Bias Pot

[CircuitButcher]

Bias points are common on the back panel of PP circuits.

If there is available real estate, what requirements/considerations/caveats are involved with installing the bias pot on the back panel?

[Phil_S]

When you make the bias pot accessible, surely someone who doesn't know what it is will adjust it for you with potentially disastrous results. Even if you use a proper locking pot, there is no guarantee it will prevent Tom Foolery from having his way with it. That's why you don't see them on the back panel.

[labb]

I built a SE amp with external adjustable cathode bias..Fun to change power tubes and play with...Go over to WWW.AX84.com, go to Classic Amp Projects, go to AX84 Single Ended Lead, go to SEL Options, go to Variable Cathode Bias options. Think you could do this with fixed or cathode biased amps..On a fixed bias amp you would for sure need to put the 1 ohm resistor in the cathode ground lead and put the external test points around it. The one that I built we played with 6V6, EL34, 6L6, and KT88 power tubes. You need to know your amp when you do this..WE always came back to the 6V6's. I did use a locking pot for the bias adjusting pot, but this amp stays at home and is played by someone that knows about biasing..Phil is correct, you would not want that feature in the hands of everyone.

[Jana]

Give a guitar player a knob and they will fiddle with it. Tell them not to adjust it and it will be irresistible, like, "what secret tone is hidden in that knob that I'm not supposed to find. Maybe this is the rock'n'roll star knob!"

The best option, IMHO, if you are going to have it external is to use one of those fender style pots with the recessed screw driver slot and put it on the top of the chassis (amongst the tubes, transformers, etc.) That way it isn't so obvious on the rear panel.

[iknowjohnny]

Thats how mine is. I have two bias pots for my dual bias setup and 3 test point jacks (1 for each tube and 1 for ground) that accept a meter probe. The pots are the type with a screwdriver slot in a very short shaft. The test points are to one ohm cathode resistors so there isn't any danger there with high voltages. Great setup. I can bias the amp so quickly anytime i want, even at a gig. why this isn't more common is beyond me.

[ToneMerc]

Here's how I did my recent JTM45 build, you can't tell from the back panel.

https://tubeamparchive.com/viewtopic.ph ... sc&start=0

TM