Marshall Jubilee 2558 Bias Question

[gibson17]

Hello all, I have a marshall 2558 Jubilee combo I recently picked up. I love it sound and have been using it to Gig. I recently checked it out to service it. I tested all the tubes and they test well on a tv7 tester. I calculated what the bias should be at with 65% dissipation. I get about 37 millivolts. When adjusting the bias the power tubes are about 10 millivolts apart. Is this circuit supposed to be self-balancing? Should I be concerned about this? Is this showing me that I need to do other service to the amp like change out the caps? Thank you all for your advice.

 

[SpHj]

Tubes will vary from one another, especially after some use. You’re only 10mA apart. I wouldn’t be concerned. Tolerances back then were probably 20% anyhow.

 

[gibson17]

Thank you for the response. When I tested the tubes they were within 2% of each other. I guess I could try to swap sides and see if the bias reading follows. When I had one tube biased up to 37, the other tube was 27, which seemed cold to me. The tubes are winged c's, so when they tested good I thought I should leave them in there. I do have a set of new el34's, maybe I'll throw them in there and see what they do...

 

[Jethro Rocker]

Try it. I can't remember where mine sits. Can always go to 39 and 29 I guess.

 

[gibson17]

Hello All, Just a follow up to close this thread out. I swapped sides of the winged C's and the low bias reading followed the tube. I swapped them back again with the same result. I then installed a new set of matched Svetlana EL34's and the bias balanced out. Thank you all for you advice and input. I appreciate it!

 

[mickeydg5]

10mA of difference or more can cause hum in the power stage.