Gibson GA-9 closet find

Discussion in 'Amp Central Station' started by King Fan, Jul 11, 2019.

  1. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yeah, the older stuff is moderately okay. But when you get up to GA-20? I think I have 5 different schematics for that...
     
  2. King Fan

    King Fan Friend of Leo's Ad Free Member

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    I like your modesty, BTW, 'pretty standard.' But throw me a line here. I'm guessing you used 25uF caps (Sprague?) to replace the paper 20s. From the form factor I'm guessing yours were 50V. Yes?
     
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    "Pretty standard" means "every amp is really the same as there are only so many ways to make a 12AX7 do the thing."

    I buy 25uF/50V usually by the 10 or 20 pack :D Spragues for those.

    Look at where it is in the circuit - it's a standard cathode bypass cap on V1 and the same for the bias on the 6v6. 25V should be plenty for either of those positions, but I have 50V on hand.

    The firecracker and F&T cover the power supply.

    Grounded cord, of course.

    Other than that, that's the only stuff you *have* to replace. The rest is just "does it work" - you might find noisy resistors or something that has drifted after you do the basic service, but that's expected with pretty much everything.

    The good news is they didn't use the wax caps everywhere like they did a couple years before, so it can still end up looking pretty original as those are unlikely to be leaky, but only takes about 5 seconds each to check with a meter if you run into problems.
     
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  4. strat a various

    strat a various Friend of Leo's

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    OK, so I have an amp i just had serviced, that is exactly this GA-9 circuit ... in a BA-9 cabinet, amp on the bottom, controls facing back. Wut?
     
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  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Welcome to Whose Gibson is it Anyway? The amps where the model numbers are made up and the schematics don't matter!
     
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  6. strat a various

    strat a various Friend of Leo's

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    upload_2019-7-11_12-31-27.jpeg

    Looks like this.
     
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  7. Wally

    Wally Telefied Ad Free Member

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    Nice looking amp. I have not seen a Gibson amp from the ‘50’s with a field coil speaker, but I have not seen everything. That Jensen is too clean not to have a serial number, ime.
    Interestingly, I just wrecked out a Jannsen piano that had a Lowery Organo system in it.....organ electronics with sensors under the piano key mechanism to trigger the organ functions....if so desired. One thing I pulled from it is a 1952 F10Q Jensen...a field coil speaker with the serial number 220238. What else????? Twelve killer 12AX7’s, 48 x 12 AU7’s, 2 x 6v6, 2X6sn7, 2x6SL7’s, and 2x6W4 rectifiers...... what a beast that one was....heavier than any organ I have moved.
     
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  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    .
     
    Last edited: Jul 11, 2019
  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Field coil speakers? I gotta get reading. I see T2 on the schematic and start to get a vibe, a feel, a field? for what might be going on. :D

    I did look up the 6sj7 pinout and then pasted it in a tidied up schematic since the inky squiggles were making my eyes vibrate...

    ga9 copy.png
     
  10. strat a various

    strat a various Friend of Leo's

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    So, am I right that they used electromagnets to make a loud speaker back in the day because the technology to cheaply build a very strong permanent magnet for a speaker didn't exist yet?
     
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  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Bleh, your image says "field coil" right on the speaker, so yup, there you go :D #ReadingIsHardSometimes
     
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  12. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    But it's important to note you can have the OT mounted to the speaker frame and have it NOT be field coil as well, as both versions sure do exist.
     
  13. King Fan

    King Fan Friend of Leo's Ad Free Member

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    OK, I admit the speaker label was a clue. :)

    On a different note, I got to looking at that grill cloth. I'd leave the 'tolex,' but the cloth ain't gonna make girls go wild.

    Do they make 'Gibson' imitation cloth? Other ideas? This isn't actually as 'salt and pepper' as the original, but it is acoustically transparent, and it comes in a *huge* variety of colors.

    cloth.jpg
     
    Last edited: Jul 11, 2019
  14. Wally

    Wally Telefied Ad Free Member

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    That T2, unless I am mistaken, is a choke. It in on the power supply line in between the first filter cap and the B1 power node.
    T1 is the output tranny, and T3 is the power transformer....in the schematic above.
    The indication of a field coil speaker are those coils at the speaker. An unmistakable indication that it is a field coil speaker are the power supply wires to the field coil that creates the magnetic force. Those can be seen in the third and fourth pics in the OP. A permanent magnet speaker with a speaker frame mounted output transformer would have only two wires coming from the chassis to the OT.
     
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  15. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Thanks, Wally. Yes, you're right, and I understand the power wires part; I noted that grundle of wires heading down to the speaker and realized I wasn't looking at any speaker leads I was used to.

    As far as T2, I was just looking at these labels:

    upload_2019-7-11_15-12-11.png
     
  16. Wally

    Wally Telefied Ad Free Member

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    King fan, I don’t know....I think that the schematic you have is drawn in an incomplete manner. Here is a BR9, which is an amp that doesn’t really move me personally. It shows connecti9ns between that 1000 ohm coil and the speaker. Sorry.....challenged I am...just a link.....

    https://el34world.com/charts/Schematics/files/Gibson/Gibson_br9.pdf

    It seems it serves two purposes. I seem to recall with at in replacing a field coil speaker with a permanent magnet, one has to consider that coil on the power supply rail....and accommodate the removal of it.
     
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  17. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    I also thought choke 2, but it's labeled as a speaker coil. I no longer have the amp here to confirm.
     
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  18. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Aha. Turns out we're all correct. It is a choke *and* a field coil. From wikipedia (see italics I add at the end):

    "A field coil loudspeaker is a dynamic loudspeaker in which the field is produced by an electromagnet rather than by a permanent magnet.

    An electrodynamic loudspeaker therefore has two coils:

    The first electrodynamic loudspeakers were produced in the 1920s, to address the problem that strong permanent magnets of the time were extremely heavy. A compromise was therefore necessary between loudspeaker efficiency, which required the strongest possible magnet, and weight. The use of a strong but relatively light electromagnet solved this problem.

    While now uncommon, field coil loudspeakers were once common in top quality console and tabletop radios and similar domestic audioapplications, particularly from the late 20s to the post-war period of World War II. In these appliances the field coil of the loudspeaker was also used as the main or only filter choke in the high tension power supply to the valve anodes."
     
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  19. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    "technically correct" is my favorite kind of correct. I'll take it.
     
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  20. Nickfl

    Nickfl Tele-Afflicted

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    Another indication is that is shows an additional coil in series with the speaker voice coil. That is a hum bucking coil that F.C. speakers needed, but as far as I know you never see those in P.M. speakers.
     
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