Best way to install adjustable bias in 5E7 bandmaster?

[brad347]

The title says it all. What do you recommend?

I was thinking of just putting one of those 10k bias adjust trimmers in series with a 50k resistor in place of the 56k resistor, but that would only give a range of 50k-60k. Is that enough range?

How is the bias pot in the blackface amps configured?

[phsyconoodler]

If you do the math to get the voltage divider to give you -40v or so,you would have V/out = Z2(56k)divided by Z1(6.8k)+Z2(56k) x V/in

So to vary the voltage I would install a 50k pot with a 27k resistor.You don't want to lose that 56k starting point,just be able to move it up or down a bit.
Take a 50k pot and install a resistor and take your ohm meter and measure it to be sure.At the center of the pot's adjustment you want 56k and when you turn it up it increases the resistance and of course the other way when you turn it down.

[brad347]

If you do the math to get the voltage divider to give you -40v or so,you would have V/out = Z2(56k)divided by Z1(6.8k)+Z2(56k) x V/in

So to vary the voltage I would install a 50k pot with a 27k resistor.You don't want to lose that 56k starting point,just be able to move it up or down a bit.
Take a 50k pot and install a resistor and take your ohm meter and measure it to be sure.At the center of the pot's adjustment you want 56k and when you turn it up it increases the resistance and of course the other way when you turn it down.

Cool.

Now my question is where do I get a 50k cermet element pot?

I know that bias voltage will burn up a carbon element pot.

I'd like to use the Fender style like this one:

http://www.mojomusicalsupply.com/item.asp?pid=18024&pg=45000&id=9021001

http://www.mojomusicalsupply.com/images/9021001.jpg

But the only one available in that style is 10k. Is there any way I could reconfigure it like a blackface amp so that I could use that 10k pot? If so, how would I do that?

Thanks!

[phsyconoodler]

A carbon pot will work fine.The fender pot is carbon.
Go to schematic heaven and look for yourself.

[brad347]

thanks.

How does the blackface setup work?

[Trout]

On the 5E7,

Sean Kilback ( Homebrew Tweaks (http://www.kilback.net/homebrewtweaks/amps/amps.htm#tweed5e7)) has vital details for adding the bias pot with pictures.

When I mod them, I use a 47K instead of a 56K, and the same 10k carbon pot as pictured here already. Plenty of range, no issues.

http://www.kilback.net/homebrewtweaks/amps/5e7/sk_5e7_chassis2.jpg

[brad347]

On the 5E7,

Sean Kilback ( Homebrew Tweaks (http://www.kilback.net/homebrewtweaks/amps/amps.htm#tweed5e7)) has vital details for adding the bias pot with pictures.

When I mod them, I use a 47K instead of a 56K, and the same 10k carbon pot as pictured here already. Plenty of range, no issues.

http://www.kilback.net/homebrewtweaks/amps/5e7/sk_5e7_chassis2.jpg

Thank you!!