Gibson GA-20

[luis]

Iīve been offered one early 50īs to restore(itīs advertised as nasty condition).Never heard any so i have not any refference at all.
How this circuit compare against Fender Deluxe or the like made at the same time. What sound do I get from it? Any "famous" user on records or live?

Thanks in advance.

[Rick Erickson]

The GA20 is one of my favorite small amps. I own the version with (2) 6SJ7's and a 6SL7 driver. A Jensen P-12R speaker is a great match with this amp. The only thing I change from the published schematic is the preamp plate resistors. I use 220K plate R's while the schmatic calls for 470K. This change is one I found in several stock amps that I have rebuilt over the years, and I think it opens up the preamp a bit with the higher plate voltage. Ry Cooder said in an interview that he uses a GA20 for his slide tone. I believe he takes a preamp out signal into a larger amp. I do not. Personally I find these amps to have a very "woody" tone for lack of a better term. Good score!

RE

[Denny_A]

Ditto

Ditto on what Rick said. I have owned 2 of these cool amps, Now I just have 1 that I will not part with.

FWIW, both of the GA-20's I have owned had a factory stock 6SC7 in the phase inverter position and 2 6SJ7 preamp tubes.

[mant]

these rock.
i do alot of studio stuff and between this and the GA9
we cover lotso turf.
these are such sleepers still.

[hasserl]

I picked up a GA20 a couple of months ago with one 12AY7 and two 12AX7's in the preamp. The circuit is similar to a Deluxe, but not the same. It uses grid leak input, and a paraphrase PI instead of cathodyne. IMO it's not quite as gritty as a 5E3, it has a great clean tone, and it sounds great when set to an edge of breakup point and you use the guitar volume to go between clean and dirty.

[mant]

the models that have the 5879's in them are cool sounding as well.

[pdf64]

Hey Hasserl, that sounds similar to the one I've got - the chassis is puched for octal small signal sockets and so the 9 pin sockets are on little adaptor plates. Anyway, if you haven't already, double check that the coupling caps aren't leaky. Mine had those cute 'bumble bee' 0.047 types, which were putting dc onto the 6V6 grids. The HT/B+ is about 50v dc lower than a regular 5E3 which softens the overdrive, but if you sub in a 5V4 for the 5Y3, you can get most of that back, and with a GZ34 it can do the mini marshall grind thing. It's a fine amp that I'll never part with, excellent plating and screen printing on the chassis 50 years down the line - Peter

[hasserl]

I'm a little embarrassed to day I haven't even opened it up yet. It has a three prong cord installed and no hum so I assume its had the filter caps replaced. I haven't played it much, a couple of hours use is all, all in my shop, not out at a practice or jam or gig yet. But it sounds so good I haven't bothered with it except to enjoy it.

One thing about it though, somebody in the past painted the cab brown, painted right over the covering! Idiot. I've contacted Rob at Buffalo Amps about recovering it for me. I'm not a cabinet guy and I don't want to learn on something like this. Rob specializes in these amps and has very close tolex and grill cloth. So I'm going to find a box to ship it in and send it off to him for that. I picked it up for such a good price I can afford to put a few bucks into having it recovered and still come out way ahead.

[luis]

Thanks all for your answers.
I guees this can be my amp,regarding your comments on the subject.

Do you know where to get tolex and grille cloth for this Gibson amps?
Iīve been checking but I did not found any source or at least didnīt notice any supplier...

[soundmasterg]

Hasserl, your GA20 sounds like the GA-20T from '58 or '59? I just fixed a '59 for a local guy and modded my uncle's '58. The only difference between the two was that my uncle's is using an alnico magnet Jensen speaker, and the local's was using a ceramic magnet Jensen speaker. There is a grounding error stock on those that causes them to hum a lot more than they should. They have the CT for the PT grounding at the filaments on one of the power tubes instead of going to a chassis ground, and the way it is setup, there is also a ground loop that goes through the preamp too. If you redo the grounds correctly, then these amps become nice and quiet. Just thought I'd pass that on.

Greg

[TD_Madden]

duct tape will lift most common paints off that "tweed" (not same "tweed" as teh Fenders used....more like woven-plastic)...unfortunately, there will be SOME discoloration.....

[pdf64]

Hi soundmasterg, re the heater CT connection to the power tubes. That confused me at first, but the Technical Info Q&A at Aiken Amps provided an expalination:-

'Q: Why do some amps connect the center tap of the filament winding, or the junction of the two resistors off the filament string, to the cathodes of the output tubes?
A: In a cathode-biased amplifier, the cathode is at a positive voltage, somewhere around 10-40V with respect to ground. If you elevate the filament "reference" above the potential of the cathode by connecting the center tap to this point, you can effectively reduce the amount of hum coupled into the tube. This is because the filament is now positive with respect to the cathode, so the cathode doesn't attract electrons (i.e. hum) from the filament. This is a very inexpensive and easy method of reducing the hum in an amplifier without having to go to a DC filament supply. In a fixed bias amplifier, the output tube cathodes are usually at ground potential, so you have to add a voltage divider from the plate supply to generate the elevated filament reference. You can experiment with the voltage level to determine the value that best minimizes the hum. Be sure to bypass the junction of the resistors to ground with a suitable filter capacitor, or you may inject some buzz or noise into the amplifier from the power supply. '

So if you reduced hum by moving that ct to ground rather than the power tube cathode resistor, there's probably something else going on that you might want to investigate - maybe a bad bypass cap? Peter

[soundmasterg]

pdf64, it wasn't the CT for the heater, but was the CT for the power transformer secondary that was grounded incorrectly. I'd have to look at my hand drawn layout again as the amp is back with it's owner, as is my uncle's, but the red wire off the PT was grounded to one of the power tube sockets pins, I think it was one side of the filament string, and this was causing hum to be picked up by the grid-leak biased 12AY7 in the preamp because the preamp wasn't grounded correctly either. Moving the PT CT to the main chassis ground point, and re-routing the preamp ground to ground in the correct order reduced the hum level to just about zero. My uncle's was wired incorrectly too, but wasn't humming as bad as this other guy's. Rewiring my uncle's made a noticeable improvement in the overall hum level, and rewiring the other guy's fixed it's hum problem, so I thought I'd pass it on.

Greg

[hasserl]

Greg and TD, thanks for the tips. I'll check on both of them. BUt the amp had no hum issues at all, so I don't think I"ll find those grounding problems. Also, seems like the date code on teh speaker (Jensen alnico) was around 56 or so, I"ll have to take another look.

Got to try that duct tape trick too!

[soundmasterg]

My uncle's was a '58 and the other guy's was a '59 and the miswire was from the factory on both. My uncle's didn't hum much more than a Fender buth the other's guy's was humming a LOT, and the rewire fixed the hum issues to where it is one of the quietest amps I've ever heard now as far as the hum level. Given that yours sounds like it is a '56, then it may not be wired the same way but it is worth a check. Good luck!

Greg