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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    2" x 1-11/16"
     
  2. EdgeGuy

    EdgeGuy Tele-Meister

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    Thanks. It's nice to see all that info on one page.
     
  3. EdgeGuy

    EdgeGuy Tele-Meister

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    OK, here is the transformer wiring. (Wiring side of course) The two green connect to the lamp. The 2 black to the switch and fuse holder, the 2 yellow to pin 2 and 8 (heater and cathode?), the 2 red to pins 4 and 6 (grids?), and the two remaining that I can't tell what color they really are go to pin 7. If I knew more about transformers I'm sure this information would help me know what I have here. I'm guessing the two remaining wires that go to pin 7 are the center taps.
     
    Last edited: May 25, 2020
  4. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    If you go back to post 6 in this thread there is a schematic of a PT, just as you described the Blk are the primary. The red are the HT. The yellow are the 5VAC heater. The green are the 6.3VAC heater. The other two are the HT center tap and the 6.3v center tap.
     
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  5. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    dkevin is probably very close to spot on.

    Amps with twice the number of tubes used about 90mA transformers back in the era of this Ga-5. These days most of the builders would replace those PTs with 115mA trannies.

    Guessing the Ga-5 may have had less than 80mA. I would say your new PT should put out between 70mA and 100mA to keep the feel of the Ga-5.
    If you can find a replacement with 3A on the 5VAC you will be able to use a variety of rectifiers which is a nice option but chances are you will only use a 5y3 rectifier so 2A is all that is required. So make life easy and find one that will bolt in to the chassis without having to modify the PT or the chassis. (If you can't find a PT with a 6.3VAC center tap, no worries, you can create an artificial one. It is very common.)
    (Also a new PT may come with extra leads for bias voltage or taps for different HT voltage. In the Ga-5 these would not be used and you would simply cap them off, wrap them up and tuck them away.)
     
    Last edited: May 25, 2020
  6. EdgeGuy

    EdgeGuy Tele-Meister

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    I had to confirm this is what I had. It's confirmed.
    I just ordered caps so the last thing to get is the transformer.
     
  7. EdgeGuy

    EdgeGuy Tele-Meister

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    The ones I've been looking at have 3A center tapped and 5V at 2A. It would be nice to have a 5U4 option for rectifier.
     
  8. EdgeGuy

    EdgeGuy Tele-Meister

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    I ordered the 30uf.
     
  9. King Fan

    King Fan Poster Extraordinaire

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    Sounds like you're well on your way. Good research by you, and lots of good researchers pitching in here.
     
  10. EdgeGuy

    EdgeGuy Tele-Meister

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    At first I wanted to try to get the original PT but that's not happening. This one seems to be the best yet and maybe even better than the original. Do you think I can swap rectifiers and bias for a 6L6 if I wanted to? It would make for one versatile studio amp. How did you wire this up with the double tap on red? I'm new to transformers simply because I never had to deal with a burnt out one before.
     
  11. EdgeGuy

    EdgeGuy Tele-Meister

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    First off, thank you for knowledge and experience and because of that, a part of you will be in this amp when all is said and done.
    I'm having an interesting back and forth with an engineer at Magnetic Components and I wanted your insight. He's saying this transformer http://www.classictone.net/40-18062.pdf is over powered for the circuit. I asked since the Valco transformer is weak on the plate voltage (http://www.classictone.net/40-18051.pdf) what does he suggest? Quote: "The next closest one that will work is our 40-18062. It's a bigger unit. The plate voltage is 610V vs the 600V shown on the schematic. Although not shown on the spec sheet, the HV winding is rated at around 100mA (see attached). Your amp will draw around one half that, so the plate voltage will run a little higher."
    Now your recommendation is http://site.triodestore.com/5F2PTS.jpg which is rated at 650V but has 3A on the 5V lines which opens up some possibilities for different rectifiers. I'm sure we've been over this but when you're learning something new, it could have easily passed me by and sometime today I'll be rereading this entire thread in search of things I missed the first time around. So, is the engineer correct, half right or off base? He went on to say re: the 40-18051. Quote: "It will work, but it will have some effect on the tone. I believe it may not be as bright sounding. This may not be audible until the amp is really cranked up.". Thanks
    Edit. I thought a higher ma rating would cool things down. Do I have that backwards?
     
  12. EdgeGuy

    EdgeGuy Tele-Meister

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    I'm starting to get a handle on this. Just starting but the fog is lifting slowly.
     
