Gibson GA8 vs GA9

Discussion in 'Shock Brother's DIY Amps' started by Mongo Park, Apr 7, 2018.

  1. Mongo Park

    Mongo Park Tele-Holic

    606
    Nov 28, 2010
    Winnipeg
    I was looking over the GA8 for a build, thinking this amp might have some nice tweedish sound with a variation from the fender camp.

    [​IMG]
    This brought me to thee GA9 with the 6SJ7 preamp tube.
    [​IMG]

    If you were to pick one to build which would it be. I would have to modify the GA9 in some way as it looks like it has a field coil speaker in the schematic.
    Cheers Ron.
     
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  2. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    The 6SJ7 would probably sound pretty cool methinks. Converting the output stage in the GA-9 to a conventional permanent magnet speaker is pretty easy. Given you're looking at B+ = 340V, then the optimum load resistance for one 6V6 would be Zout = 340/(12/340) = 9,633R. So for 6V6||6V6 at 340V, that's 9,633/2 = 4,816R (~5k) OT primary. Bias them to 100% (Start with a shared 250R cathode resistor, and see where it lands, might need to go to 220R or 270R, depending on where your power supply voltage lands, but 340V is an ideal plate voltage for SE 6V6 in Class A. Use a 20-25W OT for better power handling.
     
    Last edited: Apr 8, 2018
  3. dkevin

    dkevin Tele-Meister

    178
    Mar 13, 2011
    Seattle, WA
    2 different beasts...each unique. My personal favorite is the tweed GA-8/Discoverer amp made in the late 50's and early 60's. I have heard it described as a dual champ since it is a single ended, parallel output twin 6v6 amp. It sounds glorious at low, mid and high volume. The design is deceptively simple.
     
  4. Mongo Park

    Mongo Park Tele-Holic

    606
    Nov 28, 2010
    Winnipeg
    Thank you both for your help. Tubeswell thanks for the details on how to solve the field coil workaround, I will print the schematic and add in your notes. Yes I was thinking the same about the preamp tube, which is what caught my attention. And Dkevin thanks for pointing out that they are not really the same sort of amp. People who have the GA8 are pleased with how it sounds which is also calling my name. I think the schematic I posted is the Gibsonette, not the discoverer.
    Cheers Ron.
     
  5. Crashbelt

    Crashbelt Tele-Meister

    Age:
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    378
    Mar 15, 2017
    Cambridge England
    Can't speak for the GA9 but my 59 GA8 has the most sweetly righteous tone at the break up point of any amp I've had.

    It's my main gigging amp for pubs and clubs and at 8watts is perfect for hitting the sweet spot at the right Db levels.
     
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  6. Mexitele Blues

    Mexitele Blues Tele-Meister

    Age:
    40
    171
    Mar 7, 2018
    Westminster, CO
    That is definitely the schematic of my Gibsonette. I've only been able to find a notated version online, so thanks for posting a clean one.

    I recently blueprinted mine with the intent of building a copy with a few mods. It's a remarkably simple circuit, very similar to a 5F2 with an extra 6V6 and no negative feedback. And yes, it sounds glorious with the volume past noon.

    I'm curious if you would wire it on a board or true point to point like the original.
     
  7. Mongo Park

    Mongo Park Tele-Holic

    606
    Nov 28, 2010
    Winnipeg
    I am already leaning towards the Gibsonette as it seems to be more of what I am looking for. I have built a few 5F2a's and tried all the mods I could find, and enjoy this amp. So the Gibsonett seems like an extension of single ended amp tone. The fellow who got me started on this addictive amp stuff said build it Point to Point the first time and it will give you a better understanding of it all. So I did and he was right. As a result of this I have not moved past this PtoP thing. So yes It will be PtoP in a more square box ala gibson style. The long fender chassis are more turret friendly but with some shielded wire they are workable as PtoP. PtoP also makes it possible to get the big smoothing power source caps right up close to the tubes. In a more square box things can be placed in a more optimal location with PtoP.
    I found one other schematic and it was a very poor reproduction. I was happy to find a more readable one. I think this transformer will work well:
    file:///C:/Users/user/Downloads/PT275.2_USA_DWG.pdf

    And this for OP transformer should provide satisfaction:
    file:///C:/Users/user/Downloads/OT20SE_USA_STD_DWG.pdf
    With the 2 parallel 6V6's to keep them happy it has a 2K3 and 3K options to see what works best. As well it should drive a 12" speaker standing on its head.

    What sort of mods were you thinking.
    Cheers Ron.
     
