Loss of volume - Vox tube Berkeley

[klarsen01]

Hello all.
I have a mid '60s Vox tube Berkeley I have owned for about 15 years. I recently noticed a volume loss of roughly 20%-25%,  and slight change in tone (mainly mushy low end, lack of punch). I figured it was power tubes so I replaced them with new EH EL84s. It didn't make any difference. I've done a little work with tube amps so I opened it up and checked all the basic stuff but didn't find anything. I didn't really have time to mess with it so I took it in to a highly recommended tech. He checked it out, put it on a scope, etc. and said he could not find any problem electronically. I brought it home but the issue is still there. Does anyone have any ideas or suggestions as to what to look for?

The caps were changed about 7 years ago. and I've tried replacing all the tubes with known good ones. Tried different speakers. Even tried a different output transformer. (I have the original and a Mercury Magnetics upgrade).

One other thing - Last spring we used this as a back line amp for a garage rock festival. Sometime during the show one of the 2 EL84s went bad. I don't know how long the amp was played with the tube out but it could have been 2-3 hours. However, The problem didn't seem to show up until several months later.

Thanks,
Kirk

[PRR]

Measure plate voltages on all the small-tube stages.

[jojokeo]

AFter doing what PRR suggests, you're describing filter cap failure/aging. For the things you've tried and money spent on everything, this would be something I'd check by putting a new cap or two across (parallel) the first two main caps and see if it fixes your issue. You could simply alligator clip them for a quick and dirty test. Did you check power tube voltages and their sockets too?