Is any 50k pot usable for permanent bias adjustment?

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305
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?
 

Vanyu

Member
Messages
738
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

Silver Supporting Member
Messages
920
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

Member
Messages
8,871
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. :)
 

oneblackened

Member
Messages
1,286
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

Member
Messages
4,296
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. :cool::cool::cool:

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

The Piher's work----- and work----- and work, etc, etc.
 
Messages
305
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

Gold Supporting Member
Messages
2,457
Cermet pots are made for DC voltage/current but I’ve used Alpha and CTS pots for bias adjusters.
 




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