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nothing here?
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Had an input tube on a power amp start oscillating once; caused all sorts of unhappiness, including a nice repair bill.
WW
"A man need merely light the filaments of his receiving set and the world's greatest artists will perform for him." Alfred N. Goldsmith, RCA, 1922
Plug a good tube in and see.Lots of things can cause oscillations, anything is possible. Bad coupling caps can do it. You may have installed a new leaking cap. Try replacing your grid choke with a resistor to see if it fixes it. If it does, you may have created an oscillator.
Edits: 11/10/14
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Power supply decoupling is the probably cause . Have you changed anything recently ?
Al
Changed coupling cap and grid choke (see thread below).
Nothing to with PSU.
one channel starts to oscillate only when music plays. The larger the signal, the havier the oscillation.
I might have broken something of course... Where to look?
NC
Does the frequency of the oscillation come anywhere near the LC resonant frequency for the coupling cap and grid choke?
I'd swap output tubes L to R and then input tubes to see if it follows or changes.
dave
Changed them, and oscillation remains in one channel.
Its really low frequency, around 8-10Hz where the resonant peak of the coupling cap and grid choke is. Only one channel however.
Sigh...
As suggested, maybe a bad coupling cap? Although I find that hard to believe (VCAP CuTF).
I could replace the new Silk grid choke again with the 'Old' one (mange quest I believe). The old one has lower inductance.
If its still oscillating, I have to suspect the coupling cap I guess...
NC
I don't know the circuit, but here's a scenario:driver tube has a PSU filter (RC) which phase-shifts its output relative to fluctuations in the PSU voltage by up to 90 degrees.
Driver output goes through a CL high-pass filter (coupling cap and grid choke) giving a further phase shift of up to 180 degrees.
Output tube has a 180 degree phase shift from grid to plate.
Total phase shift is up to 450 degrees, and at 360 degrees you have an enthusiastic oscillator.
Edits: 11/10/14
Thanks,
It was always working perfectly, until I changed the coupling-cap and grid choke (different make/values).
That could also cause a different phase shift I guess. So, I will be putting the old grid choke with series resistor back and see if that helps.
NC
Swap the tubes after the grid choke. If one draws appreciable grid current that could be your cause.
dave
I swapped the tubes being driven (drivers tubes) to no avail. The same channel keeps oscillating when signal is present.
Out goes the new grid choke and back in goes the old choke with series resistor,
NC
Can you swap the chokes, then the caps left to right? If it is a bad part, it would be nice to track it down.
dave
HI,
I changed the coupling caps, put the old grid chokes back, swapped (almost) everything, but I still have the same 'issue' with the the speaker cone on my right channel 'oscillating'.
However, I might actually not be oscillation. I never saw the effects of 'rumble' on any system. What i suspect is that by lowering the coupling cap value from .47uF to .33uF my system has become more sensitive to rumble.
When I play the system from another source, the 'oscillation' is gone. The cone only moves excessively when I use the turntable (which I do all the time). The 'other source' is a computer which then plays via an inverse RIAA, soo that all components are in place.
Why the 'rumble' is only present on one channel beats me however. The coupling caps are matched.
Thanks for you help. I wanted to discuss more about the theoretical benefits of grid chokes, but I think Tre summed it up already, and there is not much more to discuss. You might be right for about 5 years already about this subject.
Maybe the low resistance DC path is not the cause of the perceived benefits of grid chokes, and its just the high reactance. Grid current as a whole might be much less of a problem in practice then in theory. Maybe thats why the 'ringing' is also not so much of a problem.
NC
I changed caps, but sill oscillation when music plays :( :(
When the amp is on for a longer time, the oscillation seems to reduce.
I'm starting suspect one of the caps in the PSU may have problems...
What a hobby...
NC
if the amp uses a PCB, might also check for cracked solder joints between the pcb trace and tube socket. especially if the effect changes as the amp warms up.
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