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Fender Bassman 100T - Strange noise

HansHarz

HansHarz

Nov 15, 2017
6
2
Berlin
www.themethusalems.de
#1
Hi,

since about three month my Amp makes some strange noise when power & standby is ON (example in audiofile). Autobias is OK.
I have already tried to jiggle the tubes (yes, softly) and also removed and inserted them again. Still the same noise.
Any clue what else to try?

Thanks,
Hans
 

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#6
I had a similar noise in my Super Bassman which turned out to be a bad guitar lead.
Before I worked that out I had planned to partially isolate the problem by taking a line out from the pre-amp output jack and running that into my laptop audio interface. I figured if the noise appeared on the interface I would know the problem was somewhere in the pre-amp. If it didn't appear on the interface but I could still hear it coming out of the speaker I'd know it was something in the power amp.
Won't solve the problem but it's an easy way to narrow down where to look.
 
#7
OK, good idea to focus the problem. I have also taken a look at the block diagram. Fact is, that the noise comes up no matter wether the amp is muted or not.
I am not an expert, but for me it looks as if the mute-switch cuts the signal AFTER the pre-amp. The noise comes also when the amp is muted. So the problem should be on the power-amp side...
Do you agree with that concusion or is there a mistake in my thougts?

Greetings,
Hans
 

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  • bassman 100t block diagram.jpg
    bassman 100t block diagram.jpg
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#8
Looking at the block diagram I think you are correct that the mute is at the very end of the preamp.
I'm not knowledgeable on the tech side, just someone who has owned a few old and new tube amps over the years. The first step in diagnosing a problem is always working through the signal path to narrow down the area to look at. I would have a listen at the tuner output, then the preamp output then the XLR output. Since you still hear it with the Mute engages it seems likely to be a power section issue but I would still take 5 minutes to test all the output points.
Then look in the narrowed down area for a microphonic tube via tapping each one lightly with a chopstick or wooden pencil. Also contact cleaner in each tube socket and re-seat each tube.
If that didn't locate the problem I'd be off to spend some money at a tech. Probably also worth posting something in the Super Bassman thread in case somebody else has solved an identical problem.
 
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