Bias resistor in a 2555 Jubilee

[Mxzx]

Is it the 47k R23? Guess not since my amp doesn't have anything in R23.

I'm trying to follow the schematic for the 2555 that I can locate, but my amp seems to be different. I am pretty sure my amp is stock.

I pulled Groove Tube GTE34LS's out of it, and they biased properly.

Only able to get 19ma with bias trim full open. JJ EL-34's.



Thanks

 

[mickeydg5]

The resistor should connect to the bias trimmer and is noted as 47k. It may be numbered R23 or R24 or R22. Sometimes the numbers are different from schematic to various boards.

 

[Mxzx]

Thanks. Mine is tagged as a USA one, would that have come with -34's or maybe 6550's?
Just wondering why it won't bias up enough?

Thanks

 

[ghostdriver]

Very normal for those amps not to bias up hot enough. Also schematics don't always match up to whats in your amp. I have no idea how these amps ended up with no real adjustment in the bias circuit. I had all of the same issues that you have. I finally found a quad of EL 34's that were hot enough to just barely be enough. I think they are JJ EL34L's.

 

[Mxzx]

This 56k look like it, R22?

 

[Mxzx]

Yep, that was it. A 33k fixed it right up.

 

[mickeydg5]

A 33k value will probably be too low for hotter tubes. So keep that in mind.