Hi
It's possible that your power supply voltage is fluctuating under heavy loading and this is getting into the low level stages as feedback and causing a low frequency oscillation.
Usually the supply to the input stages is decoupled by an RC network. Try replacing the caps or adding extra ones in parallel.
Cheers
Rob
It's possible that your power supply voltage is fluctuating under heavy loading and this is getting into the low level stages as feedback and causing a low frequency oscillation.
Usually the supply to the input stages is decoupled by an RC network. Try replacing the caps or adding extra ones in parallel.
Cheers
Rob
G'day from Oz,
I had (I would guess) the exact same problem in a tube HiFi Amp. My problem turned out to be a badly microphonic tube. Give all the little tubes (12A_7s probably) a tap. If one tube is microphonic this test will make it stand out like the proverbial "dog's b*lls".
A "wavey vibrato sound" is as good a description of the sound of a badly microphonic tube as I've seen. Once heard never forgotten. So check this out first - before worrying about power supply decoupling or anything else.
I tried to explain the sound to someone and the best I could come up with was "it sounded like a bad international telephone line you sometimes hear on the radio news when they are talking to the man on the ground in Afganistan or somewhere", a sort of hetrodyning whine.
Of course it could also be acoustic feedback into your Reberb Spring Unit, if your amp has reverb and you had it turned on.
Cheers,
Ian
I had (I would guess) the exact same problem in a tube HiFi Amp. My problem turned out to be a badly microphonic tube. Give all the little tubes (12A_7s probably) a tap. If one tube is microphonic this test will make it stand out like the proverbial "dog's b*lls".
A "wavey vibrato sound" is as good a description of the sound of a badly microphonic tube as I've seen. Once heard never forgotten. So check this out first - before worrying about power supply decoupling or anything else.
I tried to explain the sound to someone and the best I could come up with was "it sounded like a bad international telephone line you sometimes hear on the radio news when they are talking to the man on the ground in Afganistan or somewhere", a sort of hetrodyning whine.
Of course it could also be acoustic feedback into your Reberb Spring Unit, if your amp has reverb and you had it turned on.
Cheers,
Ian
hey thanks guys...my input tube is actually a 6eu7, and my guitar amp in question is actually a converted Bogen PA. I will try tapping on the tube, but I think I will try adding coupling caps...originally I had a plate voltage of 65V on both halves of the input 6EU7, I replaced the resistors with 100K ones to raise the voltage to about 180V, but there was no coupling caps, so I will try put some in, if the tap-test doesnt work. thanks for the insight boys
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