Tube Amp Archive

Recently biasing my amp and have noticed fluctuations in B+ voltage. On further research I've found that it's not uncommon for mains voltage to fluctuate 10-20V. This got me thinking that if I set the bias when mains voltage is low, it could mean the output tubes will exceed maximum dissipation when...

Cool, glad to know it's normal then!
Guess I'd better allow for this though when calculating voltages, especially since the plate and screen voltages are already above manufacturers spec. of 300V max for a EL84.

No I mean at idle, B+ fluctuates.
On different tests I've seen up to 9V difference. Can only think it's my supply, or transformer temperature perhaps? Or could it be the filter caps? - no audible hum, quite a quiet amp in fact.

Yes, might be an idea to run a bit cooler, with the elevated B+ voltages. I did check the biasing when I changed the tubes, and it was running on the hot side, but I decided to leave it since I was using an M grade EL84. Now that raises another question with B+ fluctuation I may not be getting an ac...

Spent a bit of time troubleshooting my amp, so I thought I'd share my findings. Amp was cranked way up on a gig and it started to cut in and out then eventually fizzed out to nothing. On inspection I noticed that R47 on power supply circuit feeding output tubes had burned up. Replaced resistor and t...

Thanks a lot for posting this! I too was trying to bias my blues jr and had the exact problem. Sure got me scratching my head, this post has saved me a lot of time, cheers!