rivera pubster 45 problem

[bluestom50]

This amp sounds fantastic for 4 to 5 minutes then loses clarity,sounds thin and distorted. Checked bias and it is steady at 40ma. I installed new el84s and new 12ax7s and problem is still there. I also used different known good speakers. I assume it has to be heat related problem. Any one have any idea of what I might look for. As always any help will be greatly appreciated. Thanks, Tom

[The Dude]

Try some cold spray?

[nhbassguitar]

Coincidentally, I just fixed a problem like this on a tube preamp. Turned out to be a star-ground screw that wasn't making good contact with the chassis. The chassis would get warm, then the amp would suddenly sound thin and lifeless. The screw/nut pair wasn't lockwashered, and so it was subject to loosening from repeated heat cycling. It's also a road unit that goes from gig to gig, getting knocked about frequently.

Pull the chassis, have a look around inside, and snug up every screw you can find. Sometimes a screw can appear to be only a mounting screw, but it's actually doing double duty as a ground point.

There's also the possibility of corrosion, especially if the amp lives in a humid environment. Screw-tightening may wipe enough away enough corrosion so the amp starts working right again.

The usual warnings apply re. energy possibly still stored in the power supply capacitors.

Hope this is helpful.....