  13. D'tar

    D'tar Friend of Leo's

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    A good old 5f1 champ tranny with the correct mounting dimensions is all you need unless you wish to turn this into something else.
     
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  14. King Fan

    King Fan Poster Extraordinaire

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    Heh, PTs are always hard, somehow, but this discussion has me totally confused. I was thinking what @D'tar said, that this amp needs something like a Champ or Valco PT, ideally in a standup format like the original. And that seemed to be what @dkevin was reporting in the early post about the PT he used. But now I'm seeing discussions about the schematic suggesting a higher HV would be needed. OTOH the smart guys at ClassicTone/Magnetic components seem to agree with D'tar. Probably I missed something, but where exactly are we going on this trip?

    One obvious caution we talked about earlier. Gibson made so many major and minor revisions to amps called GA-5, and was so casual about either updating the schematic or giving it any unique ID to link to a particular actual amp, that working from 'the correct schematic' is hard. Of course that's the theoretical best way, except Gibson may never have made a correct schematic. Heck, I'm not sure what the real tube complement was in this baby.

    Have we totally ID'd *which* GA5 this was? Yeah, I was pretty sure the original PT was gonna be dead -- in a fiery death. But where are we on the idea of just figuring out it's original specs? It might have shorted, it might have been overloaded by the amp, but other than hunting up some extra mA as I think @Lowerleftcoast suggested, it seems like a repro of that PT would be right. Do we know exactly what PT it was?
     
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  15. EdgeGuy

    EdgeGuy Tele-Meister

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    While I don't know what transformer is in this amp, I do have the correct schematic (see pic) and from that we can determine the correct PT. Now the fun begins. Since I'm new to this, I'm now questioning my "Ohm' test of the transformer and the PT may actually be good although I doubt it. I've since learned that testing transformers with DC isn't the same as applying AC voltage and testing but I don't have a power supply that could do that unless I find an AC power adaptor somewhere in my house and use that.....at least that's what I think. I have zero experience and it's hard to apply common sense when little of it makes sense but I'm learning. If my transformer is dead which everyone including me seems to think is the case, then it needs to be replaced keeping in mind the schematic but now I learn I have PT options that are interesting to me because this is going to be my small studio amp. It would be cool to be able to change rectifier tubes and/or stick in a 6L6 by using a different bias resistor. I could even put the bias resistors on a switch. It would be cool to have that versatility and I think others would be interested as well. I'm even looking into using one of the inputs as a re-amp input so my choice of input resistor could change for one of the inputs keeping in mind how the circuit was written and will need to be altered a bit. Right now, you only have a choice of input 1 or both input 1 and 2 but not input 2 alone. I would like to turn input 2 into a re-amp input. I have no problem changing that to turn this into a re-amp amp. So my desire is to learn fast so the people who know about transformers don't get tired of dealing with this newbie. Once I get the transformer issue handled, then I can be back in my element but my hope is to grasp transformers completely so in the future I can breeze though these issues and even design amps from the transformer up rather than just building tried and true designs. I've been able to avoid learning about transformers for 20 years because I've never had a problem like this. Lucky me but because of that, I never learned. Now I'm learning.
     

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  16. jrblue

    jrblue Friend of Leo's

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    I don't know how you can re-engineer a circuit that contains so many onknowns as it is, unless you rip it all out and start from scratch. Designs that make general sense on paper do not always, or even often, sound just as expected. Unless you're going to replace the whole circuit I would get the amp working and then mod from there. It sounds like much of the info you're getting about transformers is coming from people who are either explaining what they know and like or what, of their products, come closest to what they think you need. So I guess you need to decide if you want to replace exactly what was original, or if you're OK with trying an equivalent. I'm sure their are tranny rebuilders who could take your wreck, take it apart, and clone it -- maybe even recognie it for what it is. Try emailing/voicemailing the Freboard Journal's "Truth About Vintage Amps" podcast, and if you get on, Skip Simmons will give you as good information as anyone alive. And finally, the comments about Gibson schematics are spot on. They changed stuff all the time, particularly in that model amp, so there is never a guarantee that a schematic will match your amp.Sometimes they just threw in crap because they ran out of the spec'd part. Maddening.
     
  17. dkevin

    dkevin Tele-Meister

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    The stated footprint seems really tiny. Are you sure about the 1 11/16" dimension? I was hoping to avoid drilling extra holes in the chassis.
     
