Restoring 1948 Gibson GA-30 guitar amp(tube)


I'm looking for some advice in restoring a vintage Gibson GA-30 Tube amp, here's the schematic

GA-30 Schema

And some info on the amp,


I was thinking at the minimum I would like to recap it, and replace the tubes. But also I would like to add a grounded(3 prong) power cord as opposed to the 2 prong one it has now, which gave me quite the shock the other day after powering it up:) Flipped it around and it worked like a charm;) But i'd like to avoid that in the future if possible...

I've built some Gainclones, p2p as well as kits, and some aleph 30 class-a amps, but this will be my first tube project, do you think it's do-able for a relative newbie? Also, is there anything else I should be doing while I'm mucking around in there? Seeing as the cab is in pretty rough shape, I was thinking of keeping the speakers(jensen Alnico 5's, which are in great shape suprisingly) and then possibly removing the pre-amp section and building a mini-stack so to speak, so I can see the tubes..

Thanks in advance!
If it were mine, I would replace what is bad (electrolytics and probably tubes), definitely replace the power cord, but otherwise leave well enough alone for the collector value. As it is, it is a piece of history that is worth more than the sum of its parts.

You can get nice quality, inexpensive, alnico guitar speakers from Weber if you want to build a clone or another amp in another cabinet.

Also, you don't want to see the tubes in a guitar amp. They get moved and abused too much, and you'll just break them. Be content that you'll be able to hear them. You'll get a few people that claim that solid state and tube amps sound the same for hifi, but nobody will make this claim of a guitar amp.
You might have a shorted power transformer - or an AC wire contacting the chassis... that's the first thing I'd look at. The schematic doesn't show a capacitor connecting AC line to chassis, but they were commonly used, and will get leaky, causing the shock hazard you noticed. For testing this, I use a capacitor tester which can measure insulation resistance into the thousands of megohms with 500V applied... an ohmmeter MIGHT find the problem, but it's not a real safety test.

If you're going to USE this, you must make it SAFE - originality comes second. Or you can cut the cord off and display it...


2007-05-30 4:12 pm
This is a collectors item so keep the cab without trying to fix it. Just change the electrolytics and check the nonpolarized caps for DC-leakage. Also buy a new set of tubes if they are worn out. And off course you should replace the powercord.

Then play it!!!!!

I have the sister model GA-20, brown from -54. It plays great with both guitar and harp. Unfortunately mine lacked the original Jensen but I replaced it with a similar Weber.
I would like to keep it the way it is, but the cab is completely trashed:( The ply on the left side is broken, no handle(which is common), the leather is very frayed and worn, and a nice slash through the front cloth covering the speakers.. Anybody know where I can get the materials necessary to fix some of these things? Fixing the cab would be easy enough, it's the leather and grill that i'm worried about.

On another note, does anyone know if the Gibson logo on the front is put on with a brandishing iron and a stencil? Or is it some other method? Seems like that would be to hot, but from the pictures i've seen it looks burned on, not printed.

Also, you don't want to see the tubes in a guitar amp. They get moved and abused too much, and you'll just break them

I was thinking something more along the lines of the Hughes and Kettner amps, the heads are built like any other, only they have a clear piece of acrylic(I think) covering the face. I'll take everyones advice though and leave well enough alone, and build another amp like the Hughes after I get a bit of experience messing about with these things;)

hughes and kettner

It'll be nice to get this up and running, i'll finally have a decent amp to go with my Rickenbacker for the vintage stuff. Despite the condition of it, it still sounds great for sitting in someone's garage for 50 some years;)