Last summer, I purchased a 1979-80 Hiwatt DR504 (the "Custom 50" fifty watt, 2 input model from the Joyce era). Unfortunately, since then I've had issues with it. The main one being that after 15 or 20 minutes of moderate playing, the sound "dissipates" to a feeble that you can talk comfortably over.
Sometimes this is temporary -- and the amp will "catch its breath" and suddenly rejuvenate back to Hiwatt roar. Other times it won't, but will go back to normal after about 30 seconds of a rest on stand-by.
Right away, I took it in to a trusted local tech who specializes in British amps (Acme Guitars in St. Louis, MO), and he suggested it was the filter caps. So, we replaced all of the filter caps -- and I put some THD Yellowjackets in there as well, as the old EL34s were mismatched and ready for retirement.
This seemed to solve the problem, but within a few weeks, the problem of sound dissipation came back. At first it was minimal, but now it is chronic to the point where I can't play the amp for any longer than 15 minutes without all of my volume and tone going down the toilet.
I've perhaps found perhaps another tech to take it to -- but my options seem rather limited in this area, especially for this sort of amp and this sort of problem. So before I do, I'm wondering given your experience you might have an idea of what this amp needs. I know it's hard to diagnose an amp by e-mail without hearing it, but perhaps you have an idea of what could be causing this; and, if it is easily fixable; or if I should give up and get another amp?
Know that it seems that in the amps lifetime, someone tried a lot of Frankenstein mods to it: adding an effects loop; and rigging the normal "channel" of the amp to behave like a gain stage for the bright channel...most of these have been removed, but remnants of them still remain.
Anyway, I appreciate any thoughts you might have on this matter. My band is rather serious and I need a road-worthy, no-hassle amp that will excel at mostly Byrdsy sort of jangle with a touch of some spacey rock stuff... so maybe the Hiwatt is not the ticket for this application after all.
I hope to hear from you soon.
