Selecting a Bias pot?

[Kregg]

I've been working on a 5f6-a for a feet to the fire amp project; that is, I'm trying to build all of the circuits instead of cheating by using a drop in board. Anyway, I'm looking at adding a bias pot to the bassman build. What is the operational difference between a 10Kl, 25Kl, and a 50Kl bias/trim pot? For a bassman do I need a pot larger than 10K?

Thank you,
CW

[Ronsonic]

All depends on how much range you think you need and the size of the fixed resistor that is being replaced. Usually the new pot equals the old fixed R and is in series with a new fixed R of 1/2 the value. That gives you a range centered at the original resistance value.

Always contrive it so that if the pot failed your bias voltage would go up to full negative.

[ted01]

Always contrive it so that if the pot failed your bias voltage would go up to full negative.

Hey Ronsonic,

Could you expand on how exactly you would do this? In TUT, KO'C talks of a safety resistor strapped between the bias supply and the wiper of the pot going to the grid. How do you calculate the value of that safety resistor? What other values do I need to know in order to figure this out?

Thanks,
Ted[/quote]

[M Fowler]

Kregg,

The bassman bias resistor is 56k, use a 10k bias pot or 20K precision turn pot.

Mark