Magnatone tripping my GFCI

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workshed
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Magnatone tripping my GFCI

Post by workshed » Sun Sep 24, 2006 9:04 pm

So I bought a crazy old circa 1948 Magnatone head today at the Seatac Guitar show. It's 150 watts, 13 tubes, 12 input PA Head. I was hoping to find a way to make use of it for guitar.

Like an idiot, I took the guy's word for it that it was in working condition and didn't bother to make sure it was powering up before I left. I just got home, plugged it in (power switch off) and it instantly tripped the GFCI that outlet was wired to. Like before the plug was even all the way in the outlet. Tried flipping the switch the other way and same deal. Any ideas, or should I relegate this issue to an experienced amp tech? I'm inclined to do so and have the whole thing gone over for safety and sound quality purposes.

By the way, it was not hooked up to a speaker cab when I plugged it in... could it be that old amps like this will freak out if they don't have a proper speaker load hooked up to them?

Yes, you are correct, I know absolutely nothing about electronics.

EDIT: It appears to have been retrofitted with a three-prong cord. My first guess is that the ground wire has come off it's connection... will try to open it up and look tonight.

-Bret
Last edited by workshed on Sun Sep 24, 2006 9:32 pm, edited 1 time in total.

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Post by Phiz » Sun Sep 24, 2006 9:17 pm

I've never owned a tube amp, but I've been told they should never be powered up with out a load (speaker) connected. Doing so can do damage to the amp.

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Post by workshed » Mon Sep 25, 2006 12:14 am

If anyone wants to see photos, there are a bunch of them here:

http://www.workshedaudio.com/magnatone/index.htm

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Post by RodC » Mon Sep 25, 2006 6:52 am

It may have been working perfectly... If the guy plugged it into a normal socket.

You could have a leaky cap in there and some current it going to ground. A GFCI measures leakage to ground by compairing the current load of the Hot and Neutral. A GFCI has to be very sensitive since about .25 amps would fry you in the tub, and a typical outlet can provide 15 - 20 amps under a normal load.

Do you have a schematic?

Do you have access to a Low range OHM meter or a leakage meter?
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Post by nclayton » Mon Sep 25, 2006 8:09 am

I'm with Rod for sure. Probably the famous death cap is leaking. If you have one of those gray plug adapters that lets you plug a three prong cord into a two prong outlet, try using that and see if the GFCI still goes. It probably won't.

Ned

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Post by RodC » Mon Sep 25, 2006 9:22 am

nclayton wrote:I'm with Rod for sure. Probably the famous death cap is leaking. If you have one of those gray plug adapters that lets you plug a three prong cord into a two prong outlet, try using that and see if the GFCI still goes. It probably won't.

Ned
Thats where I was going, but if its hooked to other equipment in the studio be careful, there could be some current flow.

If it has a 3 prong plug and you want to try the grey 2 prong adapter make sure you dont hook it to any other equipment or it will provide the ground and trip your GFCI. If it has only a 2 prong plug the same could happen. Any path provided to ground will trip the GFCI.

I didnt see a polarity switch that is usualy associated with the "Death cap" but there could be a similar circuit used for RF elimination that can have a Death cap.

BE CAREFUL, if there is a lot of leakage you could be SHOCKED/KILLED if you get it fired up with just 2 prongs measure the voltage from the ground of the chasis to the ground in your house. This should be pretty low voltage.
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Post by workshed » Mon Sep 25, 2006 9:53 am

RodC wrote:
nclayton wrote:I'm with Rod for sure. Probably the famous death cap is leaking. If you have one of those gray plug adapters that lets you plug a three prong cord into a two prong outlet, try using that and see if the GFCI still goes. It probably won't.

Ned
Thats where I was going, but if its hooked to other equipment in the studio be careful, there could be some current flow.

If it has a 3 prong plug and you want to try the grey 2 prong adapter make sure you dont hook it to any other equipment or it will provide the ground and trip your GFCI. If it has only a 2 prong plug the same could happen. Any path provided to ground will trip the GFCI.

I didnt see a polarity switch that is usualy associated with the "Death cap" but there could be a similar circuit used for RF elimination that can have a Death cap.

BE CAREFUL, if there is a lot of leakage you could be SHOCKED/KILLED if you get it fired up with just 2 prongs measure the voltage from the ground of the chasis to the ground in your house. This should be pretty low voltage.
Thanks guys. I think I'm going to play it safe for now and take it to my tech to have it gone over. I am not confident enough in my meager electronics knowledge to take a chance with that much potential voltage. Rod, you are correct, I have not seen a polarity switch on the amp anywhere. And this house, being really new, seems to have a GFCI wired into every circuit.

So the amp should fire up with out a speaker load, correct? I cannot see how the lack of a speaker load would cause it to short out, but also understand that the amp should not be run sans speaker load.

-Bret

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Post by RodC » Mon Sep 25, 2006 10:01 am

I never bring up a tube amp without a speaker load, if there is noise in the circuit that is a signal and it will be amplified, thus you want a load on the output. However, you should be OK, Im just over cautious.

