Tube amp volume drops completly after 20 minutes

[leftu2]

Hey guys, I picked up a 1993 VHT pitbull  2x12 combo on vacation. As i stated the volume will drop completely after 20 minutes or so (all tubes till glowing), turn the amp off for about 15 minutes and back on...amp works perfectly. The time of the volume drop takes about 4-5 seconds.
Everything works as it should otherwise. I contacted them (yep, they won't give out schematics!!) and they said it sounds like a thermal regulator issue. This might be a stupid question but can they be calling  a voltage regulator a thermal regulator? I see 1 voltage regulator in the amp (7812CT), see the close up pic. Nothing on the boards looks burnt/bulged.
Any other possible causes? Supposedly the amp was re-tubed but the preamp tube in V1 is a different brand than the others (JJs)
Would a bad preamp tube die off after getting warmed up?

[glass54]

Hi Leftu2
My first concern would be heat damage esp around the 3T regulator (U8?) and CR9 and CR10. Further inspection would be required on both sides of PCB, but the joints on those components look poorly wetted and borderline fractured. It would be good to monitor your low voltage power supplies to confirm. Can't tell anymore from the photos at this stage.
(Fender and others have experienced similar problems when using Power Resistors and 5W Zener Diodes or Regulators on their PCB based amps).
Its a bit of a pain to remove and inspect these PCBs, but can be rewarding. I have done many Fenders and Edens with similar faults.
Kind regards
Mirek

[drew]

I contacted them (yep, they won't give out schematics!!)

For an amp that's nearly 30 years old . . . 

[leftu2]

I contacted them (yep, they won't give out schematics!!)

For an amp that's nearly 30 years old . . . 

That's exactly what I thought. Their repair service is cheap though, a flat $75 + parts, customer covers all shipping. Though sending the chassis to California from tn and back won't be cheap. I think they do the repairs cheap to offset the fact that they won't give out schematics. Steve sold the vht line in jan 2009 and supposedly the new owners will give out schematics for products made after that.