Gibson GA-20 - Real or Clone, Any Experiences Or Opinions?

Discussion in 'Shock Brother's DIY Amps' started by danlad, Jul 17, 2019.

  1. danlad

    danlad Tele-Meister

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    So I couldn't sleep. Started to wonder if it was time for a new amp build. Have probably most of the parts for an 18 Watt type. Minimalist or Lite IIb appeal to my sense of elegant simplicity. As do cathode bias output sections. Maybe a Matchless Spitfire. But with 6V6s because I can't be arsed with tiny valve bases. They are neither elegant nor simple now that I need glasses.

    Still couldn't sleep so read a bit of my Valve Wizard book and saw the cascode section. Seems a whole lot more elegant and just as simple really as the 'what the hell shall we do with the leftover triode?' parallel input stage. And I have some 6N1P valves gathering dust which are reckoned to work well there. So a (fake) pentode into a long tail. Isn't that just an old style AC-15? Fair enough, but maybe there's something else I can try. Be a bit more creative. Do something different. Oooh, paraphase inverters. Rough and off balance just like I'll be if I don't sleep soon. And definitely simple. So simple I can draw the schematic in my head right now.

    Let me Google 'pentode paraphase guitar amp' for any tips...

    Pentode (well, cascode) -> Spitfire tone controls -> Paraphase -> 6V6 cathode biased output section.

    Oh, it's a Gibson GA-20.

    Has anyone built one? Or got an original? Even just played one? Any thoughts or suggestions?
     
  2. radiocaster

    radiocaster Poster Extraordinaire

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    6N1Ps have tiny bases like the ECC83/12AX7.
     
  3. Nickfl

    Nickfl Tele-Afflicted

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    Which GA-20 are you looking at? There are a couple of versions of that amp that meet your pentode into a paraphase PI requirement. The GA-20T has a 5879 pentode input on one channel and grid leak biased 12ay7 inputs on the other. That amp has intrigued me for a while now, though I haven't met one in person, it is somewhere on my bucket list of builds.

    There is also an octal preamp GA-20 with grid leak biased 6sj7 inputs and a 6sl7 paraphase inverter. That also meets your requirements, but is probably a pretty different sounding amp than the GA-20t mentioned above.

    Either way, it sounds like a promising build!
     
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  4. danlad

    danlad Tele-Meister

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    True enough, but a tube in the hand is worth two in the bush. I'm willing to put myself through the hassle to save on a few pennies by using up what I have in the toolbox that doubles as my parts bin.
     
  5. danlad

    danlad Tele-Meister

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    What, there's another one?? Will investigate for more inspiration. Am guessing it is T for Tremolo.
     
  6. danlad

    danlad Tele-Meister

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  7. FenderLover

    FenderLover Friend of Leo's

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    Ive got two GA-25's from 1948. Not what you asked about, but still pretty cool. Very similar to that posted above. No trem, but pentode input, paraphase PI and a 12" / 8" speaker combo.

    Not very much clean headroom on these if that's what you were looking for.
     
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  8. danlad

    danlad Tele-Meister

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    Perfect!
     
  9. dkevin

    dkevin Tele-Meister

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    I have 2 early GA-20's (all octal),1 Mid-50's GA-20 and a GA-20T same vintage. If I had to choose one to build.....I would build them all!
    One of the early GA-20 amps has the "no headroom" issue but the other one is very loud and has plenty of headroom. One of these days I'll take the time to find out why.
    The mid 50's GA-20 was a CL find. The seller bought it in a pallet of items and blithely informed me that the power switch smoked when he turned it on so it (the power switch) needed to be replaced for an easy fix. I told him he had smoked the PT and that is what was wrong. I considered the chassis/cabinet and speaker to be worth his asking price and rebuilt it with a Magnetic Components PT that was spot on size-wise and voltage wise.
    The GA-20T, I would advise you to re-engineer (as Victoria Amps did to the GA-40). The layout is an issue because the components are mounted to both sides of the board and are a PITA to service. The amp is very much like the GA-40 (Chocolate Thunder) but uses a 12AY7 in the mic channel and a 5879 in the instrument/Tremolo channel. This amp is beautiful and a joy to play! So you can see my dilemma..They all sound great and worthy of reproduction.
     
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