Trim pots - what’s special about them?

[Steve Blackdog]

Hi all.

I am thinking of replacing a damaged 20k bias balance trim pot. Is there any reason why this cannot simply be swapped out for a standard 16mm 20k linear pot?

if I move it to the chassis with a knob on it, I won’t have to pull the chassis every time I want to bias new valves.

cheers

steve

[g1]

No reason you can't.
I would highly recommend you NOT put a knob on it. Too easy to get moved by accident. Recessed shaft that has slot requiring a screw driver (like classic Fender style) or locking type are the best options.

[Steve Blackdog]

No reason you can't.
I would highly recommend you NOT put a knob on it. Too easy to get moved by accident. Recessed shaft that has slot requiring a screw driver (like classic Fender style) or locking type are the best options.

that is very good advice- also too tempting to keep twiddling!

[Mick Bailey]

Some (as an example) older Fender amps from the 90s (blues deluxe etc) don't have adjustable bias and when requested I locate a 16mm pot close to the output tubes. I cut the splined section off, leaving about 2mm of plain shaft projecting and cut a screwdriver slot across it. Once set if it's for a customer I put a on dab of thick red cellulose paint I keep just for this type of thing. With some amps it's sometimes worthwhile installing 1 ohm cathode resistors and externally accessible test points if you install an external bias pot so that you never need to open up the amp for regular tube swaps. Unless you use a bias probe, that is.

[Steve Blackdog]

Some (as an example) older Fender amps from the 90s (blues deluxe etc) don't have adjustable bias and when requested I locate a 16mm pot close to the output tubes. I cut the splined section off, leaving about 2mm of plain shaft projecting and cut a screwdriver slot across it. Once set if it's for a customer I put a on dab of thick red cellulose paint I keep just for this type of thing. With some amps it's sometimes worthwhile installing 1 ohm cathode resistors and externally accessible test points if you install an external bias pot so that you never need to open up the amp for regular tube swaps. Unless you use a bias probe, that is.

That is the solution I have been looking for. Thanks!