Music man Bias

[Valvehead]

oops----didnt have my magnifiers down there...

heres what i have

Pin
1 23v
2. ac
3. 739v
4. 370v
5. 23v
6. ac
7. 65v
8. 23v

[g1]

Any chance you got off count on the pins? Pin6 should be empty, pin7 should be AC (heater), pin8 should be 65V (cathode).

[J M Fahey]

Also thinking pins do not match.

In any case, some voltages (regardless of stated pin number) are meaningful-

Pin
1 23v
2. ac
3. 739v
4. 370v
5. 23v
6. ac
7. 65v
8. 23v

* ac - ac are clearly ends of filaments. Should be on pins 2 and 7
* +23V : fixed positive grid bias
* 739V: plate
* 370V : screen
* 65V : cathode
which means that *actual* power tube bias is 23-65=-42V (minus because grid is less positive than cathode which is same as more negative)
also plate to cathode voltage is 739-65=674V ... ouch !!!!
in any case, screens are at reasonable 350V so that helps.

[The Dude]

If those voltages are all correct, I'll stick with what I said in post #24.

[Valvehead]

so would the JE1692 transistors maybe cause that bias imbalance ? its about 14mv difference from tube to tube (ive swapped tubes)

[g1]

Maybe. But those resistors need to be close also. And are you using a pair of matched tubes?
So if you stick with a pair of tubes and switch them around, the low reading stays with the socket?

[Valvehead]

i stuck in 2 new transistors--bias is great now.

Im going to stick with the Sovtek 5881's for now ...unless the amp eats them quickly !

[The Dude]

By comparison, tubes are cheap. It's not that I'd worry about. I'd be worried about a catastrophic tube failure that takes out the OT. At least make sure you're not over-fused!