1950 Gibson GA20

[Randy Magee]

I couldn't resist posting a picture of this amp that was in for repair last week... it's been in a closet since 1957.

[Phil_S]

Your instinct is right on. No gut shots? It's a beauty.

[Randy Magee]

The guts looks terrible with that true and very confusing point to point wiring... another thing that I found very odd was that they put in the speaker, put the back of the cabinet on, then covered it with the leatherette looking stuff. You can't get the speaker out of this thing without cutting some material and disassembling one of the back panels. It sure wasn't made to be easily worked on...

[Phil_S]

After you pull the chassis, you should be able to remove the speaker. It might take some careful handling to get it tilted just right. I assume it is in good condition and it can stay put? You don't want to take that speaker out just to see if you can.

I expect the inside is not orderly in the way we think of a board-built amp, but it might still be a thing of beauty to those who can understand it for what it is.

[Structo]

I like how they used to put the output transformer on the speaker back in the day.

[bluesfendermanblues]

Which particular jensen model is that speaker? doesnt look like your average P12R or P12N

[Randy Magee]

Phil, even after the chassis is out, there is no way you can get the speaker out... in fact you have to take the speaker loose and move it a couple of inches down to get the chassis out. The speaker, a Jensen P12R, was still in good shape. I found some documentation to the effect that some of the earlier models had this problem with not being able to get the speaker out without cutting the leatherette and popping the board that covers the chassis out. Here is a kind of poor shot of the guts before anything was done to it...

[Phil_S]

OK, I understand, it's painted into a corner permanently. Somehow, with a Gibson, I'm not surprised. Thanks for the gut shot. It's fugly.

[wyatt]

After you pull the chassis, you should be able to remove the speaker. It might take some careful handling to get it tilted just right. I assume it is in good condition and it can stay put? You don't want to take that speaker out just to see if you can.

I expect the inside is not orderly in the way we think of a board-built amp, but it might still be a thing of beauty to those who can understand it for what it is.

Nope.

I had one of those 1950 models and he's right, they finished the cabinet after installation. These are the earliest GA-20's right as they transitioned from the bottom-mounted chassis-ed GA-6. And I believe they over-engineered it. I think it was changed early, probably before the end of 1950.

I fixed the electronics in mine and have to say...meh. It's really a big, ol' meh of an amp. Only the Mic channel has any real gain, it can overdrive pretty good, but the tone was always lifeless. Honestly, it's the plain and dull cousin to the GA-40, which is a much fuller-sounding amp.

[Randy Magee]

Wyatt,

The little booger makes a great slide amp with a humbucker... doesn't sound too bad with a P90 either...

[Phil_S]

I fixed the electronics in mine and have to say...meh. It's really a big, ol' meh of an amp. Only the Mic channel has any real gain, it can overdrive pretty good, but the tone was always lifeless. Honestly, it's the plain and dull cousin to the GA-40, which is a much fuller-sounding amp.

I've never been in the same room as an original, but I built a single channel clone of the circuit. I used a number of things I just had lying around, so it isn't totally faithful to the original. I suspect using different iron may be a key difference. I ended up with a different PT than what you see in the pictures here http://home.comcast.net/~psymonds/GA20.htm The OT was purchased on eBay and appears to be a pull from a Hammond organ chassis 8K:8.

I think the main deviation was in the treatment of the 6SJ7. I changed the dropping resister in the B+ ladder and I may have tweaked the voltage divider on the plate/screen supply (not sure about the divider). Initially, the plate voltage was under 100V and it didn't sound very good. I did what I could to raise the plate voltage. When I got it to 140V the whole amp livened up quite a bit. It is very Hendrix-like, no pedal needed, and I really like it a lot.