Fender bassman 100 blowing fuses

[g1]

do I take the resistor out and the center leg and one of the other legs go where the resistor was? what do you do with the third leg? after installed how do I set it? Thanks[ATTACH=CONFIG]28287[/ATTACH]

Yes. And you can connect the third leg to the middle leg. Set it so it measures around 3K to start. Then use your bias probe and adjust it till you have around 25mV reading. See how it sounds. If it still doesn't sound good you can increase the idle current to more than the 25mV reading, doubt you will need higher than 40mV. The lower you can run it and still have a good sound, the longer your power tubes will last.

[Jam]

Whats the best way to solder it in? because it replaces the 3.3k resistor?

[52 Bill]

Because you need to reduce the voltage going to the bias circuit, I would consider leaving the 3K3 resistor in circuit and just insert the trim pot in series with it at one end. This way when the trim pot is set to zero resistance the amp will be the same as if there was no added resistor.

I would unsolder the resistor where it connects to the diode at the fiber board side. Then I would solder one end of the trim pot resistor into the eyelet. Then I would connect the loose end of the 3K3 resistor to the other end of the trim pot resistor and to trim pot wiper terminals.

[Jam]

Ok I have set around 25mV and I think it sounds good. So if I have any where from 25mv to 40mv I will be in good shape? Is the bias the last step? Thanks

[g1]

You should be good then. The last step is to re-set the output matching/bias balance control.
No signal, master at zero, adjust for minimum hum.
Re-set the bias balance any time you adjust the bias.

[Jam]

Thanks So you do the hum setting with the balance control even if I set the two pair when bias meter was hooked up?

[The Dude]

If both sets of tubes are the same mA and hum is not a problem, you are good.

[52 Bill]

Thanks So you do the hum setting with the balance control even if I set the two pair when bias meter was hooked up?

A little bit of explanation here, the trimpot that you added sets the idle bias level. The hum balance pot balances the output tube bias. It was designed to allow the use of unmatched output tubes.

I would suggest that you start by setting the balance control to its' midpoint. Then using your bias meter adjust the bias control so that the tubes draw an amount of idle current that is both safe for the tubes and sounds good to your ears. Then after the bias is set and stable, adjust the balance control for the least amount of hum.

[Jam]

The amp sounds good. I just want to thank everyone for their help and sorry for all the questions. Thanks again

[Jazz P Bass]

"sorry for all the questions"

It's the questions that do Not get asked that keep you from learning.

Glad that you got it resolved.