Fixing an Epiphone SoCal 50

[thejoe]

Hey guys,
I was playing through my Epiphone So-Cal 50 about a month ago and it wasn't sounding right. Had that crackle and distortion that an amp makes when you're shutting it down. I put new tubes in it because I figured it was about time anyway and they were fine for the first 20 minutes of me playing. I didn't use it again until a few days later and now it seems to have the same problem. I notice that one of the tubes glows blue with my strumming.
I modded this amp about 2 years ago. I did the work myself but I was walked through the process by a guy in another forum who pretty much designed the mod for me. Wasn't too much. Swapped some capacitors out and such. So my ability to work in a tube amp is not my concern I just don't know the theories of how to go about diagnosing what the problem would/could be. Any help is greatly appreciated. Thanks,
Joe

[bluesfreak]

Did you adjust the bias after replacing the tubes? Statics and crackling usually denotes either failing caps or overloaded tubes, e.g. too hot biased tubes.

[thejoe]

I did adjust the bias when I did the mod because I changed it to accept JJ KT77's. The new tubes I put in are also JJ KT77's so I didn't adjust anything.

[bluesfreak]

New tubes do require a new bias setup if you want to do it perfect unless you can 100% ensure that the new KT77 do have same parameters as the previous ones. Can you swap back to exclude a failing tube? Depending on the mod it could be also that some parameters do ecxceed save operating range (e.g. grid voltage/current) leading to arcing, etc. As a simple check you can play the am in the dark and check if you see some arcing sparks somewhere when strumming, the blue glow is normal so don't get irritated by that...

[thejoe]

I opened it up and turned it on with the lights off and just as I switch from standby to on I saw a spark from D11. Right after that pretty much all sound went away.
I've attached the schematic. Thanks for your help.

[bluesfreak]

D11 is one of the rectifier diodes for HV so if there is an issue you will loose B+.
Root cause could be physical damage, early lifetime failure or a overload situation caused e.g. by a bad filter cap (C37/C38). As this diode is a standard type (1N4007) I would first just replace it and then recheck if B+ is stable now and now addtl orcing does occur...

[thejoe]

Thanks I will do that. Silly question, i know...How do I check the B+ voltage?