1966 Gibson GA20-RVT Reverb

[fester gomez]

howdy all, Have a '66 Gibson GA20-RVT EL84's and the reverb is crazy, she needs a taming, if any of tall have an idea please let me know. When I got the unit she had zero reverb, replaced the faulty transformer, available pretty much only from Mercury. The reverb works well just sometimes it takes off, if I have the tank outside of cabinet no problem. I have tried all the padding and isolation, just think it may need taming on the return side. thanks in advance Fes

[g1]

If you have no problem when the reverb is out of the cab, then it is not an electronic problem, rather a physical one. Adjusting the circuit would be masking the underlying problem. Does the tank mount at the bottom of the cab, or near the chassis?
Sometimes these issues are caused by proximity to the speaker magnets. Are the speakers stock? Can you move the tank around in the chassis and find a location where it does not oscillate?

[Chuck H]

Listen to the g-one. It's an acoustic problem. And as far as I know there aren't any replacement tanks. I've acoustically buffered a couple of snarky reverbs. It feels like a losing battle. Bubble wrap can help. A combination of bubble wrap and some other cloth padding has worked best for me. Don't screw the pan down tight to the wood. Let it sort of float. And, as mentioned, try different locations such as are available. And good friggin' luck.

[g1]

And as far as I know there aren't any replacement tanks.

In case this may help others with Gibson reverb issues, I'm attaching a list of Gibson tank replacements I found somewhere on the web. Not to be taken as gospel, especially ones marked with ?, but may prove helpful.
Also a pdf with explanation of the codes and impedance & resistance readings on page 9 of pdf.
In the case of the tank in the GA-20RVT, it should read around 200 ohms at input and around 215 ohms at the output of the tank.

[Chuck H]

Great!!! The thing about vintage reverb tanks is...

They're all eventually going to be microphonic. The damping material used in the transfer tubes gets brittle with age and is no longer made. That, I'm told, is one of the factors in the closure of the Accutronics company and sale of the Accutronics name. They just couldn't make tanks anymore. And as anyone in the know will tell you, new reverb tanks are a crap shoot at best and just crap at worst. It's an ongoing issue. With new digital reverbs being used in many (if not most) manufactured amps there's a diminishing availability of quality reverb pans. I can't say how much this mysterious (and possibly toxic?) damping material plays into it, but I do know that buying a reverb pan has become a white knuckle affair.