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Guitars, Gear, Tone!
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PostPosted: Fri, Feb 09, 2018 8:30pm 
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Hack

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Hi.
I've replaced *all* the tubes with new ones. There was 1 bad power/output tube, that was causing the HT fuse to blow. I'm using a known-good speaker cable. Hums even with no guitar plugged in, and hum increases when volume is turned up. It sounds great, otherwise (with a guitar plugged in and playing!)! I've poked around inside the chassis with a wooden stick - looking for a possible bad ground, somewhere - but I'm not sure what my next step is. The hum sounds like when you touch the end of a guitar cable. This amp head appears to be 'barely used' - as everything on it looks pristine! Please advise. Thanks!


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PostPosted: Mon, Feb 12, 2018 10:16pm 
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anton1117 wrote:
Hi.
I've replaced *all* the tubes with new ones. There was 1 bad power/output tube, that was causing the HT fuse to blow. I'm using a known-good speaker cable. Hums even with no guitar plugged in, and hum increases when volume is turned up. It sounds great, otherwise (with a guitar plugged in and playing!)! I've poked around inside the chassis with a wooden stick - looking for a possible bad ground, somewhere - but I'm not sure what my next step is. The hum sounds like when you touch the end of a guitar cable. This amp head appears to be 'barely used' - as everything on it looks pristine! Please advise. Thanks!


Those newer Marshall tend to take out the hum balance pot when a power tube fails which can cause hum. Take a look at the pot. Measure each side of the filament AC supply to see if each side is about the same.

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