Gibson GA-40 layout, anybody?

straight8413

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5
Does anybody have a layout for the 1957 model of the Gibson GA-40 amp? I have 2 schematics but have never seen an actual layout...the original used both sides of an eyelet board, if I remember correctly. I had a '57 GA-40 back in the late 80's which I wish I still had today so I think I might build a clone if possible. Sweet sounding amp for cleans with a pretty cool vibrato.
 

straight8413

Member
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5
Thank you...looks like the author of this is substituting a 6DR4 tube for the original 6SQ7 in the tremolo circuit.
 

bullet6759

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2,860
Does anybody have a layout for the 1957 model of the Gibson GA-40 amp? I have 2 schematics but have never seen an actual layout...the original used both sides of an eyelet board, if I remember correctly. I had a '57 GA-40 back in the late 80's which I wish I still had today so I think I might build a clone if possible. Sweet sounding amp for cleans with a pretty cool vibrato.
when cranked or pushed The dirt is killer.
 
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4,784
Straight8413, this thread made me pull the back off of my circa 1955 GA-40....two tone covering...Les Paul amp. I had pulled it out of 'the l in e up' a while back to give it a look because it wasn't what I thought it should be. Hmmm.....one of the original Cornell Dublier caps has a broken lead....that might be a problem, eh? Anyway, yes those boards are loaded on both sides. So, the layout would have to be a double page affair, right? I have never had the board out of the amp to take a pic of the underside.
Fwiw, when you said 1957 GA-40, I was wondering IF your old amp was even similar to mine....but the tube compliment must be the same because you mention the 6SQ7.

Oh...and welcome.
 

straight8413

Member
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5
I used to crank mine all the way and run a OD pedal in front of it for a little boost...pure tube sickness. I quit a band and left it at our practice space until their next show, and when I met them at the show they had just left it outside in the rain. I brought it that night to another practice space where it was promptly stolen.

Steppin' Wolfe I would just have the entire board re-capped...the resistors are probably Allen Bradley's and they tend to hang in there but I'm betting the cap values have drifted over the decades, even if they aren't bulging or looking hinky. I'm gonna have to look at this layout vs. a schematic because a couple of things look kind of off to me
 
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straight, when I replaced that broken cap the other day, I did a quick in circuit test of some resistors....they were within spec. I had a '52 Pro in which I had to replace every resistor on the board because they had all drifted up in resistance to a great degree. I had replaced all of the PIO tone and coupling caps as a matter of course when I did the e-caps...but the amp was very dull. After replacing the resistors, it came to life.
I agree that pulling the board and replacing the old tone and coupling caps in this amp might do it a world of good. I have considered doing just that...but...lots of projects, right?
 

HotBluePlates

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9,406
T...looks like the author of this is substituting a 6DR4 tube for the original 6SQ7 in the tremolo circuit.
The 6DR4 is a 7-pin tube that's straight-up 1/2 of a 12AX7. The 6SQ7 is an octal that's roughly like 1/2 of a 12AX7, but also has the two diodes.

Oddly, some places want silly money IMO for a 6DR4. Sticking with 7-pin, you could just use a 6AV6 and ignore its two diodes. Same basic thing, but you can find NOS all day for $3-5.

I myself built a clone amp using the 6AV6 instead of the original circuit's 12AX7, as the original circuit had a 2nd channel that I didn't need and that was purposely hobbled in the original amp. I skipped the 2nd channel & the 2nd 12AX7 triode.
 
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I am not familiar with the 6D4 tube. MY research shows that it is indeed a single triode, 7-pin tube. It purposes are different from a 12AX7 tube's designed intent....relay, sawtooth oscillator or RF noise generation being the target uses for the 6D4.
 

dkevin

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Messages
91
Found one drawn by Jon Currier in 2006...posted on this forum I believe...It shows the dual sided board and the components mounted on each side. I googled Gibson GA-40 component layout and went to images.
 
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This one???



NOw.....trick question. He has these two layout labelled 'back side' and 'front side'. does back mean to the front of the amp and front mean to the back of the amp??? LOL...... I see what is what. It just occurred to me that those labels could be a bit misleading IF a person didn't grasp what he was intending. Would 'top' and 'Back' sides make more sense? 'Visible' and 'hidden'???
 

dkevin

Member
Messages
91
It helps if you have an amp/example to compare the layout to..until you see one, you can't imagine how in the #$@%^^% someone could visualize and execute a layout like this...and expect that it could be reasonably repaired. If is wasn't for the unique tone of this amp, I doubt anyone would want to replicate it or repair it for that matter.
 
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It is nice how he 'folded' the thing. IF one sits and looks at this drawing, one can 'see' the circuit. working on it is another thing, right?? LOL....
One might think that his is the fellow who did it......
https://www.linkedin.com/in/jon-currier-25777112
https://www.mylife.com/jon-currier/jon_currier

IF one took the time, one might see a way to mount everything on one side of a board. It looks as if the tone and coupling caps are mounted on the 'other' side. Folding that backside over on top of the top side places things so that one could assess whether or not that could be done.

thanks for the heads up, Dkevin.
 




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