  18. tubegeek

    tubegeek Tele-Afflicted

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    Here's a suggestion that's out of the box just a little bit:

    If you could get the use of a high voltage adjustable power supply, and if you could get the rest of the amp set up to work OK, you could try various voltages by ear and decide what you like best, and THEN try to match that up as close as possible with an existing transformer and the various options for rectifiers. In other words: try find a stationary target/goal to shoot for and work backwards from that rather than do a lot of speculation that will never be conclusive.

    Some info about power transformers that may be useful: the amount of current a power supply delivers into a load is determined by the demands of the circuit it feeds, within the limitations of the transformer's capacity to deliver that current at the expected voltage.

    If a transformer is rated for too LOW a current, attempting to draw more will tend to limit its lifespan and performance and it will also put out a somewhat lower voltage than it is spec'ed to provide under its rated load. It may overheat and cough out its potting material, it may even catch fire, but these are extreme circumstances. (At some point this happened somehow to your amp, probably not due to "normal" operation though.)

    If it's rated for a HIGHER current than your circuit is actually drawing, a couple of things can happen: the output voltage will tend to be higher than expected, and the transformer will tend to run cooler with less stress over its lifespan, and its ability to handle a varying load will be different from a transformer that is being stressed beyond its capacity. Different how? Test it out and see.

    All this is in the context of a device that was probably NOT way over-rated originally and whose imperfections are - possibly - a desired part of its sound. So we can conceivably make it "too good" if we apply basic precautions too rigorously and lose some mystery mojo - conceivably.

    I don't get the sense that you want to line up three candidates for a transformer, wire up each one, and compare. But the adjustable power supply could give you a first-order feeling for what the options might sound like and it wouldn't be terribly hard to set up. If you found the right supply it'll have a low voltage supply for the heaters built in, replacing that function from the dead transformer. What it won't really be able to do too easily is give you a feel for the way the rectifier would influence the results in terms of dynamics and behavior when pushed. A series resistor could approximate that a little bit maybe but it'd be a little different.

    Better than just guessing though?
     
  19. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    You might get a used one at a good price. A current ebay item:

    *2 Vintage tube stereo amplifier transformers Stancor PC8406 & RCA cloth leads*

    The stancor PC 8406 PT 325-0-325 40mA, 5v @ 2A, 6.3v @ 2A. Mounting - stand up 2"x 1 11/16".

    :cool::cool::cool:
     
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  20. EdgeGuy

    EdgeGuy Tele-Meister

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    I misspoke when I said "circuit change". The original intent was to fully restore the amp to it's original state. That's not happening because the power transformer is unavailable so, plan B. I still don't know if the output transformer works, or the speaker, or the tubes. So far I've ordered replacements for every electrical component in this amp. If I have to replace the speaker or output transformer, what's left of the original amp? The dream of restoring it has morphed into creating the ultimate small studio amp. Regarding the circuit, almost nothing will change. It's still all 1957 Gibson and will sound that way with the 5Y3 and a 6V6. All that will change in order to allow a 6L6 to work is a bias resistor value change to make sure the 6L6 is comfortably biased. I may add a 3-way switch so the 6L6 can be biased for different rectifier tubes. I have a bias tool which makes quick work of it but you can bias without it as well. For the amp to except a 5U4G or 5U4GB rectifier tube and others, the 5V line needs to be at 3 amp instead of 2 amp and a couple of the suggested replacement transformers already come with 3A 5V lines. Again no circuit change. The only circuit change that I might make is to separate the two input jacks from each other and I doubt I'll have too. I'll know as soon as I check the output of my Antelope Audio interface re-amp lines. It's probably fine as is because re-amp outputs are designed with guitar amp inputs in mind. So there will be no actual circuit changes at all. The idea of putting a switch in the circuit so I can switch from 6V6 to 6L6 is a small simple job that can be undone in 5 minuets and involves tapping a switch into a circuit that will remain unchanged. I intend to also put the output transformer on a output jack so I can use other speaker cabinets. This is a common thing studios do and can be undone in less time than it takes for the soldering iron to get hot. So I kind of misspoke when I said I was changing the circuit. I'm really not and it will remain all 1957 Gibson GA-5 with an option for something different.

    I found 3 different GA-5 schematics before I found the one that matched my amp. It's a prefect match so my amp is actually a Skylark in a Les Paul Jr. cabinet. I'm just waiting for the people who are experienced with transformers to chime back in and then I'll order a transformer.
     
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