I was on the road for a while, so today was my first opportunity to open up the amp and take a peak. What a mess. Some of the soldering looks like it's been done by a 5yr old. The tube sockets are mounted on stilts.
Anyway, visual inspection didn't really bring any results. I took a few measurements and these put the biasing VR on the suspect list. I can't get a steady reading from it. There's also a crack in the PCB near the VR, although I don't see an immediate correlation there.
All 9-pin sockets feel a bit loose, but I suppose that's just wiggle room.
Respect for those in this thread who opened this soab and lived to tell the tale.

Update: Somebody told me about a similar experience with a Twin where a pre-amp tube turned out to be the culprit ("It burned red instead of orange").
So I replaced the preamp tubes with some old 12ax7/12au7 I found laying around. Played for about 10 minutes. Left it standing for 30 minutes. Played again for 10 minutes or so, and just as I started gloating in triumph it dropped its volume again. This time I did not touch any pots when it happened.
Patching the effect loop did not make a difference.
Granted, the tubes I put in where from an old Hammond organ and I cannot vouch for their state, so before I deep-dive in the amp I want to do another test with some fresh bulbs.

Quote from: g1 on May 27, 2024, 01:54:20 PM

Quote from: Frank75 on May 27, 2024, 08:18:36 AMAs soon as I moved the volume pot I lost, say, 80% power.

Have you cleaned the pots?  If not, do so. 
If you already have cleaned them, it sounds like the volume pot is defective or needs to be re-soldered.

A dirty pot was my first thought too so I did give it quite a few twists and turns. No crackles, no pops, no jump scares.
Potfluff doesn't seem consistent with losing volume over time. Strange though that moving the pot seemed to trigger the defect, so not excluding any options at this time. Including coincidence 

Received the new tubes set today. They seemed to do the trick. After half an hour or so, I still had full volume...until I adjusted it. As soon as I moved the volume pot I lost, say, 80% power.

I was reading back in this thread to see if there's something I should know about biasing (is there?) and came across this comment from Mprall00:

Quote from: Mprall00 on January 29, 2019, 12:40:17 AMReplaced r17 with the 50k pot for gain.  It sounds good now.  Incredibly loud.  It was oddly quiet stock for 30 watts

That sounds a lot like the reverse of what I am experiencing. At start up it is by al means incredibly loud.

Anyway, while replacing the tubes I noticed that the sockets were not screwed to the chassis so somebody's already been messing around with this amp. I'll add this one to my project list.

I recently purchased a TSA30H head for a small price. According to the seller the volume goes way down after a little while. After reading rave reviews on this amp and figuring the problem sounds like just some bad tubes I went ahead with the deal.
Then looking for a schematic I stumbled on this thread and started thinking that my small price wasn't small enough.
Anyway, plugged it in today, hooked to a V30 cab, and I'm quite happy with the sound. Yes, there's a slight hum on ridiculous volume, but not more than on many an old Fender. After a few minutes I got the opposite of what I was expecting. The volume shot way up giving me a slight heart attack and some angry looks from the missus. Played for another 10 minutes or so without further issues. Put the guitar down and left the amp to warm up for half an hour. Sure enough, after that the volume was extremely low.
So I pulled the tubes for visual inspection and much to my surprise it had one 6L6 and one EL34 inside. Now I understand that you should not put EL34's in an amp set up for 6L6, but could that be the whole reason for the soft-loud-soft sequence?
I'm trying to get my head around it, but I just don't understand tube behavior well enough.
I also understand that asking this on a solid state forum is sort of blasphemous, but this seems to be the only thread on this amp in the whole world wide web.
One way or the other, I ordered a pair of 6L6's for proof and pudding but that will take a few days.
I was also wondering if using an EL34 can do damage to the rest of the circuit and if so, what I should be looking for?