Gibsonette GA-8 inspired first build

Discussion in 'Shock Brother's DIY Amps' started by Mexitele Blues, Feb 13, 2020 at 1:18 AM.

  1. Mexitele Blues

    Mexitele Blues Tele-Holic

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    Hello all, now that I have been reading and learning for a while I think it is time to get my hands dirty. In March '18 I found a 1960 Gibsonette on CL and fell in love with its thick, single ended antics.

    Gibsonette GA-8 Schematic.png

    After servicing it and seeing how simple it was inside, the wheels started turning. So here is my plan for a modernized Gibsonette circuit. 1) Reign in the bass a touch and shore up what is left. 2) Allow for external speakers. 3) Switched NFB with a low pass setting for hairy highs and clean lows. (No idea if this will work or sound good!)
    Brunette Schematic.png


    I'd like to try out some other mods when finished, both proven and ill-advised, so this will be a mule of sorts, in a Princeton Reverb chassis with plenty of room and extra holes. This project's goal is to get a feel for the process and theory rather than turn out a polished amp on the first try.

    So there you have it, no finished tag board, no beautiful cabinet, only a schematic drawn by a noob who knows just enough to be dangerous. All tips, critiques, and comments are welcome!
     
  2. Splodgeness

    Splodgeness Tele-Meister

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    There are people on here far better qualified than me who could comment but unless I'm reading your circuit diagram incorrectly, it looks like when you have no external speaker connected, you have a dead short to Ground across the Output Transformer..... (and also with a 4ohm external speaker, assuming that the top jack is incorrectly labelled as 8 ohm external)

    :)
     
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  3. Mongo Park

    Mongo Park Tele-Holic

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    I would separate the common cathode resistor/cap on the 6b6 power tubes. That way you can bias them separately.
    You could bias one hot one cool which would give you more harmonic distortion And get more depth from the parallel power tubes. Somewhere around 470 ohms is standard and something higher and lower would separate them.
    Cheers Ron.
     
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  4. Squawker

    Squawker Tele-Meister

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    I always thought the mismatched g1 resistors were an attempt at the same. On the original they're 0/470, in the suggested schematic they're 470/1K5.

    Also, you could keep the common cathode circuit and just not use matched valves (which is historically correct) :)
     
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  5. Mexitele Blues

    Mexitele Blues Tele-Holic

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    Thank you Splodgeness, I will give it a second look. The idea behind the speaker jacks is that if there is no internal speaker plugged in, the OT shorts to ground. The tips of both jacks are connected so that whatever secondary is feeding the external jack also feeds the internal. Then if 8 ohm loads are plugged into both jacks, the 4 ohm secondary carries the load in parallel.

    Hi Ron, I recall you were buying transformers for a similar build last year. Have you begun the build yet? Playing with the power tube balance is absolutely on the agenda after I get it going and hear the 'baseline'.

    My Gibsonette has its original Gibson branded RCA tubes, which indeed don't have any markings to suggest matching. I have a pair of mismatched JJ 6V6 which will most likely find their way into this amp.

    Any suggestions on the transformers? I see I have an email from Amplified Parts touting a 10% discount on Hammond for the next week so I should probably take advantage of that.
     
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  6. Squawker

    Squawker Tele-Meister

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    My GA-8 clone thing has a Hammond 125DSE. It's quite a chunk of iron - Hammond are notoriously overengineered, but it's a general problem with Single Ended OTs that they need a big air-gapped core to handle the standing DC current.

    Edit: Ha - I see now that is what's on your schematic too :)
     
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  7. Splodgeness

    Splodgeness Tele-Meister

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    My apologies ........ I guess I misunderstood the labelling on the diagram. I think what you are proposing then is that the intenal 8ohm speaker connects to the lower jack (labelled "internal"). Then a single external speaker would/could be plugged into the upper jack to effectively connect the speaker pair to the 4ohm transformer tap. What threw me was interpreting the upper jack as the one where a single external speaker could be plugged in with no internal speaker connected, which in this case wouldn't work........... :)
     
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  8. Mexitele Blues

    Mexitele Blues Tele-Holic

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    What did you use for your PT? The 290XXA is the Pro Junior replacement spec. I'm open to others if they will work better.

    Have you got a build thread on your GA-8?

    That is correct, no apology necessary. I wasn't sure how exactly to clearly label the intent on the schematic. That seemed the most practical application to me for an external jack as opposed to an impedance selector.
     
  9. Squawker

    Squawker Tele-Meister

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    Oh, now you're asking.... I can't remember, but I'll look this evening. Might even have been salvaged from something else. I don't have a build thread - there were some comments on the AX84 forum years ago when I was debugging some motorboating, but that's about all.
    Will get back to you later.
     
  10. Mongo Park

    Mongo Park Tele-Holic

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  11. Squawker

    Squawker Tele-Meister

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    I think the PT was bought from ELFA which is a Scandinavian version of RadioSpares or Farnell. Made by Swedish Tranformator-Teknik, model 56-003-09. 260-0-260, 3.15-0-3.15 (all heaters) and 0-5-6.3 winding (used for the lamps). Unsure about the current ratings.

    That 290XAX looks good, as does the 370DAX (or its 270xx equivalent).
    Or something like this https://www.tube-town.net/ttstore/toroidal-80va.html - that's in Germany, but something similar might be available closer to you.



    upload_2020-2-13_21-14-58.png

    upload_2020-2-13_21-15-32.png
     
  12. Mongo Park

    Mongo Park Tele-Holic

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    Hammond 290xxa is plenty big enough. It would take some looking into but their is a way to pull both 6V6 and use one 6L6 but you need the transformers to run it. The 290xxa would do fine but the OP transformer would have to be able to cope with it as well.
    The build I did is 100ma, which runs this amp just fine.
    Cheers Ron.
     
  13. ArcticWhite

    ArcticWhite Tele-Holic

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    Why did you decide on a solid state rectifier?
     
  14. Squawker

    Squawker Tele-Meister

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    There is no musical advantage to a valve rectifier in a single ended amp. They don't sag like P-P so there's no natural compression/browning out from the power supply. Does provide a softer startup, but it's not a biggie.

    Solid state saves space and heater current, and can let you use a lower voltage rated power transformer.
     
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  15. Mexitele Blues

    Mexitele Blues Tele-Holic

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    Squawker hit it on the head. Space, simplicity, no need for a tube.
     
  16. ArcticWhite

    ArcticWhite Tele-Holic

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    Ha! I didn't know that.
    Why don't single ended amps sag?
     
  17. ArcticWhite

    ArcticWhite Tele-Holic

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