Ampeg B25 howlin' wolf *solved*

[xtian]

My client's Ampeg B25 was in poor shape. I replaced filter caps, tubes, power cord, B+ dropping resistors, bias circuit. Up and running OK now, with Volume controls off. Voltages on each triode measure OK. However, turning up volume on either channel produces a loud howl oscillation, even with gNFB disconnected. I have tried one other 7199 phase inverter tube, same result. No howling if I remove the PI tube.

Any suggestions?

Ampeg b25.pdf

[xtian]

IMG_2370.jpg

[xtian]

Also does not oscillate if power tubes are pulled (but PI is left active). I see 66VRMS on each power tube's grid before clipping from the PI.

[Stevem]

That terminal strip where the ground connection is made from the filters and the solder connection there looks pitiful!

What does the resistance read from that point to ground?

Are any of the interstage coupling caps leaking DC down stream?

[maxkracht]

I have worked on a few Ampegs from, I think, the same era where all the tropical got baked off of the tropical fish due to being right next to the tubes. Just metal foil with a tiny bit of cracked paint left. Don't think I found any that were truly bad, but suspicious enough to change. No luck with a chopstick?

[xtian]

@Stevem, I think your comment about the ground lug was spot on. I made a more secure connection to chassis, and my troubles are...mostly...gone. I still have some hum when Volume turned up on either channel, but I don't see where I can improve the grounding scheme any further. The coupling caps do not show leaking DC.

@mackracht, thanks for chiming in.

[Stevem]

Good news there!

The daisy chaining of the the grounds in these amps is a pretty good source of 120 hz Hum also.
Break all the grounds dealing with each tube down into just one ground run from each tube and then drill for a bolt down lug for each near the main PT ground.

This should make a good improvement.