Gibson GA30-RVT Reverb Tank

[Jack W.]

We have a Gibson GA30-RVT Invader combo in with no reverb tank. The circuits have been restored but any tank tried howls or squeals. Turning reverb control off or footswitch stops the feedback. Swapped several tubes, etc..Tried most AB's EB's & suggested FB's. Does anyone know which is the correct replacement for this amp? (& yes , tank is physically isolated & bagged, as well as postioned).
Thanks.

[g1]

That schematic didn't work so I'll repost it.
V7,V8, and V3 are all in the reverb circuits, did you try swapping all of them?
How about the cathode cap C37 at V3B ?
The info I have says tank is 4FB2A1C

[Jack W.]

Thanks for the assist on the attachment! I ran across a page that suggested the 4FB tanks but had a (?) after it so I wasn't sure. Yup did the tube swapped with V7, V8 & V3. Replaced the bias components including C37 & other related caps. Wiring layout in chassis is still in original position as well. It just gives a better advantage in trouble shooting when you can be certain the main part is the correct one in the ckt. I've seen the RCA cables to the tank do odd things, so that's my next approach , (w/the 4FB2A1C installed). They can get real microphonic or act up in other ways. I'll post results of the successful solution. Again thanks for the tips.

[The Dude]

Are both send and return grounded? If memory serves, the return should not be?

[g1]

I ran across a page that suggested the 4FB tanks but had a (?) after it so I wasn't sure.

The info I have is not from the factory so it may also be questionable.
The ground scheme may be different like The Dude suggested. If you are using a mod tank, try adjusting the ground jumpers.

[Enzo]

The schematic g1 posted shows the reverb return as floating, but clearly the drawing is wrong. There is no complete circuit without grounding the return line. Note on the drive side, the transformer and pan input are grounded. See the connection dot midway down that ground lead? Now look at the output coil on the pan, at its bottom end is another connection dot that goes to nothing. On each of those dots, there is a tiny flake of drawing left where a line was drawn from one to the other. Thus both input and output are grounded. A horizontal line should join those two dots.

Now, if you wired your circuit up exactly like that drawing, you have all that input stuff on an unterminated grid - a giant antenna system!! I wouldn't doubt it would howl. Could that be your issue?

[Jack W.]

Enzo's correct. The schematic I tried to attach has the connection between those two nodes. On the amp I have, the reverb Xformer sec is not hard grounded. The return cable shield IS grounded to chassis thus grounding the grid input ckt to V3B AND the xformer secondary via the tank. I rechecked the RCA cables only to find the return plug still had corrosion that Deoxit didn't cleanup. Used a dremel to restore 100% contact. Seems to be working fine. I'm using a MOD 4FB3A1B tank & both RCA jacks are factory grounded. I couldn't find a 4FB2(?) & I had this one in the shop. It gets quite "cavernous" at a depth of 4+, but I can adjust that internally when I fix the tremolo.
Thanks for the guidance.