Is any 50k pot usable for permanent bias adjustment?

[gerrickreidenbach]

I have a Silvertone 1484 arriving next week and the mods for this amp presented in GW's book All About Vacuum Tube Guitar Amplifiers caught my eye. He recommends using a cermet pot for the bias adjustment mod but will any pot work?

 

[oneblackened]

Yeah, carbon comps work fine. The amount of power dissipation is pretty minimal.

 

[Vanyu]

Really doesn’t matter too much what kind of pot you use.

My go-to’s are just regular metal case pots, like CTS or similar. I’ve heard that the small trimmers give out over time, better safe than sorry

 

[Tron Pesto]

While the material type of the pot doesn't really matter, I find the function of the pot to be important. I find that a multi-turn pot the way to go. While it will depend on how/where you wire the pot into the circuit, the voltage can jump quite a bit with the tiniest adjustment with a single turn pot.

Bourns makes high quality trimmers and pots. I've had success with the these types and also these. Whether I use a trimmer type or pot depends on the situation. There are plenty other manufacturers/types available.

 

[zenas]

Did GW mean a multi turn pot by any chance?
A normal 50k pot like the kind on the front of an amp will be pretty sensitive for bias. The 10k pot in a Fender is pretty sensitive for that matter, but doable.
Seems like GW talked about a 50 turn cermet pot in his book. But it's been years since I read it and 50 turns seems kinda high.

That one Tron linked is 15 turns, maybe I got 15 and 50 mixed up.

 

[jthomas666]

Is this amp fixed or cathode biased?

https://www.thegearpage.net/board/index.php?threads/biasing-a-silvertone-1484.592088/

If it's cathode biased, I am pretty sure that you can't use most standard potentiometers (i.e., like you would use for a volume control). The pot would need to handle the high voltage. With a fixed bias, a standard pot would be ok.

 

[oneblackened]

Is this amp fixed or cathode biased?

https://www.thegearpage.net/board/index.php?threads/biasing-a-silvertone-1484.592088/

If it's cathode biased, I am pretty sure that you can't use most standard potentiometers (i.e., like you would use for a volume control). The pot would need to handle the high voltage. With a fixed bias, a standard pot would be ok.

high current, rather. The voltage drop across a cathode resistor is maybe 15 volts but it's significant current flow. Regardless, 1484s are fixed bias with a resistor instead of a rheostat.

 

[gldtp99]

I still use the Piher type single turn trim pots for bias adjustment.

They work for me.

Here's what Mouser has in stock in the 50k value:

https://www.mouser.com/Amphenol/Pas...0ytZ1yzvvqx?P=1z0spgxZ1yzmno6Z1yzuvtwZ1z0zp68

If I'm replacing a stock Fender bias pot or if the chassis I'm building an amp into uses the stock BF Fender style bias pot, I'll use these sometimes:

https://www.tubesandmore.com/products/potentiometer-fender-10k-linear-bias-screw

I never got into using multi turn trimmers for bias adjustment---- The single turn pots seem to work just fine for me.

It's just a bias pot----- why make a big production out of it ?

The Piher's work----- and work----- and work, etc, etc.

 

[gerrickreidenbach]

The mod went well. I placed the pot on the back panel of the amp for the ease of external adjustment. If it is accidentally changed in any way it is easily reset.

 

[VICOwner]

Cermet pots are made for DC voltage/current but I’ve used Alpha and CTS pots for bias adjusters.