Gibson GA30RVT No Reverb

[earache]

Hello

I've been looking at a Gibson GA30RVT Invader that has very weak reverb. I believe that the only reason I can hear reverb at all is because of the vibrations from the speaker are vibrating the tank. A shake of the combo creates a "crash" so I know the output side is OK.

The three tubes involved with the reverb are V3, V8 (both 6EU7's) and V7 (a 12AU7).

Both channels work, so the input tubes of both of those stages are at least operational. As a functional test, I swapped V1 for V8 - No Change.

Swapped in a different 12AU7 at V7 No Change.

Swapped V2 and V3 - No Change

There is a measurable resistance at both the input and output jacks of the reverb tank, so that is operational. It is an old Gibbs tank so is likely to be the original. The resistance at both jacks was around 175 ohms

Checked the reverb input cable -seems OK

Cleaned the tank's input jack and related RCA cale end - No Change

The reverb footswitch works and can shut off the "crash" that I get if I shake the amp

I can measure continuity across the reverb transformer's input side


Any advice on what to check next? The schematic is linked below:

ga-30rvt_invader.pdf

Thanks In Advance!

[Jazz P Bass]

Measure the Vac signal going Into the reverb transformer.

Now measure what is coming Out of it.

It should be roughly double the voltage.

[earache]

Would I need to input a signal to perform this measurement?

[Jazz P Bass]

Vac stands for Volts AC.
Which is your signal, yes.

[nevetslab]

I assume the DCR measurements you made of the Gibbs Reverb tank was with the RCA cables disconnected. 175 ohms at each end indicate it's high Z tank at both ends. With the transformer drive shown off the 12AU7 to drive the input to the tank, usually the tank is a low input impedance, typical of the Accutronics 4AB3C1B used in the Fender Twins & Deluxe Reverbs.

A simple test would be to use an RCA cable to bypass the reverb tank. As the only reverb control on this amp is on the Drive side of the tank, start with the Reverb control all the way down, and slowly turn it up with signal going thru the amp (Guitar will work if no test signal is available). Apart from some filtering in the reverb circuit, there will be a lot of gain from this circuit. This would be a way to verify both the input and output of the reverb circuit is working. You should also be able to use the RCA output that normally drives the tank as an output from the Ch 2 Preamp/Volume control (via Reverb circuit). If no scope or meter available, you could plug that into Ch 1. The Reverb volume would be the signal level control to feed Ch 1. All alternate ways of verifying you have valid signal drive to feed the input to the reverb tank.

Since you're getting Reverb crash when you shake the tank, you assume correctly that the return side of the circuit is working. I have had old reverb tanks give me typical DCR readings on input and output of the tank, yet not get any response from the tank.

Does this Gibson GA-30RVT have a lever to lock the tank down when traveling, to keep the reverb tanks' fragile spring s clamped down> You don't find that on current products, but was common in older products. If it does, make sure it's not engaged.

You could also try swapping input and output of the tank, just to be sure. Of course, you can also replace the Reverb tank.

[earache]

Nevetslab

I swapped in an old reverb tank and found the reverb signal to be much improved, so I ordered a new tank.

I was fooled by the fact that there was an old Gibbs tank in place, thinking it must have been the original.

Thanks for the help everyone!