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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    In a prior thread and some related 'now what' questions, y'all helped me figure out a buddy's ?1955 Gibson GA-9; octal preamp, parallel single-ended 6V6s, field coil speaker, all original. A few 'before' pics:

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    With good advice here, we decided to do the minimum to make it safe and playable, no external work, etc.

    Other than new resistors to replace a 470 and a 220K that had drifted badly (both were floating over tube sockets -- the PTP here means almost everything is), most of the revisions can be glimpsed in the overhead. SJT power cord with strain relief. DPST hot/neutral switch. Stuffed new Nichicon 22uf/50V inside the old bypass caps. New filter caps with split grounds. And a lot of dust and debris removed.

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    The PTP wiring made revisions tricky, since the 3 non-recto sockets essentially do all the work of a board; many socket lugs have up to 6 wires attached under an overhead defensive perimeter of wires and floating components. The whole chassis is narrower than the board in a small tweed: 3". Most of the resistors were surprisingly intact in value. All voltages came in nicely. I still need to put a p-clamp on the power cord and tighten up the springs in the sockets, which may be the source of some occasional static on loud notes. But it sounds extremely sweet -- awesome tone, in fact.

    I can add a few details later, but first I'm gonna complete the last few steps and then let my buddy give it a test run. Thanks to all of you for your help on this.
     
  2. D'tar

    D'tar Friend of Leo's

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    Great job KF! That looks fantastic! Good for another 20 years!
     
  3. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Thanks, D'tar! I owe you for good ideas on this and probably every other project I've ever done. :)
     
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  4. muscmp

    muscmp Tele-Afflicted

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    wow,looks great.
    mine is a 58, completely different animal.

    ga9.jpg
     
  5. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Yeah, Gibson never stood still on a design for long. The early GA-9s like my friend's are barely described on the Gibson Garage page, and they don't seem to even mention the later ones. What are the differences? Did I read they changed to a 12ax7 preamp, dropped the field coil, added a tone knob? I guess the preamp change would alter the breakup threshold?

    That cab and handle are cool for sure.
     
  6. Mexitele Blues

    Mexitele Blues Tele-Meister

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    The OP's GA-9 with the changes you mention would make it functionally identical to my GA-8 Gibsonette. Not sure about the later GA-9s but you are correct about their longevity of designs.
     
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  7. strat a various

    strat a various Friend of Leo's

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    I'm still tryin' to figure out why Gibson jammed a GA-9 circuit into a BA-9 cabinet, in the case of my amp. People say, "Oh, they did stuff like that", but no one I know of has seen one before.
     
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  8. muscmp

    muscmp Tele-Afflicted

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    wallace marx has put out a gibson amp book that is pretty good. while it is not always correct nor inclusive(that may be partially gibson's fault), it did not include my version of the ga9 that has tweed covering, 10" non field coil speaker, 12ax7, 5y3 and two 6v6. the cabinet is definitely larger than the 53 br9 that i have and it replaced the br9 with the b9 student amp/lap steel set. this is my br9 amp and br9 lap steel.

    br9-set.jpg
     
  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Last things first, this afternoon I decided to look at that stove-in grille cloth. The speaker was easy to de-mount, and the baffle (¼" plywood) just as easy. Beyond the baffle, the cloth turned out to be sitting on a stiff wire mesh, and the wire mesh was bolted down by the baffle.

    IMG_2808.jpg

    It's hard to totally flatten wire mesh that's been stretched, but it was fairly easy to bow it outward rather than inward and then pop the mesh in and out until it was mostly even.

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    I vacuumed the front while it was dismounted, and though it's still torn and dirty, I think it looks better than in the 'before' condition seen in the initial pics.
     
  10. King Fan

    King Fan Friend of Leo's Ad Free Member

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    More detail? Why not a rare easy solution?

    I've often posted puzzles and problems here, and made at least my share of bonehead booboos. So I wasn't too sure what to try when initial fire-up showed some fairly loud crackly static that seemed to increase with louder notes. I'd cleaned the input jacks, and the field coil means there is no speaker jack. It persisted with tube swaps, and didn't vary with chopsticking all wires and solder joints. That's good -- getting a soldering iron to these crowded socket lugs varies from nerve-wracking to impossible.

    But after messing around with the amp, the static was gone for a while. It gradually came back while playing.

    What does it all mean? Comes and goes. Varies with vibration. What about tightening the 62-year-old socket springs? Looks like they need it... Tubes are much tighter to seat... and... the static is gone.

    Sometimes intermittent problems are the hardest to find and fix. But sometimes the intermittency (is that a word?) is a clue.
     
  11. tubedood

    tubedood Tele-Meister

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    I am new still to a lot of these older amplifiers.

    I am curious on how the "field coil" speaker sounds or responds different than say it's cousin of the era an Alnico? If I recall FC was a little before the move to permanent magnet speakers?
    I dig old school tech I am intrigued about this one!

    Dale
     
  12. King Fan

    King Fan Friend of Leo's Ad Free Member

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    I’m a field coil know-nothing myself. Good question — I don’t know if they saturate like alnico or not. I will say this one seems to have lost none of its vintage-Jensen mojo. The cone looks factory fresh, and the speaker sounds great from bass to treble. This circuit isn’t blackface scooped, exactly, but it is much less mid / bass heavy than my small tweeds, with a lot of tight bass and a whole lot of rich tenor to soprano. This is an amp where tele bridge pups can sparkle, sizzle, and sing.

    One thing for sure, the field coil means I won’t be testing this speaker plugged into my other amps.
     
