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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 see many RCA 5y3's from the same era on ebay. I'll get one that tests strong.
     
  2. EdgeGuy

    EdgeGuy Tele-Meister

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    I moved several times since my heart surgery and a lot of stuff got lost including a bunch of tubes, x3 SM-57s, and my safety glasses. I'm sure it's all sitting in a box at a friends house in their garage and it got forgotten about. I'll get new glasses.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The PT you have is probably fine. You may have to create an artificial CT but that is easy and does not interfere with the circuit at all.
     
  4. EdgeGuy

    EdgeGuy Tele-Meister

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    The "Tuff Stuff" looks good.
    I never thought I would ever be able to identify those wires. Now I can look for the exact replacement transformer. I may or may not use the one I ordered. If not I'll just put it back on ebay.
    I used to have a variac but it died and I haven't needed one in years. Now I need one so the lightbulb trick will save me from buying one.
     
  5. Nickfl

    Nickfl Friend of Leo's

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    For what it's worth the light bulb limiter and the variac are not really doing the same thing. The light bulb limiter protects your circuit by preventing an excess of current from running through it in the event of a short, it also is a very easy way to test for a short in the circuit because the light bulb will not light up fully unless there is a short. A variac would be used to change the voltage supply to the circuit but does not necessarily protect from over current conditions. Of the two the light bulb limiter is probably the more commonly useful as the circumstances that would call for a variac are limited but a light bulb limiter should probably be used every time you start up a circuit after making modifications to it just to test that something hasn't gone wrong.
     
  6. EdgeGuy

    EdgeGuy Tele-Meister

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    I've decided to restore the amp. I'll just find the original transformer now that I know what I'm looking for. All I need to know is who are the 6 companies Gibson bought transformers from in the 1950s. This transformer will be in their old catalogs.
     
  7. EdgeGuy

    EdgeGuy Tele-Meister

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    I have the parts in the garage and barn to build one. I just have to find the stuff.
     
  8. theprofessor

    theprofessor Poster Extraordinaire

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    One of the best places to get tubes, in my experience, is from Brent Jessee's ebay store. He's a tube dealer who sells NOS tubes, but he puts things on ebay that are ANOS. He tests them thoroughly and guarantees them. When you buy from Brent, you're not rolling the dice, as with most others on ebay. He's got several 5Y3's in the store at the moment. There's even an RCA from 1956.

    ("Audiotubes by BJR" is the name of the store.)

    http://www.ebaystores.com/Audiotubes-by-BJR?_rdc=1

    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=223460892180
     
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  9. EdgeGuy

    EdgeGuy Tele-Meister

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    That's the seller I need.
     
  10. EdgeGuy

    EdgeGuy Tele-Meister

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    For the interested. The inside of my transformer. 20200531_200003_HDR.jpg 20200531_195739.jpg
     
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  11. EdgeGuy

    EdgeGuy Tele-Meister

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  12. EdgeGuy

    EdgeGuy Tele-Meister

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    MM confirmed that my transformer is at 70ma max, "just like the original". I think they know what I have.
     
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  13. tubegeek

    tubegeek Friend of Leo's

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    Is it physically the same size? Footprint, lamination dimensions, stack height? That'd be a VERY good sign if everything was close to the same.
     
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  14. EdgeGuy

    EdgeGuy Tele-Meister

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    MM doesn't give out much data which is frustrating. Even the photo on their site isn't the photo of the unit. Their footprint is 2" by 2" (mine is 2" by 1-11/16" so it won't fit. Even with the 1/8" extra play, their transformer will cover the screw holes by 1/8". I could use thinner bolts but let me see if I can find the real one.
    When I OxyCleaned the wires and finally saw what they were, I realized nothing I had been looking at previously was close to what I have. MM knows what I have and while they don't give out much information, they got it right because they know my transformer. It's obvious they know my transformer but made it too big. So now I can look for an original old replacement. Now I'm looking for a 300-0-300V, 70ma, 5V with CT, and a ground wire smaller than 2" by 2". Armed with this knowledge, I may find the original transformer.
     
  15. EdgeGuy

    EdgeGuy Tele-Meister

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    MM doesn't give out much data which is frustrating. Even the photo on their site isn't the photo of the unit. Their footprint is 2" by 2" (mine is 2" by 1-11/16" so it won't fit. Even with the 1/8" extra play, their transformer will cover the screw holes by 1/8". I could use thinner bolts but let me see if I can find the real one.
    When I OxyCleaned the wires and finally saw what they were, I realized nothing I had been looking at previously was close to what I have. MM knows what I have and while they don't give out much information, they got it right because they know my transformer. It's obvious they know my transformer but made it too big. So now I can look for an original old replacement. Now I'm looking for a 300-0-300V, 70ma, 5V with CT, and a ground wire smaller than 2" by 2". Armed with this knowledge, I may find the original transformer.
     
  16. tubegeek

    tubegeek Friend of Leo's

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    "Knowledge Is Good" - Emil Faber
     
  17. EdgeGuy

    EdgeGuy Tele-Meister

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    Who is Faber?
     
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  18. tubegeek

    tubegeek Friend of Leo's

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    Founder of Faber College, home of Delta House. His statue with this motto is the opening shot of one of the funniest movies of all time. That's all the hints you get.
     
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  19. EdgeGuy

    EdgeGuy Tele-Meister

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    I'm learning about Faraday/electrostatic shields in transformers since the GA5 transformer has one. These shields help with grounding RFI so it doesn't reach the secondary and get into the low voltage DC lines. In other words, the grounded transformer in the GA-5 was no accident and although it's effectiveness is in question when the secondary has a CT to ground, it's what Gibson engineers decided on. I could always disconnect it to see if it makes a difference.
    It looks like Mercury Magnetics has the only transformer for the GA-5 with my circuit and if I use thinner bolts to bolt it to the chassis, it should fit without drilling new holes.
     
  20. EdgeGuy

    EdgeGuy Tele-Meister

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    I saw Animal House in the theater. It was wild when it came out but it seems so innocent and tame now.
     
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