'77 Ampeg V4B Transformer Repair

[ajunea3]

I'm SOOO glad that others also had difficulty with hum and the OPT. It was grating on my mind from the start. As others have said there are so many causes that you have to get the amp to TUBE tech for a proper diagnosis. Could very well be as simple as a bad output tube. See if any are glowing red on the plate. Keep us informed of your progress.

None of the output tubes were glowing red last time I checked. I rolled some different preamp tubes as well as some old functional output tubes that I had laying around, and the hum persisted in every case. So it's definitely not a tube issue.

The guy I took it to is by no means a master tube amp guy. The next guy I will bring it to, however, has quite the reputation for being the local "old tube amp" fixer. I would've brought it to him first if it weren't for the fact that he just moved his shop 45 minutes farther down the road.

Again, thanks for all the help guys. This is why I love Talkbass!

 

[WingKL]

It's not very hard to determine if the filament supply is the cause of the hum. Simply wire a switch temporarily so it can interrupt the filament supply. Let the tubes warm up as usual, hear the hum and interrupt the filament supply. If the hum goes away immediately when you throw the switch, you found your culprit. Of course, this is something for your tech to do and not something you would try if you're not familiar with working on tube amps.

It looks like the V4B shares the filament supply winding shares the same transformer as the B+ supply. The hum may be 120hz interference coming from the rectified B+ supply.

Whoah! dejavu