A friend gave me his burnt out 1957 Gibson GA-5 Les Paul Jr. amp.

Discussion in 'Amp Tech Center' started by EdgeGuy, May 22, 2020.

  1. EdgeGuy

    EdgeGuy Tele-Meister

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    I've come up empty on what values the power transformer has and I refuse to pay Mercury Magnetics 3 times the normal price for a transformer that won't fit and might not be the right one. The only markings on the transformer case are 'GA 5". The case cover style is an old Hammond style but Hammond says they don't know what it is. The Amp seems to be an early Skylark transition from a Les Paul Jr. I intend to lovingly restore this amp and use it in my recording studio. I've restored many classic amps in my time and never have I had such problems finding a transformer. Does anyone know what this transformer is?
     
  2. dkevin

    dkevin Tele-Meister

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    http://www.triodeelectronics.com/5f2potr3270m.html

    5F2PTS.jpg I used this PT to replace a GA-5 transformer that burned up. It gave the voltages I needed according to the schematic. Check the schematic for voltages and especially to match the footprint of the original PT.
     
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  3. Peegoo

    Peegoo Friend of Leo's

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    Contact Amplified Parts and Weber and ask if they can source a transformer. I'm sure both places can cross-ref a PT for you. You could also get that rewound.
     
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  4. EsquireOK

    EsquireOK Friend of Leo's

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    Sorry I can't help with your inquiry, but I wanted to say that IMO it's worth fixing unless it'll run you into the hundreds of bucks. I love mine. Found it in an abandoned house in the desert. Put in a new WGS speaker and had some dried up components replaced, to the tune of about $150 total. Looks like hell, but sounds like heaven. Crank that baby up to 8–10, play a Tele through it, and use your guitar’s volume and tone knobs to control the amp. Great sounding amp (not too different from a '50s Fender Champ, from what I have been told).
     
    Last edited: May 22, 2020
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  5. Dacious

    Dacious Poster Extraordinaire

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    Do a Google search for someone doing radio repair in your area. I have a guy specialises in doing that - he's rewound 1 paper former trans created me a new 240 volt power trans for my Superchamp from E-I bits, plus redone a deluxe reverb output and all of them look original.

    Cost me $70 for an old push-pull PA head that had one primary out. Used the old lacquered wire. Looked original with proper colth covered wire
     
  6. King Fan

    King Fan Poster Extraordinaire

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    Yep, these are great little amps. Highly rewarding to restore.

    I especially like the PT diagram @dkevin shared there -- if that nailed his voltages, it's certainly very mid-50s (can you say Fender Champ?) so there should be a *lot* of PTs in that vicinity. (OK, if the GA-5 uses the bias tap, it gets a bit harder, but the PR and similar amps borrow bias voltage straight from the rectifier -- Doug Hoffman has a page on that.) Check out the ClassicTone 40-18019 for example -- wow, even a heater CT!

    upload_2020-5-22_20-7-45.png

    Have you restored Gibsons before? If so, you'll know this: Gibby often changed schematics within model names, or numbers, or both, and even more often changed actual builds before they changed the schematic. You might share a few gut shots -- and if you have a matching schematic, share that too. Some folks here know a lot about these little guys.
     
  7. PCollen

    PCollen Friend of Leo's Silver Supporter

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    I found a Gibson GA-5WT amp at the curb, waiting for trash pickup. But it is solid state, not tube. Has an 8" UTAH speaker. It works and sounds OK. I use it for a "porch amp".
     
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  8. Engine Swap

    Engine Swap Tele-Meister

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    Have you tested the transformer? What is the issue?
     
