Gibson GA20 restoration.

[luis]

Hello all.

I finally got the amp we talked at http://music-electronics-forum.com/s...highlight=ga20

I plugged a stratocaster into the amp and got sound but very tiny,I guess no more than 2 w at full volume. My goal is getting it to full power so I would ask for your help. I´m posting some amp pics as actually is,you´ll see all caps have benn changed,some to Illinois yellow.....Also.. original Jensen P12R speaker has a tear on the upper side of the cone and transformer as well as chassis are rusty....

Here they are....










Please let me know.....

[Sweetfinger]

try a different speaker.

[luis]

So your guess speaker hole causing loss in power.Will try a cab I have 1x12",must I disconect actual Jensen speaker?

I´m uploading GA20 schematic above to show how it should be....

[MWJB]

Could be the hole in the cone, but also the cone may have simply become detatched from the voice coil. Jensens of that age have a habit of just falling apart.

Yes, disconnect the Jensen from the 2 wires that attach it to the output transformer that is bolted to the speaker frame, extend those wires to the new/test speaker. Be aware that the 3 primary wires from the amp's chassis to the output transformer are carrying the highest voltages in the amp.

If changing the speaker makes no difference then a DC voltage check (by a suitably qualified experienced person) at the tubes is a good idea.

Out of interest, with the amp unplugged from the wall measure dc resistance from each 6V6 pin 3, to pin 8 of the rectifier.

The amp is very rusty, this is generally not a good sign. Rust means the amp has been exposed to moisture and they don't like that.

[luis]

I have tried the speaker with another amp and I can say this is a beautiful sounding one (have to fix the hole however) and not causing power loss,so I must look elsewhere...
Would any preamp metal tubes causing this?
I´m thinking the trasformer would not be OK,but ther e are no signs of burning.....How would I be able to test the original trafo?

[MWJB]

"Would any preamp metal tubes causing this?" Maybe? It would be a good idea to try new tubes (whether metal or glass). If the problem is the same on both channels it is likely to be later in the circuit than V1 & V2.

A quick & dirty test is to measure the DC resistance from pin 3 of each 6V6 socket to pin 8 of the rectifier (this is the dc resistance of each end of the OT primary, to the centre tap). Maybe expect to see 150-300ohms per side? The actual figure is not that important (unless it's "0" or open load), you just want to be sure that there is not a mismatch of more than 15%.

To properly test the OT you need an AC voltage source (variac/signal generator) and 2 voltmeters. Obvious signs of burning are a concern, but many OTs fail without any visible sign.

I would eliminate your tubes as a possible problem first, then check dc voltages at the tube sockets.

[52 Bill]

MWJB is right, check or change the tubes. Then check the power supply voltages through the entire amp.

Also, if you didn't do the recap job, double check the work that was done to see if it was done correctly. Heck, double check it even if you did do the recap.

That amp has seen a lot of moisture. Every single metal part has to be suspect of corrosion and oxidation. It may even be a corroded wire somewhere in the signal path that is causing your problem.

[Casey4s]

I rebuilt a GA-15 Circa 1957, several years ago that looked almost identiucal to that one on the inside (maybe not as nuch rust). It belongs to a life long friend and he inherited it from his Gand Dad.

I started testing the components and virtually every carbon comp was way, way out of spec. and had to be replaced. The PT and the speaker mounted OT were both good and the Jensen speaker looked good but had a bad VC rub. All the electrolytics were shot too.

Long story short, I wanted to try to save as much of the original amp as possible so I put it in a new cabinet, trimmed out most of the wiring harness in one piece, includng the pots, switches, and jacks and put all of the original stuff in storage. I just rewired and replaced just about everything in the amp. It sounds great and my friend is happy his Grand Dad was impressed and that is all that really counts I guess.

I would suggest that you CLEAN as much of that rust out of that chassis as you can and make sure all of your grounds are solid. Then read every one of the old resistors with a meter and replace as needed. There is a good possability that you may need to do almost as much replacement as I had to do to get it functioning at an acceptable level.

Best of luck to you, and let us (at least me) know how this turns out for you.


Here are some photos of the GA-15, they are in reverse order so go to the last one and scroll backwards.

http://s238.photobucket.com/albums/f...&paginator=top

[luis]

Thank you all for your answers, have been great for help.

I do knew this was going not to be easy to fix it, but I liked to try and own a "vintage" Gibson amp and I guess I´m going to learn a lot with this amp.

I was thinking about cleaning the rust on chasis inside and outside. Not so sure if it´s OK cleaning speaker back with anything but a lightly damp cloth.

Which product can I use?

Can anyone put here or send me inside pics from his GA20 to have refferences?

[luis]

Just got 2 6SJ7 (cristal late models) used and swaped original metal ones. No sound this time,just a loud hummmm no matter which input I plugged the guitar...Maybe a microphonic pair?¿

[MWJB]

That would be bad luck, if so the vendor will replace if that is the case.

You're not going to make much headway in this project if you don't start measuring things and eliminating obvious causes of problems. So refit the metal 6SJs and get someone suitably experienced to note down voltages from the tube sockets. What DCR readings did you get at the OT?

[Rick Erickson]

I've had similar experiences with 6SJ7GT (glass bottle) tubes. The plain old 6SJ7 is my preference in a GA20. Not sure what the problem is with the GT's but in my GA20 they hummed like you describe. I have had very good luck with the 6SC7GT's in Tweed Deluxe amps. I much prefer glass bottles in those amps.

RE

[luis]

Good news.

I put metal tubes back again.
I also replaced 6V6GTs for a NOS matched pair after talking with a knowldge friend of mine and...Bingo! Now there is power enough and all channels working. This amp seems to like neck humbucker better than bridge.

However...Sound is not good enough still.Amp is farting when plugged a Les Paul and there seems to lack some musicality and pleaseant warm sound.
I own a Mission 5e3 amp wich sounds really beautifull and everybody loves it when try it.Young guys not educated on this kind of amps get very susprised whan listen to it...

Will proceed measuring voltages in a few days when have a while....

Regarding cosmetics, I´m trying to find a source for faux binding with no sucess.The original one is falling off.....