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Thup-thup-thup and static coming from my power amp

Posted by Wilbur Pan on December 12, 2016 at 14:17:41:

This isn't really a from-scratch DIY tube amp, but I'm trying to rehab my tube amp, so I thought this would be a good place to get some guidance.

I have an old Anthem preamp/power amp combo. It's about 18 years old, and seems to be your typical EL-34 based push-pull power amp. A while back, static started coming from the speakers. When I checked it out, one of the resistors in the power amp was cracked, which was an issue I've had in the past with this amp. I've attached a picture of the schematic for this amp. It's R54 that failed. I'm pretty sure the purpose of the resistor is to provide a way to check the bias on the power tubes.

I sent the power amp to Parts Connexion, which is affiliated with Anthem/Sonic Frontiers back in the day. They replaced the resistors (R53-R56), the power tubes, and some of the driver tubes.

When I got my amp back, it was working well, but over a two week period it's developed this fast thup-thup-thup-thup-thup-thup sound, with occasional static. This is a new sound to me. I've swapped all the tubes I can between the left and right channels except for a 12AX7 that's the first input channel tube, and the sound still persists.

Any hints as to what may be wrong? By process of elimination, it looks like if it's a tube problem, it's that first 12AX7. Or could something wrong have happened when the resistors were replaced?