  9. EdgeGuy

    EdgeGuy Tele-Meister

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    I think this is it! http://www.triodeelectronics.com/40-18051.html

    The values look about right and it even has the same screw spacing and covers and in fact, I think I will be able to re-use the old covers but that's to be seen.
    The original transformer leaked out a lot of waxy substance which is on the bottom of the case. Almost half the interior of the chassis is blackened by fire and some of the caps are blistered on the outside from over heating and are impossible to make out but the resistors I can make out and that is what will tell me if this is the right transformer. The volume pot is completely rusted which I've never seen before in an amp with zero water damage and the chicken head will not come off. Someone may have crazy glued it on so I may have to hacksaw it off to get the pot out. The output transformer looks remarkably good and it's codes are clear. I'm going to bring this little guy back to life.
     
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  10. EdgeGuy

    EdgeGuy Tele-Meister

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    You know, It looked so bad I never considered testing it. All the wax is gone so, should I?
     
  11. EdgeGuy

    EdgeGuy Tele-Meister

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    I've never had a Gibson amp before. I've restored mainly Fender with a lot of Ampeg (the 50s tube rectified are my favorite), Rickenbacker and some other odds and ends. Gibson documentation hasn't helped but people here have and I'm finding out that what you are saying is true. I haven't seen a schematic yet that matched this amp because it's a transition amp. Part Skylark, part Les Paul Jr, and im sure part whatever parts they had on hand. When I have time I'm going to have to document all the resistors because that will be most reveling. I need some time to do a real forensic investigation. I just hoped someone might know about the transformer. As it turned out, I'm pretty sure I found the exact one.
     
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  12. Engine Swap

    Engine Swap Tele-Meister

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    I have seen plenty of transformers that have been stressed, but still work fine. A short in a cap or a rectifier could be the root cause. Transformers can be repotted with an appropriate wax, or modern potting compound.

    I would test it if it were on my bench.
     
  13. King Fan

    King Fan Poster Extraordinaire

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    Good job on your research. Sounds like you’re well on your way. Now (hint) this thread seems tragically lacking in pics...


    Sent from my iPad using Tapatalk
     
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  14. tubegeek

    tubegeek Tele-Afflicted

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    (seconded)
     
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  15. EdgeGuy

    EdgeGuy Tele-Meister

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    And here are the pics.
    I almost forgot. 20 years ago, this amp was given to a music shop for repair. 1 year later, they never repaired it but they did screw with it. The power cable was cut out of the chassis and the plug removed and soldered to the speaker terminals. It appears they used the speaker as an extension speaker. A mystery 2 foot long coax cable was soldered to a tube socket and was connected to ground. Why? On top of all that, the Gibson logo was stolen but not to worry. No charge. I have more pics of the weird repair attempt or nonattempt if anyone is interested. 20200523_020932.jpg 20200523_020926.jpg 20200523_020901.jpg 20200523_020857_HDR.jpg 20200523_020845.jpg 20200522_144616_HDR.jpg 20200522_144447_HDR.jpg
     
  16. tubegeek

    tubegeek Tele-Afflicted

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    That there is a problem child from a broken home for sure.

    Also, re: Edison plug on the speaker:

    I'm reminded of Spinal Tap, and the thin line between clever and stupid.
     
    Last edited: May 23, 2020
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  17. King Fan

    King Fan Poster Extraordinaire

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    Hey, great pics. Thank you!!!

    It does look like the PT burned up. Or do they burn down? Either way.

    If you restore a lot of amps, you're probably way ahead of me, but I really enjoyed a much simpler restore on my buddy's (similar-era) GA9, with some details in this thread from some good suggestions (eg, ways to replace or restore the big firecracker multi cap) in this thread.
     
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  18. EdgeGuy

    EdgeGuy Tele-Meister

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    This one burned up and down. I have only once had to deal with a dead transformer and it was Chinese crap so I just replaced it with a new American built one. I have no experience with repotting transformers......yet.
     
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  19. EdgeGuy

    EdgeGuy Tele-Meister

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    Here are the pics before I removed the wire madness. Clever? Stupid? Something in-between? IDK.
     

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  20. tubegeek

    tubegeek Tele-Afflicted

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    The whole ancient-Jensen aesthetic is wonderful but I can't say I really expect much of that amp to work, or at least, I wouldn't bank on it. Have you tested the speaker?
     
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