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Early Gibson GA-9 Voltages Inquiry

Discussion in 'Amp Tech Center' started by K Teacher, Jun 7, 2020.

  1. K Teacher

    K Teacher TDPRI Member

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    Greetings,

    I am interested in Single-Ended, paralleled wired tube guitar amps; particularly the early Gibson GA-9 model (circa 1953).

    I have been searching the internet for information on this amp model, but had very little success.
    Additionally, the schematic available on many sites is blurred and difficult to read.

    For these reasons, I redrew the schematic and I added notes from the information I pieced together from forums, YouTube videos, ect… and calculations using tube manuals datasheets (see attachments).


    The questions:

    • If you have access to an early Gibson GA-9 in good working condition or have one on the bench, could you please write down the voltage figures, so I can check to see if what I got is correct? Please, include mains and filament voltages.

    • Also, if is not too much to ask, could you measure the field coil speaker resistance and the output transformer winding ratio?

    • Please, include any other information about the GA-9 you think might be important; for example: which filament pin (2 or 7) is grounded, power resistors, speaker and/or transformers identification number, etc.


      Thanks in advance for your help.


      Attachments:


      VOLTAGES TABLE
    upload_2020-6-7_20-16-9.png




    REVISED GA-9 SCHEMATIC


    upload_2020-6-7_20-15-0.jpg
     
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  2. muscmp

    muscmp Tele-Afflicted

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    welcome!

    i have a ga9 but it is the later version. yours may be closer to the br9 that it replaced. i have a 53 br9 that has the same tube complement.
    gibson tube amp group on FB may be of help.

    https://www.facebook.com/groups/234008453370077/

    play music!
     
  3. K Teacher

    K Teacher TDPRI Member

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    Hello muscmp,

    Thanks for your response.
    The BR-9 circuit is very close to the earlier GA-9 amp… but it does not have the 6V6's cathode and plate key voltage values, as well as, the 5Y3 output voltage (B+).

    Probably, the power transformer is the same. Do you have the ability to measure those?

    Thanks again,
     
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  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Welcome! I don't have a GA-9 I can measure, but I restored a '55 for a buddy (details here) and IIRC it may be sorta like yours, a transition model from the BR-9. I did measure voltages at one point, and FWIW wrote these down.

    volts1.jpg

    As noted, that was with the original tubes. I recall I later put in a new 6SJ7, but not sure if it was for specific reasons or just to make sure we had a 'good' one.

    Your schematic looks better than the one I came up with, but in case it could help somehow, here's one I marked up to show the R and C values on the pic, some tube pinouts, and a few other notations.

    my GA9 schematic markup.png

    Let us know how you come along -- if my buddy's is any clue, these are great little amps.
     
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  5. K Teacher

    K Teacher TDPRI Member

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    Hi King Fan,

    Thanks for the detailed information you posted; it helps a lot.

    I studied the voltage figures you posted... The higher B+ values look correct and I believe, are result of 122VAC mains instead of 117VAC... not big deal.

    What I observed is that, if you divide delta voltage across the speaker field-coil (365-297 = 68 V), for the calculate total amp idle current ((0.9/1)+(16.7/0.204) = 82.3 mA), the field-coil resistance comes to approx. 830 ohms instead of 1000 ohm as indicated in the original schematic. Same happens using the values from the original schematic.

    I could not find any info on the Jensen F10-U... but I suspect that the schematic is using a per-production value and never got corrected...

    Why is this important?
    To estimate the new bias resistor and OT impedance in case you need to replace the field-coil speaker for a permanent magnet one... It does not feel good to me to install a 1 kohm-30W resistor, which is going to generate a lot of heat, inside of a crowded under chassis space...
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yeah, I knew nothing about field-coil speakers when I got in there, and knew only a little more (like why there's that inductor symbol on the scheme) afterward.

    There's also some interesting discussion on the web about how these are pretty polite amps given their 2x6V6 SE power. I didn't get into that much, either, altho somebody suggested the 500K volume pot could be replaced with a 1M? I didn't know enough to decide if that made sense -- and given it was my buddy's amp, I wasn't gonna mess with the original values -- and especially didn't want to blow the original speaker, which sounded very nice at age 65...
     
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  7. K Teacher

    K Teacher TDPRI Member

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    Hi King Fan,

    I never had the opportunity to work with a field-coil speaker myself...

    Regarding the volume pot, it is part of the 6SJ7 load... the optimum value for the next stage resistor (in this case the volume pot) is about 2x to 3x the previous stage plate resistor, which for this circuit is 220K... beyond that the gain increment is negligible...

    Additionally, there is a maximum recommended value for the 6V6 grid resistor (500K for Class-A, with cathode bias), which if exceed, could affect the tube bias... so 500K pot is fine for this circuit...

    Also, the 6SJ7 does not have enough gain (approx. 197 in this case) to drive the output tubes to full power... to do that you will need approx. gain of 800, which can be easily obtained using the 2 halves of a 6SL7 (dual triode) in a configuration similar to the one used on the Valco 510-11...

    However, I would not modify a classic amp circuit... I would use a pedal to get the extra gain needed instead... but that, of course, could be too much for the vintage speaker...
     
  8. K Teacher

    K Teacher TDPRI Member

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    BTW, do you happen to remember if the GA-9 field-coil speaker has smooth or ribbed cone?
    Thanks...
     
  9. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks for the insights on the circuit. As to the cone, this is the best I can do...

    D7550F0E-4CD0-450B-BC46-D2CDB24D262C.jpeg
     
  10. K Teacher

    K Teacher TDPRI Member

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    Hi King Fan,

    Thanks for the close-up pic. It looks like a smooth cone to me...
     
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