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Gibson GA-20 transformers

#1 User is offline   dkevin 

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Posted 01 September 2018 - 10:37 AM

I am looking for a power transformer and output transformer for a Gibson GA-20 with nine pin tubes in the preamp. I think the amp is a 1956 version and the PT currently in the amp is not only incorrect but it is also fried. The OT is an off-the-shelf replacement and I would prefer to have original transformers if possible. The PT is stamped 'GA-20-P" . The OT markings are not readily evident from the pictures I have. The OT is often "lost" when the original P12R speaker is fried and replaced. It was riveted to the speaker frame. Thanks for your help!


https://imgur.com/a/T1ZZnVB
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#2 User is offline   dkevin 

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Posted 12 September 2018 - 07:30 PM

Update


https://imgur.com/a/KKTolZg
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#3 User is offline   dkevin 

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Posted 14 September 2018 - 06:52 PM

The beast lives. Voltages very close and sounds very good! More testing and then I'll wrap this up...meanwhile, look at some more pictures...

https://imgur.com/a/xcEyRck

This post has been edited by dkevin: 14 September 2018 - 06:53 PM

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#4 User is offline   dkevin 

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Posted 16 September 2018 - 06:27 PM

&nbsp; The PT and OT worked out great! The amp sounds very good! I've got the chassis reinstalled into the cab with the warehouse G12Q and it sounds really nice! I had to tighten up the octal socket pins. The third time I played through the amp, I turned it on and ....no sound. I checked my rail voltages and they were all high and not dropping into normal operating range. I chopsticked the output tubes and when I touched the 6V6 plates (3) I got a audible pop. I checked the solder joints and they were all clean. So I removed the tubes and looked at the pins inside the sockets. They were spread out so I closed them up a but with a small machinist's scribe. The amp started and played from then on with no more symptoms.I am making a back cover for the chassis innards...the seller said he lost it somewhere. I happen to have a GA-20T that has the back cover so I'll just copy that one. The two amps are of the same vintage but the circuits and layouts are vastly different. The 20T is a horror story to work on with components mounted on both the upper and lower surface of the component board. It is the sort of Gibson amp that leaves techs shaking their heads and swearing never to work on another Gibson amp. Compared to the 20T, this rebuild of a GA-20 is a "walk in the park".<div><br></div><div><br></div><div>https://imgur.com<div><br></div><div><br></div></div>

This post has been edited by dkevin: 16 September 2018 - 07:05 PM

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#5 User is offline   dkevin 

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Posted 16 September 2018 - 07:07 PM

https://imgur.com/a/O59kPkb
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