Thumping from left speaker when volume is low

[dru85]

Hi i just finished building my S-5 Electronics Tube Amp Kit. Everything was working fine, but a few days later the speaker started thumping or a pulsating like a heart from one of the speakers. When I touch the red or positive connection the pulsing stops and the music plays fine. Is this a grounding issue? Thanks in advance.

[Geek]

Feedback is getting in. Normally I'd say supply decoupling motorboating problems, but if you touch a connection and it stops, the LF oscillation may be a symptom of an ultrasonic one. Redress the chassis wires maybe?

[richwalters]

Now the difficult ones..Had sim effects but 1st stage tube was picking up AM garbage signals and demodulating them into low level audio. My fault was an excessively long speaker cable picking up nearby AM signals and returning it via front end via global feedback loop. Adding step response to global feedback loop solved the problem.

There is a more serious snag that troubles some tube amps.....your amp might be sensitive to phase shift aggravated by high order 12/18/24 dB oct loudspeaker crossover networks and "just kicks into oscillation" at low level. This was common problem with a well-known Dutch amp. The fact that the other channel doesn't make unusual noises may simply be bad-luck with input tubes...Have you tried swapping input tubes as one could be "taking off" ?

If you have a oscill'scope Tackle the problem methodically on each channel, a dummy load and signal genny, check interstage caps are correct value. It isn't unknown for the UL stage to oscillate unknowningly and this would show up in cathode currents-check them anyway.

richj