Gibson GA-9 closet find restoration

Discussion in 'Amp Central Station' started by King Fan, Aug 1, 2019.

  1. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Today's detail is... power cord replacement.

    The old power cord went through a grommet in a 3/8" hole and had an overhand knot tied inside the grommet by way of strain relief. One AC wire went straight to the fuse tip (good) and the other to the switch (not so good). I won't rehash proper power wiring here, but if you're interested, the faculty here gave me a graduate seminar in this thread.

    After my seminar, I decided to upgrade to EU / international safety standards and use a DPST to switch both hot and neutral. I've been curious about this, it is modern best practice, I see Rob now including it in his layouts, and I just wanted to try it. Plus it is easy, elegant, and practical to have a switch lug act as tie point for the white neutral wire.

    I got a 16ga SJT 3-prong cable because... sturdy. It is possible a slightly slimmer SVT cord would have worked as well, and been a bit easier to work with. But sturdy won.

    Here's the old switch with a bit of power cord still attached, and the 3/8" hole where the grommet had been. I'm loosening the PT bolts to drop the PT, which otherwise sat right in the path of drilling out the power cord hole.

    old.png

    Some folks suggest you can insert a 3-prong cord through a grommet, and maybe with SVT you can, but this cable, no way. Besides, a proper strain relief is better, and although I wasn't gonna make a double-D punch in this crowded corner, a standard Heyco relief fits in a round 5/8" hole -- and a P-clamp on the chassis prevents rotation if some jerk decides to twist your power cable....

    So let's drill. Even a good sharp step drill can seize in the work, so proper clamping is needed to prevent your chassis from becoming a propeller. A little blue tape tells you when to stop.

    IMG_2541.jpg

    Strain relief pliers are the tool most likely to reduce swearing in your shop. On this SJT, it still took a tiny bit of a squeeze, but no air was blued with profanity.

    IMG_2542.jpg

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    Before inserting the cable, I'd stripped off enough outer insulation to expose 3-4" of the green, white, and black wires inside. Stripping outer insulation without cutting the wires' individual insulation, and without cutting yourself, is not my favorite job, and I'm not gonna pretend I don't sort of reinvent the process every time I do it. If you're doing it for the first time, don't ask my advice; buy some extra long cable and practice with a few YouTube or DIY tutorials until you find a method you like.

    Next step: Safety ground (earth). Experts suggest the green wire be left longer than the black or white, so in the event the amp hangs from its power cord, the green will be the last to pull loose. Remember that jerk who was twisting your power cable?

    A swage-on ring terminal is actually industry standard, but I solder over the swage for grins, and because... shiny.

    IMG_2545.jpg

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    Finally, best practice (and UL code) says to anchor the safety ground to an anchor that's mechanically and electrically separate. In this tiny space, though, I was reluctant to drill more holes, and decided to use the fallback of a PT bolt. But in any event, two keps nuts with loctite on both -- and crank 'em down tight.

    IMG_2552.jpg
     
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  2. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Finally, the new (Carling) DPST switch. The hot + neutral switching works like this:

    upload_2019-8-4_10-11-28.png

    The PT primaries both attach to the lower poles of the switch. My heat shrink job on the hot-to-fuse-tip is a bit like that too-short t-shirt your wife wants you to throw away, but it prevents a casual finger making contact. Capping the whole fuse tip would be better, but the protruding lug didn't look easy to cap.

    IMG_2593.jpg

    And here's the whole sector:

    IMG_2803.jpg
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Nice. Here is how paranoid I am. I would have taped over that socket for the drilling process.
     
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  4. King Fan

    King Fan Friend of Leo's Ad Free Member

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    And you would have been *smart* to do so. Metal filings tend to create vagrant shorts that can be insanely hard to track down. Don't ask how I know.

    My second-best method was to deploy my shop vac with a small dust nozzle over everything in the neighborhood, but simple blue tape on the sockets would have been better.
     
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  5. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Last detail. I obviously wanted to replace the 1955 filter caps in the old C-D cardboard firecracker. The firecracker, big as it was, was too small to stuff with three F&T caps.

    IMG_2446.jpg

    The best approach I'd seen was this close-as-possible modern firecracker by @corliss1. Since no one makes a 20-10-10 combo nowadays, he'd still had to piggyback another cap on his. And I didn't want to be a copycat.

    ga9 restored.jpg

    But I liked about his that he was able to re-use the single hole the original firecracker clamp bolted into. I tried to map out tag strips, but none of the available topologies would fit 3 caps without several more holes in the chassis. And the space, that looks so empty in pics, is really quite small -- about 3 x 3." I've seen the caps laid on a diagonal to space them on shorter tag strips, but it didn't appeal to me.

    Filter tag strips.png

    I also wanted to split the caps' grounds if I could, and luckily the original firecracker had long-ish wires going off to the B+ connections, and to the original ground on V3, so I had some freedom as to orientation.

    In the end I decided just to mount the caps with RTV silicone. Very happy with the result -- really secure, no new holes, and a space-saving topology that used the original wiring but also allowed split grounds with just one added piece of black wire to the input jacks where the preamp is grounded.

    IMG_2648.jpg

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    Heh. If I post this many pics to show a simple mini-restoration, you've gotta be thankful I didn't build a Bassman. Next time!
     
    Last edited: Aug 5, 2019
  6. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Heh, didn't know anything about this, but cruising YouTube today I found Uncle Doug (of course) has a video showing at least one of the transitional amps between BR-9 and GA-9. Does your amp look anything like this, inside or out?

     
  7. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Muscmp, I see Uncle Doug's hybrid amp looks like the BR-9 in your post, at least on the outside. He finds a totally different (?earlier) cabinet that also belonged to the BR-9 label at some time.

    As we discussed above, Gibson was clearly guided by Emerson: Consistency is the hobgoblin of small minds.
     
  8. muscmp

    muscmp Tele-Afflicted

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    that looks exactly like my br9!

    br9.jpg
     
  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Cool, I thought so. Does yours have the transitional GA-9 chassis inside like his in the video?
     
  10. muscmp

    muscmp Tele-Afflicted

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    i have the 6sj7 version as uncle doug does. mine is from 1953. i think i'll contact him to see if he has a pdf of that schematic. i have a gibson service manual for most of the gibson and epiphone amps and the one for the br9 shows the 6sn7.

    my ga9 is a 58 and uses a 12ax7. the book shows a 6sj7 but i have the schematic for the 12ax7 version also. both amps have the 10" jensen speaker.

    here is the ga9. due to the 12ax7 it is a little more gutsy than the br9.

    ga9front.jpg
     
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  11. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Yeah, that’s totally consistent with their inconsistency.

    Have you had a chance to watch the video? He has some kind of manual and schematic for the amp. But yours is 1953? His is early 55.


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  12. muscmp

    muscmp Tele-Afflicted

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    yes, i watched it to make sure mine was one of the transition models. i watch all of his videos figuring i can learn something each time. i guess i could use screen grab to capture a pic of the schematic. it appears that gibson gave it at least two years before changing the amp.:lol: the wallace marx gibson amps book states that the br9 changed to ga9 in 54 but that may not be correct either.
     
    Last edited: Aug 13, 2019
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