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Thread: Fender Bassman Ten - very low volume.

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    Member PfeifferElectronics's Avatar
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    Fender Bassman Ten - very low volume.

    A customer brought me a fender bassman ten from the late 70s. The complaint was hum after it warmed up. When I opened it up, it had all original components. So I proceeded to replace all the electrolytic caps and put two new 6L6 power tubes in it. Fired it up and it sounded great! So I proceeded to play for about 5 minutes, then volume dropped to nearly non-existent. BTW, the 6L6s are taking 60mA each which seems a little high but there is no bias pot. I checked the bias voltage to be -49V. Any ideas?

    Thanks

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    Bent Member Chuck H's Avatar
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    Dirty contacts and/or pots?

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Preamp tubes?

    So, what are the tubes actually dissipting? Did you check the bias supply components & replace those electrolytic caps? Humming after a warmup ALWAYS makes me suspect a bias supply problem first, because if it IS, it's likely the most expensive if not caught & fixed. If the tubes have redplated beforw, they may be permanemtly damaged and now grossly mismatched.

    It may not have a bias adjustmemt pot, but that doesn't mean it can't be adjusted.

    Justin

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    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
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    Bent Member Chuck H's Avatar
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    Here's the schematic.

    https://schematicheaven.net/fenderam...n_10_schem.pdf

    You sure don't need 60mA current for each tube. You should be able to increase the bias voltage by increasing the value of the 15k resistor off the balance pot. The schematic say's -59V (I think, it's pretty blurry). Even that might be low considering the higher modern wall voltages.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Don't forget the joker g1's Avatar
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    Over-current tubes and loss of sound, I'd be looking for possible oscillation, or maybe output transformer issues.

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    Member PfeifferElectronics's Avatar
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    She's hummin now! I changed out the 15K resister on the balance pot to 20K. This got my bias current to 42mA. I need to do this anyway and I don't think this was the problem. However, by accident, while I was swapping that resistor out, I noticed the bass pot on the studio channel was very loose. I tightened it down to the chassis. Now everything is working great.

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