Firing it up without a speaker load should not cause it to trip your GIFCI
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Post by brianroth » Mon Sep 25, 2006 10:52 pm

Tripping a GFCI sure sounds like the unit has "death caps" from the AC mains leads to the chassis.

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Post by nclayton » Tue Sep 26, 2006 9:46 am

Sorry...I hope it was clear (but I don't think it was) that I was only suggesting using a ground adapter as a test. Wouldn't want you to actually use the amp that way. I didn't even mean to imply you should turn it on with the ground adapter, just plug it in. Really though, it's an unecessary test anyway since it's pretty much guaranteed that it will solve the GFCI problem. On the other hand, you could go ahead and use it with the ground lift adapter, and you'd be as safe as thousands of other people out there using old tube amps with bad death caps (which is to say, constantly on the verge of electrocution).

Taking it to a tech is easy and a safe bet, but it's really almost 100% sure that it's that ground ref. cap. You could just take a photo of the inside of the amp like you did with your ROSAC, and we could tell you which capacitor to snip out and save you 50 bucks. Assuming the three prong cord is installed correctly, that's really all it would take to fix the problem and make the amp perfectly safe, most likely. And if it DOESN'T solve the problem you'll know as soon as you plug in the amp again....no harm done, easy come easy go. Then take it to a tech.

Ned
Last edited by nclayton on Tue Sep 26, 2006 10:00 am, edited 1 time in total.

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Post by workshed » Tue Sep 26, 2006 9:49 am

nclayton wrote:Sorry...I hope it was clear (but I don't think it was) that I was only suggesting using a ground adapter as a test. Wouldn't want you to actually use the amp that way. Taking it to a tech is easy and a safe bet, but if you can verify it's the ground cap by plugging it in with the adapter, then you could just take a photo of the inside of the amp like you did with your ROSAC, and we could tell you which capacitor to snip out and save you 50 bucks. Assuming the three prong cord is installed correctly, that's really all it would take to fix the problem and make the amp perfectly safe, most likely.

Ned
Good call! You can see a whole gaggle of photos here:

http://www.workshedaudio.com/magnatone/index.htm

They ran the new AC cord into the chassis, then hooked it up to the remainder of the old cord!

Also: How can I figure out what impedance of speaker cabinet I need for this thing?

Thanks guys!

-Bret (off to look at some potential studio space for lease)

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Post by nclayton » Tue Sep 26, 2006 10:41 am

Okay, well it looks like the assumption that the 3 prong cord was hooked up exactly right might have been a little too optimistic. It's hard to tell for sure what's going on in the photos but I don't see a good candidate for a death cap.

On the other hand, what I THINK is the new power cord (coming out of on the bottom of most of the interior pics) only has a black and white wire with no green wire and the white running directly to the chassis. I don't think that's all there is too it, though, or else I don't think your gfci would trip. Even if the neutral is connected to chassis, the chassis would have to be grounded for there to be a possible current imbalance. Do you see a green wire coming out of the power cord?

Here:

Image

Is #1 a chassis connection? It sure looks like it is, but can't be 100% sure. Also, I'm guessing what happens is inside that shrink wrap the green wire (to the transformer) is soldered to the white wire and a second white wire that comes out the other end and attaches to the chassis. Is this what it looks like to you?

Is #2 the new power cord? I don't see a green wire coming out here. Oh wait, from what you said earlier I guess #2 is actually the old power cord and the new power cord attaches to it outside of the chassis. If the latter, there must be a green wire coming out of the new cord and screwed or soldered to the chassis somewhere?

I think what needs to happen is the white wire in #1 needs to be detached from the chassis. If you wanted to you could do a half assed job of that with a snipper and a roll of electrical tape, but it might just be better to take it to a tech and have him redo the whole power cord. It sounds like the new cord on there now is not so professionally installed in the first place. Plus this is a pretty old amp with some pretty old caps and it might be best for it to be powered up on a variac just to be on the safe side. So....

Ned

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Post by workshed » Tue Sep 26, 2006 11:26 am

nclayton wrote: On the other hand, what I THINK is the new power cord (coming out of on the bottom of most of the interior pics) only has a black and white wire with no green wire and the white running directly to the chassis. I don't think that's all there is too it, though, or else I don't think your gfci would trip. Even if the neutral is connected to chassis, the chassis would have to be grounded for there to be a possible current imbalance. Do you see a green wire coming out of the power cord?

Ned
Ah, I didn't get the chassis connection into the photos. They ran the green wire off the new 3-prong cord right to the chassis next to the big transformer on the top (next to the tubes). Then they split the black and white cords inside of the wiring compartment and hooked it into the remainder of the old 2-prong cord. It does appear to tie off/ make contact with the chassis in a few spots, which could be part of the overall problem. Looks like I'm going to let a tech sort this one out. :-)

Thanks for the information though. I wish I had more time to monkey with this amp and the Rosac, but I guess I need to acknowledge my skill and time limitations.

I'm guessing the tech should be able to figure out the impedance requirements too.

-Bret

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