Matching tubes pot in old Fenders

[slideman82]

I wanted to know what does this pot do, is it a kind of bias adjust pot? Also would like to know how do I know that output tubes are correctly matched.

Thanks!

[Enzo]

There are two kinds of balance pots on Fenders.

One kind is a wire wound 100 ohm pot across the 6VAC heater wiring with the wiper to ground. This is adjusted by ear. Turn up the amplifier to hear as much preamp noise as possible. Now adjust that control for least hum.

The other kind is the bias balance control. The purpose is to try to balance un-matched power tubes. Fender never bothered to install matched sets of tubes. The adjustment was also done by ear. In this case, we turn the preamp controls all to zero, so we only hear hum from the power amp stage. Again, adjust the control for least hum.

Today, modern tubes are easily available in matched sets. Many of these older "bias balance" controls on Fender amps have been rewired to be bias level controls. The ones that were balance controls could not adjust the amount of bias, it could only adjust which side got more than the other. So assuming both side tubes were very close, we rewire to we can vary the amount of bias, but now both sides got the same. It would be easy to add a control and have both balance AND level, but I never saw anyone bother.

[slideman82]

Originally Posted by Enzo

The other kind is the bias balance control. The purpose is to try to balance un-matched power tubes. Fender never bothered to install matched sets of tubes. The adjustment was also done by ear. In this case, we turn the preamp controls all to zero, so we only hear hum from the power amp stage. Again, adjust the control for least hum.

That's what I was talking about! I know the hum balance pot (in fact I have to change it, it was burnt!), and I know bias is fixed, but didn't know that I had to listen hum to adjust it! Thanks! I'll try it today!