Ampeg tripping GFCI outlet

[drewl]

Got this old Ampeg Super 800 amp from 1949 that customer says trips the GFCI outlet.
Checked it out and yes it does.

Amp works okay, has three prong cord, grounded to chassis with the hot going to the fuse, then switch to power trans and neutral going straight to trans.

Sorry, couldn't find schematics
The power switch, pots and jacks are on a separate plate with main chassis below.
Upper plate grounded, no hum from this thing at all.

Do I need to install a double pole power switch?

Do guitar amps trip a GFCI?
Never tried it before, had to drag this thing into the bathroom!

Thanks

[The Dude]

Does it have a death cap? If so, clip it out and retry.

[drewl]

No death cap
I found a post from some sight about the current leakage from a power transformer can be a few milliamps above the GFI trip point of about 4ma.
I'm gonna have to do some research

[The Dude]

If there are any RFI caps across the AC line, they can also cause GFCI tripping. I had a similar problem in this thread.

https://music-electronics-forum.com/...2000-trips-gfi

Edit: Since it's a 1949 amp, I doubt there are RFI caps, but worth a look.

[Greg Robinson]

You'll really need a Megger (insulation resistance tester) to properly check this. If there are no death-caps/class-Y caps (won't be class-Y safety rated line-to-ground caps in an amp this vintage unless they've been fitted in the last couple of decades by someone else) to provide an AC path from line-to-ground, then it's quite probably insulation breakdown. The transformer primary should be floating with respect to the rest of the circuitry - ie several Megaohm DC resistance to chassis. You might not be able to measure any DC path with the low test voltages of a multimeter, that's where the Megger comes in. It applies a high test voltage (typically 100/250/500/1kV test options) to check for any insulation breakdown. Anything in the primary circuit could be responsible - the switch, the fuseholder, any line-power neon indicator, or worst case, the transformer primary windings to its core.
If you don't have a Megger, then start with a multimeter, you might get lucky and find the responsible part.

[Jon Snell]

There should be absolutely no leakage between the primary and the secondary windings of any mains transformer!
If there is leakage, it has failed and needs replacing.
I would check the neutral to earth connection firstly. You should get infinity on an old style x100 Ohmeter. (AVO 8, Simpson etc). DVMs cannot read this correctly due to such a low voltage.
Disconnect the primary live and neutral, then carefully, test again.

[drewl]

I think we have a Rotron megger around here, I used to repair them in the 80's.

Might have a hipot at work
They used to trip on really humid days when testing motor wire insulation

[Chuck H]

I would think that if there's enough errant current on a grounded chassis to trip a gfci then the amp would be blowing mains fuses as well. But maybe not? If the transformer checks out then I would re examine the grounded AC conversion. Especially the actual ground contact. And I would check for voltage on the chassis testing from the chassis to a remote ground. If there's anything wrong with the AC ground connection even a slightly leaky power supply cap might trip a gfci.

[drewl]

Think I got it
Death cap from fuse holder to ground hidden under some stuff, one of those old black ones with the colored bands, .22uf.
Removed it and it seems okay
My bulb limiter at work has a GFCI, brought it home so I didn't have to drag the amp back and forth to the bathroom

[Justin Thomas]

And this is why we call it a Death Cap & why we get rid of them...

Jusrin

[drewl]

Yup
The Dude wins the Diagnosis Discovery Sweepstakes!
Johnny tell him what prizes he's won!

[Chuck H]

Yup
The Dude wins the Diagnosis Discovery Sweepstakes!
Johnny tell him what prizes he's won!

You forgot the:

BING! BING! BING! BING! BING! BING!

In a perfect world Alex Trebek would announce it

[Justin Thomas]

A new CAR!

Jusrin

[Leo_Gnardo]

Without the chassis grounded with the AC cable, what you had there was a widowmaker. Somebody was bound to get a whopper of a shock what with the old 2 wire cable and a 75 year old leaky/shorted death cap.

Good call Dude! Best prize is saving lives and injuries. And annoyances. Thank you.