wavy vibrato-like sound when amp cranked - diyAudio
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Old 15th May 2005, 03:44 AM   #1
9am53 is offline 9am53  Canada
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Default wavy vibrato-like sound when amp cranked

hey guys, when I turn my guitar amp past 6 or 7 I get a wavy kind of like vibrato sound...could this just be my cheap speaker, or could this be the amp?
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Old 15th May 2005, 04:22 AM   #2
rpapps is offline rpapps  Antarctica
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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
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Old 17th May 2005, 01:19 AM   #3
9am53 is offline 9am53  Canada
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thanks I'll check it out.
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Old 17th May 2005, 03:31 AM   #4
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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
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Old 19th May 2005, 12:23 AM   #5
9am53 is offline 9am53  Canada
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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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