connect the pot as a rheostat

This way, if the wiper fails, the bias voltage snaps to the more negative side and shuts down (or really chokes off) the output tubes.

Very good point.


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