    Last edited: Aug 3, 2019
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  13. King Fan

    King Fan Friend of Leo's Ad Free Member

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    A fun detail this time, but one where I still have a lot to learn. I decided to re-stuff the old cardboard bypass caps. Reasoning:
    1. Tiny new ones would look odd. (If you did use new, the longish Sprague 25uF / 50V caps like @corliss1 used would look best.)
    2. Short new ones would leave too much exposed lead to re-use the wonderful old brown spaghetti.
    3. It would be fun and I'd learn something.
    Credit: Most of what I learned about this topic came from antique radio sites. I especially liked this one (search down the page for 'Restuffing Paper Capacitors) on an amazing set of pages called "Phil's Old Radios."

    Step 1, cut the old leads flush with the end caps to save as much spaghetti as possible.

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    Step 2: Heat old caps in a hot oven (200-250°F x 20 min) to get the waxy goo to melt. Hold with silicone hot pad, push out contents (a huge Phillips screwdriver works well). While still hot, wipe down outside of tube with paper towel to clean.

    Step 3: Wrap new caps in something to snug 'em inside tube. Some folks use stiff paper; I used high-temp electrical tape.
    NOTE which end of the new cap is positive. in hindsight, I shoulda marked that lead with black marker, but strict attention to position lucked out this time.

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    Step 4: Find something to cap ends. Some caps have re-useable cardboard ends. In our junk drawer I found these small felt bumpers / glides from the hardware store that were just the right size.

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    Step 5: Punch a hole in the center of the end cap, slide into place.

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    Step 6: Center the new capacitor inside the old tube. Re-cap the ends with melted wax.

    This is the step where I struggled. Some folks re-use the melted wax from step 1, but here the stuff was almost resin-hard and barely melted at 250°F. It started to solidify while you handled it. Pouring or molding seemed right out, so I melted some beeswax (sold at the hardware store for its highly-useful role in lubricating threads on wood screws) colored with a bit of tan furniture crayon from the junk drawer (tan Crayola would work too).

    It'd be handy to have a small ceramic crucible like we had in chemistry class, or a small stout metal cup with a pouring spout -- something that would keep the wax melted and make it easy to pour. Pouring, done well, would give the smoothest results -- I only succeeded partially, and even then only on one of four ends.

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    So I had to hand-mold and finger-shape the wax, which doesn't give a uniform shape or smooth, shiny surface.

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    But I think the final results, from a polite distance, look OK. I especially like the re-used brown spaghetti. Spaghetti is a useful, even vital item in PTP where component leads take the place of wires and often crowd together. Wish I'd had some for that new resistor!

    IMG_2813.jpg
     
    Last edited: Aug 3, 2019
  14. Wally

    Wally Telefied Ad Free Member

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    Good job, King Fan. I am curious why you left that old bias resistor there? Perhaps there is nothing wrong with them when they look like that, but it is my practice to remove them while I am dealing with the bypass cap. Something in me makes me think that there is a reason why they put the material around that resistor, and when that material has fallen away to some fair extent, I don’t want to wonder about it again....so I replace them. Ommv.
     
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  15. tubelectron

    tubelectron Tele-Meister

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    Nice and Tidy job on this "Gibsonette" amp...

    Refurbishing the caps guts is a good idea - I never thought about this : well done !:cool::cool::cool:

    -tbln
     
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  16. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Excellent question. Yup, that old 7W 200ohm resistor looks both fuzzy and funky. But it tested at 204 ohms, and I figure if it's lasted this long, it might as well keep on keepin'. This isn't my amp, and my buddy may keep it or move it on, so I figured the more I could leave alone the better for 'vintage-ness.'

    Funky? Functional? You're right, there's some erosion of the coating. If it was my amp, and built on a board or tag strips, I would replace it. But floating a modern 10W 200ohm wirewound there wouldn't be much fun.
     
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  17. muscmp

    muscmp Tele-Afflicted

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    very well done. i don't think i'd go that far but i do understand that it is closer to original.

    play music!
     
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  18. Wally

    Wally Telefied Ad Free Member

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    Re: Field coil speakers....
    Filed coil speakers were used until permanent magnet speakers became ‘affordable’. ‘Affordable’ back then might shock most of us. I know I was taken back a bit when I read the catalog prices for say a Jensen P12N back IAST some point in the ‘50’s.....iirc, it was at least $325...not adjusted but a real number. Go ahead and do a cost inflation calculation on that!
    Anyway, my longest experience with a field coil was with a ‘52 5B5 Pro I restored. When it was all said and done, that was a great amp. It was sort of ‘as new’ operational condition as far as the reconed F15N speaker, new tweed...because the original covering was long gone, and but for the input resistors every electrolytic, tone and coupling caps because paper in oil does not stand up over all of these years, and resistor was replace...by necessity. Great big, full tone cleans at volume. Smooth break up when pushed. I loved that amp.
    I have played a few other field coils after repairs. I find good tones in those old amps in most cases. I just got to know that 5B5 very well....got familiar with it. Hot amp for the jump blues thing, Jazz, swing, rock and roll.....whatever. I liked it. Somewhere in the recesses of my tonal memory lives that sound....for surely I heard some of that on that old country I heard when I was very young...those tweeds were all over the early rock and roll and all of the country...damn, the years role by, don’t they.

    Okay, I have slept on things. Maybe that $325 was not correct. However, a few years ago I saw a pice list, and the price of one was astronomical...much higher than one would pay for say a Weber clone of a vintage age Jensen Alnico. So, don’t hold me to that number above.....perhaps in today’s money it would have been a $400-500 speaker?
     
    Last edited: Aug 4, 2019
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  19. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Wally, you're a poet, a scholar, and a gentleman. Thank you.
     
  20. Wally

    Wally Telefied Ad Free Member

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    Well, thank you.....two out of three isn’t bad. My parents tried to raise me up correctly, and I have had one poem published.
     
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