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FDP Forum / Amp Mods, Repairs, and Projects / Weird 5E3 clone hum

66gent

USA

Dec 15th, 2018 09:46 AM   Edit   Profile  

Well, the hum is normal 60 cycle. It's what makes it happen that's weird. Amp is a Mission 5E3 kit I built about 12 years ago and was fine when I put it away. Hadn't played it in 2-3 years until recently. When I started using it again there was a bad hum, and the normal vol didn't work - it was on full blast regardless of what it was set to.

Replaced pot and I thought all was good until I discovered if I plug into the normal channel the hum comes back, much better than before but definately still there. However, when I plug into bright channel it goes away. If I plug into norm again it's back again. When I say plug in I mean even if I plug in and remove the cable. The hum is either back or gone regardless if there is something currently plugged in, and only on the normal channel. The norm volume makes it louder and lower, and it's gone if I turn the vol down completely.

Where do I start? The fact that it happens regardless of which jack per channel I use makes me think the issue is away from the jacks but I don't know what I'm doing with troubleshooting. FWIW, jacks are just grounded to chassis, and tube swaps did not solve the issue.

Thanks in advance

Peegoo
Contributing Member
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Results not typical.

Do not try this at home.
Dec 15th, 2018 07:52 PM   Edit   Profile  

Sounds to me like it could be a poor ground on a pot.

If you pull V1, does the amp still hum?

66gent

USA

Dec 16th, 2018 05:47 AM   Edit   Profile  

Thanks for response. Hum happened with both old pots and newly installed pots. It does go away when V1 is pulled

Leftee
Contributing Member
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VA

Caught snipping the Bright Cap
Dec 16th, 2018 07:48 AM   Edit   Profile  

I'm with the Goo, but in broader terms. There's a ground issue before or at V1.

At any rate, that's a good place to start.

(This message was last edited by Leftee at 09:49 AM, Dec 16th, 2018)

66gent

USA

Dec 16th, 2018 10:19 AM   Edit   Profile  

Looking at the schematic, the only things before V1 are the jacks and the resistor/cap that comes off the V1 cathode to ground. Am I reading that right? Does the schematic indicate signal flow? I wonder if individual grounds from the jacks would help. I guess it wouldn't hurt.

Peegoo
Contributing Member
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Results not typical.

Do not try this at home.
Dec 16th, 2018 08:47 PM   Edit   Profile  

That’s what I’d try. Make sure to tie the ground to a central common ‘star’ ground point. This prevents noise created by ground loops.

Hammond101
Contributing Member
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So. Cal. USA

Dec 17th, 2018 10:47 AM   Edit   Profile  

You might try cleaning the grounding tabs on the input jacks before you venture into other components. A while back Switchcraft built many grounding jack with a residue that would not allow the ground to complete when the plug was removed.

Sometimes there is not enough tension on the grounding tabs and the need and "adjustment" in addition to cleaning.

FDP Forum / Amp Mods, Repairs, and Projects / Weird 5E3 clone hum




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