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Gibson-like project

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  • #1

    Gibson-like project

    I am looking for another project to work on and I am hoping that some of you can help me refine my search.
    I have just finished up a rebuild of a Gibson GA-5 Les Paul Jr. amp. It uses a 6SJ7 tube as the preamp tube. I love the sound of this amp so much I would like to build a larger version of this amp from scratch. The one I would like to build would use 6SJ7(s) in the preamp and twin 6V6's for output tubes. I also would like to use a 5Y3 rectifier.
    I have found several amps (Gibson of course) that use these tubes like the GA-8/9/20/30 etc. I wonder which of these amps you have heard/played thru and what you liked about them. I am really in search of a bigger brother for my Les Paul Jr. amp.
    Got any suggestions????
    Thanks!
    dkevin
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  • #2
    Gibson-like, eh? Well, another fella with good taste on this forum... If you're looking for a big brother to your other amp, maybe consider the GA-50? I know you mentioned 6V6's, but the cathode biased 6L6's are not that loud in the GA-50, the plate voltage is not that high. It also has separate bass and treble which was a 'professional' approach back in 1948 when those started out.
    I have a pair of GA-25's as well (6V6's) but the tone circuit is a little too odd for some folks. Me too frankly - I rewired one of them into a more conventional tweed style.
    You probably couldn't go wrong with the GA-20 and it's master tone control if you're stuck on 6V6's. Study where the tone pot is, and realize you can try different tweed ideas there too if you look at what Fender did.
    The reason I like my GA-50 aside from the separate bass and treble is the 'tweed vibe'. Low plate voltage with higher current gives a different feel and sound than a more modern high plate voltage and with lower current.
    I have both types, because my mood changes. After all, what serious guitarist only has one guitar? There is no single amp that does everything well. Lastly, I like the tweed EQ circuits because they roll off the highs and lows while leaving the mids alone. Completely different sound than a Fender, with it's mids scooped out. If you play jazz - you're home.
    Happy hunting.
    Black sheep, black sheep, you got some wool?
    Ya, I do man. My back is full.

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    • #3
      I just finished rebuilding a 1952 Gibsonette with that same tube layout and I love it. I swapped the field coil speaker for a 10" I took out of a Gibson Medallist 4x10 many years ago and it is a real sweet sounding amp.
      I need to put a post elsewhere on the site. The amp had recently been recapped and the tech wrapped the multi section cap in black tape so it would fit snug in the clamp. Trouble is, he covered up the specs so I don't know what to replace it with in future.

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      • #4
        52 Gibsonette speaker

        Hey Resonator Guy,
        When you removed the field coil speaker and substituted a permanent magnet speaker, what changes did you make to the amp to account for the missing resistance of the coil?
        I notice on several old Gibson amps they cite a 1000 ohm coil that feeds the speaker and the remainder of the power rail.
        Thanks!
        dkevin

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        • #5
          DKevin, I had read in one of Weber's books that you had to do precise measuring of Henries, ohms etc when replacing a field coil speaker with a big resistor and capacitor, so I carefully cut the electro-magnet part of the old speaker off and mounted it and the output transformer to an extra thickness of wood I had glued to the inside of the cab. The old speaker still had its Jensen label and I think it looks okay in there.
          The main other work in fixing the amp up was repositioning and gluing the wood supports for the chassis, cleaning and regluing the old tolex, which was up at every seam and corner, finding a replacement handle, and retubing it. There was about 57 years of nicotine on the amp and I almost started smoking again!
          I am real happy with the results, both appearance and sound.
          Attached Files

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          • #6
            Wow!

            So, you re-used the original field coil (seen inside the cabinet ) and remounted the output transformer (just peeking out from the top of the cabinet) too. Was the output transformer originally mounted on the chassis or was it always affixed to the cabinet?
            Did you have a chance to measure the resistance of the salvaged coil before you hooked it up to the circuit? I wonder if it is the 1000 ohm resistor shown in the early Gibsonette schematic?
            How does she sound? (drool....drool...)

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            • #7
              Dkevin, the output transformer was originally attached to the speaker, so I had to find a place for it inside the cab.
              I have a matched pair of NOS Brimar 6v6s, a NOS RCA JAN CRC-5693 preamp tube , and a NOS GE 6087/5Y3GTB in it.
              The pre-amp tube is a heavy duty replacement for a 6SJ7. I tried a couple of 6SJ7GTs, but I liked the sound of the metal ones better. This one is in a red can.
              I didn't measure the resistance of the coil but I will next time I have the thing apart.
              Oh and it sounds magnificent. If I played jazz, this would be the amp to use, but it sounds good with blues or Neil Young running through it.

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