Gibson GA20 VRT.

[Mixfish]

Ok. Thanks for letting peek into the garage.
I don't know much. I buy old amps, that I love.
I play them for a week or 2 and they "smoke".
I try to fix.
Then they go on the shelf.
I have a garage full of old tube gear form scopes to radios.
So, I purchased a AD3 and want to learn enough not to be so dangerous.

Screenshot 2025-04-11 011606.png

https://el34world.com/charts/Schematics ... _20rvt.pdf
I have replaced 10 caps and have high ripple.
I am trying to get the reverb working, it had no tank and did not seem to have ever had one and I used a newer Mashall that makes a lot of spring noise but no guitar reverb.
The tremolo is very weak but oscillates.
One tube V4 is unmarked but seems to be a 6EU7.

About to test some tubes but some were kinda new.
The amp is working but seems weak and lacking bass.

Any advice on any aspect of this would be appreciated.

Fish

[Phil_S]

Fish,
It's your lucky day. I own one of these. I worked on it extensively when I first got it. It has been in a closet for at least 10 years, so memory is all I've got on it. There are at least three known versions of this amp. There are two commonly found schematics. I have a few things that may help. My amp has what I'll call schematic #2 glued to the cab. Last I had it out of the closet, digital cameras weren't great and I couldn't get a decent hi res picture. So, I manually made a copy. (Can't guarantee accuracy, but I believe it is a competent copy.) I'm attaching my layout and schematic and hope these will help.

I do not know what type of reverb tank this amp uses. It is a lot of trouble to a clear what's in front of where it is stored and with other things in life also getting in the way, it would be a while until I can get to it. Sorry I can't help with this.

Ripple is only related to the filter caps. From my schematic and what I remember, there is one 3-section filter cap. Replacing that should cure the ripple. If you had leaky coupling caps, that, too should help, but you need to get the filter caps done. It's my recollection that I used a JJ multi cap as a replacement.

The tremolo uses a "roach" that is no longer available. I've looked extensively for a replacement. If the tube is weak, replace it. If the roach isn't working well, then you'll need to make your own from a bulb, a light-dependent resistor, and heat shrink tubing. Sorry, I don't know the specs.

The attached indicates what tubes are in my amp. V4 in my amp is a 12AU7. Unfortunately, V4 on the schematic you posted says it's a 6EU7. Look at the tube socket wiring. The heater connections will tell you which tube belongs there. Tube wiring diagrams are easy to find on the internet if you don't have them.

I'll point out that the only difference between a 6EU7 and a 12AX7 is the pin-out. If you can't find a decent 6EU7, you might consider rewiring for a 12AX7. Some will say this is a crime, but I'd disagree. Also, some will say the two tubes are not fully equivalent and on this, I disagree; just look at RC-30 for the tube specs.

The amp lacks bass. Yes, it does. There are a few ways to attempt to reshape the tone. I'd start by changing the cathode bypass caps on V1 from 5uf to 20uf.

There is also an excessive "ice pick" character which I believe is caused by the notch filter both channels. This may contribute to what you identify as lacking bass but is really over emphasized treble. On the Normal channel, I think this is either C4 or C6 and on the Reverb channel either C10 or C12. I think C4 and C10 are the ones to target. (References to my schematic.) Add a jumper wire across the cap to test the result.

[Mixfish]

Thanks Phil.
I will study them closely.
I need all the help I can get.
Awesome.

[Mixfish]

So far great.
I replaced bigger caps and it came to life. and the 3 in your schem,38, 6 and 15.
I have ripple at new diodes of 7VAC.
But pulling tubes drops it.
Trying to replace a 6eu7 I don't seem to have.
Anyway, thanks for the pictures.

[maxkracht]

Kindof a longshot, but I worked on one of these years ago where there was a broken wire on the reverb transformer. The RCA cable was connected via spade connectors, if i remember correctly, and there wasn't much holding those lugs to the transformer. Easy to snap the wire in the transformer, also fairly easy to remove the paper and reconnect the wire. Checking the resistance of the secondary of that transformer would tell you if it is still connected. Spring noise means the return is working, so your problem is before the tank.

[Mixfish]

Thanks Max
I will check it out.

[Mixfish]

I was wondering if I should just try and convert one socket to a 12au7? And skip the 6EU7.

[maxkracht]

12ax7 is a closer match. If you don't have or want to buy 6eu7s it's a good idea.

[Phil_S]

12ax7 is a closer match. If you don't have or want to buy 6eu7s it's a good idea.

12AX7 is a perfect match for 6EU7. Same tube, different pin out. Some will think this is heretical mod. As Max says, it's a good idea.

[Mixfish]

I ordered a socket to convert for now and put the 6EU7 in later if I find one. Socket 1/2 the price of a tube.


Thanks

[Phil_S]

Why not just rewire the socket that's already in the chassis? Something wrong with it?

[maxkracht]

Why not just rewire the socket that's already in the chassis? Something wrong with it?

I believe he means keeping the amp stock and using a 6eu7 to 12ax7 inline adapter

[Phil_S]

Why not just rewire the socket that's already in the chassis? Something wrong with it?

I believe he means keeping the amp stock and using a 6eu7 to 12ax7 inline adapter

IMHO, that juice isn't worth the squeeze. Can't decide for others. Whatever floats his boat.

[Mixfish]

Thinking to rewire all 3 sockets.
But with a closet full of tubes, I feel I will find a 6eu7 eventually.
And trying to get this amp straightened out with a few problems and no rev tank.
Now , traded a reel to reel for more tubes!

Thanks.