Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: musicfixerupper on June 06, 2012, 04:13:45 pm
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Working on a AB763 build and I am getting guitar sound thru the speaker with the volume on the amp turned off. Even when i ground the wiper of the volume pot manualy i still get the same amount of sound. So I removed all the coupling caps and still sound. The only path left around the volume pot i think is through the plate resistors. Can this be?
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Working on a AB763 build and I am getting guitar sound thru the speaker with the volume on the amp turned off. Even when i ground the wiper of the volume pot manualy i still get the same amount of sound.
There must be one or more wiring errors. I'm assuming when you say "sound" you mean you hear a guitar plugged in the input jack.
If you got sound with the volume off, I'd guess the lug of the pot which should be grounded isn't. But then you still get sound when the wiper is grounded. So that volume pot couldn't be properly connected to the grid of the 2nd stage of that channel, or grounding the wiper would ground that grid and prevent any sound from getting through.
... So I removed all the coupling caps and still sound. The only path left around the volume pot i think is through the plate resistors. Can this be?
That couldn't happen normally. So, do you think you might have wired one set of input jacks to the "other" channel? Like the Normal jacks to the Vibrato channel?
Don't worry about having made a wiring mistake. I just built an amp, and made 3 or 4 wiring errors. The only thing that separates me from a beginner is I acknowledge the issue is due to my error faster, and find the wiring fault faster. :laugh:
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I've had the same problem with a pre-PI master volume, where removing the wire to the input of the PI coupling cap (it was a LTP invereter) did not help. Shorting the PI input did not help either. I never figured it out... They layout was pretty 'textbook', as was the grounding.
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The tube on the other channel is removed, the guitar still comes through. Grounding pin 7 where the wiper is connected does not stop the sound. Its likes the signal is going directly from plate to plate bypassing the tone stack and volume all together. Grounding pin 6 after the cap of coarse, does stop the signal though.
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My $$ on what HBP said.
Brad :icon_biggrin:
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The tube on the other channel is removed, the guitar still comes through. Grounding pin 7 where the wiper is connected does not stop the sound. Its likes the signal is going directly from plate to plate bypassing the tone stack and volume all together. Grounding pin 6 after the cap of coarse, does stop the signal though.
Is the "sound" as though there was a normally functioning amp (otherwise), is are you saying that something faintly gets through (as in cross-talk).
Cross-talk is possible, but very uncommon. I've never seen it happen in a vintage Fender amp to an extent to enable sound to get through when the volume is all the way off.
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For me must've been some kind of cross, and sounds like that what the OP was describing too.
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Either cross-talk, or the pot ground lug isn't connected to the resistor trace inside the pot maybe? (so the pot wouldn't function as a voltage divider)