330 Volts on OT Frame! Is it Toast?

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330 Volts on OT Frame! Is it Toast?

Postby vez on Fri Jul 28, 2017 1:55 am

My Gibson GA30's OT is mounted on the speaker basket and the whole thing is reading 330 dcv. Initially I can't see any B+ wires shorting on the frame. Can transformers short in some way that would do this? Other than that everything works fine, the amps sounds great except for some crackling sounds that aren't loud. Is my OT toast? Have you ever seen this happen?
Thanks, Rick
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Re: 330 Volts on OT Frame! Is it Toast?

Postby Gooch on Fri Jul 28, 2017 10:24 am

Sounds like a short in the winding to the core. Just check it with an ohmmeter, from the frame to one of the OT primary connections (with power off of course), it should read open.
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Re: 330 Volts on OT Frame! Is it Toast?

Postby vez on Fri Jul 28, 2017 4:51 pm

Gooch wrote:Sounds like a short in the winding to the core. Just check it with an ohmmeter, from the frame to one of the OT primary connections (with power off of course), it should read open.

Can it be fixed? I guess it would have to be rewound. That sounds difficult. I love the tone of this amp.
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Re: 330 Volts on OT Frame! Is it Toast?

Postby vez on Fri Jul 28, 2017 6:42 pm

Gooch wrote:Sounds like a short in the winding to the core. Just check it with an ohmmeter, from the frame to one of the OT primary connections (with power off of course), it should read open.

The reading from the plates to the frame is 448 ohms and 766 ohms. Do I need a new OT? Or can I fix it?
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Re: 330 Volts on OT Frame! Is it Toast?

Postby pdf64 on Sun Jul 30, 2017 7:25 pm

The OT needs to be rewound / replaced.
Unless used with an isolation transformer, the amp is unsafe to use as is.
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Re: 330 Volts on OT Frame! Is it Toast?

Postby vez on Tue Aug 01, 2017 5:28 pm

pdf64 wrote:Unless used with an isolation transformer, the amp is unsafe to use as is.

Hi pdf64, Yes I found this out the hard way.
I was shocked, SHOCKED to find over300 volts on my speaker basket.
What would a isolation transformer do? save the rest of the circuit?
I was thinking of putting the OT in an ABS enclosure mounted inside the cabinet.
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Re: 330 Volts on OT Frame! Is it Toast?

Postby pdf64 on Wed Aug 02, 2017 11:06 am

Sorry, on review I think that an isolation transformer wouldn't help, as the issue is the high Vdc between the OT/speaker metalwork and the amp chassis. If a dog or kid has a nose around back there whilst you're not looking, it may be fatal.
I guess you could try putting the OT in an insulated case. Mouser or a hobby electronics supplies shop will likely have a suitable plastic box, doesn't need to be very big given the size of Gibson OTs :D
The thing is that even if a single short to the frame is 'get roundable', it's possible / likely that there's another weak spot not far behind.
If that happens, a short across or between windings (ie via the frame) will render the transformer useless.
So you may as well look to get it re-wound / replaced.
Re-winding is likely to be 5 to 10x the cost of replacement eg Hammond equivalent.
Merren might to willing to do it?
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