Baffling Tube Preamp Problem--Rhythmic Thumping after Tube Replacement

GREETINGS and here's the update. I went to my electronics guy and he said "Take my Dynaco Tube amp and test your preamp".

Basically it worked fine from both the A and B outputs.

So czarivey said "So far the issue seems like tubes and caps going away in the power amp, not preamp."

And that does seem to be the issue given that my preamp worked fine with other solid state and tube amps.

So we are going to test/replace the capacitors and anything else on those amps. I have no idea WHY the power amps went bad suddenly, given that the preamp definitely had bad tubes, which is why I brought it in. Maybe some bad signal triggered the capacitor failure in the power amps.

Making me think about selling everything after I get it all functional again. Or, if it's functional again, maybe just keep it running for another 30 years? 

If B supply resistor blows, that huge voltage will be applied at bias cap and destroy it. It acts like circuit protection in a such weird way that cap will go bad as well. Upon visual you will see that cap bubbled, but B supply resistor is the first item to inspect.  

I have a schematic. I like the idea that the B section Capacitor is the problem. It wasn't the original problem, the tubes were the original problem. but maybe in transport it was compromised. I think that it's also possible that the tubes and/or capacitors in the amp became damaged when putting the bad signal through them after I got the preamp back. It is frustrating but I think based on what you guys said and what I have read, this is likely the problem given the age of the amps. Thanks again for your feedback.

@oddiofyl they don't mind providing schematics of the amps. That's how I fixed mine when I had it. After fixing and observing unacceptable built quality, I've decided to sell my MB100. 

Best to find someone local that can repair.  VTL, Luke and Bea are horrible people to deal with.  One of the worst in the biz....

From a diagnosis perspective, it seems odd that BOTH tube amps would suddenly develop the same problem at the same time. Thus, I like the hypothesis that there's a bad cap in the B section of the preamp.

And perhaps something was dislodged during the handling of the preamp taking it to and from the tube shop.

I don't know that these amps were so sensitive. But they have been in service for many years. 

What do you mean by "They will not fix it"? Can't these parts be replaced with something similar? I'm hoping my local guy can fix it, but if not there are some places I found online that works on VTL (High End Audio Repair in Brooklyn, don't really know anything about them.).

The original problem was definitely bad tubes in the preamp, but perhaps that resulted in some cascading problems down the line.

I've fixed Manley. They have very primitive bias circuits without much of protection. Extremely poor design for all of his units including the BIG ones.

So far the issue seems like tubes and caps going away in the power amp, not preamp.

Compact 80/80.  All of the Manley designs....

If it is a Stereo 50/50 or 100/100 they probably will not fix it at this point.   I had a similar problem with an older VTL , it was bad caps

In VTLs bad tube can take away few elements around it such as resistors and caps. Test/replace, because your new tube and circuit board are in jeopardy.

Yes, A outputs were fine with solid state but my amps kind of freaked out when I plugged the tube amps into the solid state output. One of the 5881 tubes fried and then a fuse blew. So that experiment didn't work out so well.

I don't know the manufacture date but I got these in the early 90's and they've been mostly trouble free. It does seem like the problem is limited to the B output section so I will recommend that he look at that next. By caps do you mean capacitors? I'm not that up on the parts situation. 

Weird thing is that everything was basically fine (albeit noisy) until I took it in and we put new tubes in. Maybe the handling caused something that was dried out ended up failing or becoming disconnected. The oscillating sound was on both speakers so I know it's something in the preamp.

Thank you for this and any other advice. Really trying to avoid having to send it somewhere.

Sounds like it’s oscillating—motorboating. Could be caps are dried out in the B section. Have you tried the A outputs?