  8. Mexitele Blues

    Mexitele Blues Tele-Meister

    Age:
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    171
    Mar 7, 2018
    Westminster, CO
    It's a little weak in the bass so I'd go for a 12" speaker and bump up the first two filter caps to 22uf. Despite the schematic, it had a 20+20 firecracker for those when I brought it home so I don't think another 10% is a stretch. Maybe an NFB loop & switch to clean things up every now and then. Then an extension speaker jack, and that should do it.
     
  9. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    This thread points out the need to discriminate between different amps that happen to carry the same model number. The GA-8 is a different amp from the GA8T just as these two amps are different from that GA9 shown above. The GA8T has a push/pull output with a cathodyne phase inverter.
     
  10. dkevin

    dkevin Tele-Meister

    178
    Mar 13, 2011
    Seattle, WA
    I have used an Allen amps output transformer as a replacement for a tweed GA-8. It is the TO-11C output transformer. The two 6v6's are running in parallel, single-ended.
     
  11. Mongo Park

    Mongo Park Tele-Holic

    606
    Nov 28, 2010
    Winnipeg
    Wally you are perfectly right. I am a victim of a transient internet self education. This leads to large gapes in knowledge. Thanks to this sort of form and the people who have chimed in I would be lost.
    With that in mind do you think the 2K3 and 3K on the output transformer is usable, from what Dkevin posted.
    Cheers Ron.
     
  12. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Mongo, to keep things straight....you are thinking about building that GA9,right? Interestingly, I have one of these in for work right now. It will end interesting to see if it adheres to the schematic, or if it veers off the track as do many Gibson amps.
    For my money, I would opt for a different amp...but that is your call. The GA-8 schematic shown in your post is closer to being a bigger 5F1si play do to the fact that the preamp and driver is more akin to the Champ...more modern than the grid leak biased GA-9. Frankly, model names don’t keep things straight for me with Gibson amps, either. If you are thinking about building the GA-9 with the octal preamp/driver, then tubeswell has you on track, I would think. In fact, if you are going for the first GA8 schematic there, his advice holds there as well.
    That GA9 is a different amp from anything you have encountered, I am thinking...unless you have played a 5C1 or earlier Champ.

    https://schematicheaven.net/fenderamps/champ_5c1_schem.pdf
     
  13. muscmp

    muscmp Tele-Holic

    982
    Jul 1, 2008
    california
    i have a 58 tweed ga9 that uses one 12ax7, two 6v6 and a 5y3, with a 10" jensen. sort of a bright amp that plays better with a gibson humbucker guitar than a fender single coil. i usually combine it with something like the ga8t i've shown below.

    https://muscmp.wordpress.com/2010/11/10/356/

    i also have the ga8t discoverer that is a great amp but nothing like the earlier ones. it is a later black tolex version from 65 with a great tremolo.

    https://muscmp.wordpress.com/2010/10/27/gibson-ga8t-discoverer-tremolo-amplifier-2/

    play music!
     
  14. Mongo Park

    Mongo Park Tele-Holic

    606
    Nov 28, 2010
    Winnipeg
    Wally, from your advice I will go for the GA8, and leave the 6SJ7 for another day. I can only build one amp at a time and the GA8 has some good reviews. Thanks for taking the time to help me out with this. Cheers Ron.
     
  15. Mexitele Blues

    Mexitele Blues Tele-Meister

    Age:
    40
    171
    Mar 7, 2018
    Westminster, CO
    Mongo, I have my schematic noted with the pin locations of all the P2P connections. If that would be helpful to you let me know and I can upload it.
     
  16. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Cool. Promise us you’ll post a build thread!!!
     
  17. Mongo Park

    Mongo Park Tele-Holic

    606
    Nov 28, 2010
    Winnipeg
    Thank you all for your help.
    GA8 it is, iron is on its way.
    Mex-Tele Blues it would be great if you would post that info, all help id appreciated.
    Photos is part of the build fun.
    Cheers Ron.
     
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  18. Mexitele Blues

    Mexitele Blues Tele-Meister

    Age:
    40
    171
    Mar 7, 2018
    Westminster, CO
    Here you go. Keep in mind there are no turret boards or strips in this amp, it's all done on the tube sockets. I annotated this with a mind to build one the same way.

    Gibsonette GA-8 Schematic with P2P pinout.png
     
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  19. Mongo Park

    Mongo Park Tele-Holic

    606
    Nov 28, 2010
    Winnipeg
    Wow, a dream come true, thank you very much. All of this is great it is like the PtoP turret board everything is all laid out for me ready to go. i just gots to press print.:)
    cheers Ron.
     
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