Gibson GA15RVT reverb transformer

[nashvillebill]

FWIW, I have the Epiphone EA-15RV (schematic matches the BA-15RV and is also similar to the GA-30RV) which drives the reverb tank similar to the way you drew up earlier, I believe.

The attachment is the schematic glued onto my amp, it's pretty rough looking but at least it was there!

[Resonator Guy]

And here is the schematic for a GA15 RVT from the Gibson service book.
IMG.pdf

[52 Bill]

I have a whopping 270VDC on the plate after that 1.5k resistor on the B+ rail which seems high (need to check the schematic to see if the voltage is there) but maybe that was correct. The transformer primary was only 1.4K and now I have a 1.5k resistor there so unless the transformer was magically dropping more voltage that part of the circuit should be equivalent.

I think you are confusing transformer impedance with the resistance of the primary winding, they are not the same thing.

Adding any resistance to the circuit between the coupling cap and the tank input will only reduce the signal. Follow Leo's circuit and adjust the cathode resistor to get the tube voltages in a safer area.

[cminor9]

Cm9, I was at the cottage all day yesterday. I have added some more photos and can't find any leads to the speaker driven reverb tank at all from any transformer outside the chassis. This was the strangest reverb I have ever worked on and the amp didn't need anything but the leads put on correctly to make it function, so I am not intimately familiar with it. I last opened it up about 5 years ago. The smallest right hand transformer has two terminals on it but they have never been used, to my eye. I have put the chassis back in but if I can assist let me know. I did measure across those two terminals and got, with the meter set at 2K ohms, a reading of .439.
[ATTACH=CONFIG]29101[/ATTACH] [ATTACH=CONFIG]29102[/ATTACH] [ATTACH=CONFIG]29103[/ATTACH]

Thank you! Yes, this is a strange amp. The schematic shows a roach type trem but there no roach in there.

So wait, the reverb works but there was nothing connected to that transformer? So weird!

[cminor9]

Man, you all have left me a wealth of info! Thank you so much!

I am going to try the ampeg way again. If that doesn't work for me I'll try the way on the schematic resonator guy left.

Gibson must have really changed this amp up over the years. My specimen matches no schematic I have seen, and everyone seems to have some slightly different. Strange indeed.

Either way, I'll post result of my experiments. I cannot say thank you enough for all the ideas thus far, folks!

[Resonator Guy]

Yep, the amp works perfectly. The reverb and trem will never match a Fender, but they do have a nice vibe. I have never before or since seen a reverb that is speaker driven. When I got the amp it was all unhooked, but once I got it set up right, it works dandy.

[cminor9]

Tried the ampeg method again, nothing.

There is no AC signal coming out of the plate on v3b (a 12au7). Shouldn't there be? I mean, that's what we are trying to let pass through with the cap, right?

I wonder if this tube is bad. Don't know if I have a spare...

Edit: known good tube, no AC signal. Hm. I may have something else going on here. I have no assurance that the reverb even worked before I got the amp. Hadn't been used for a while.

[cminor9]

Yep, the amp works perfectly. The reverb and trem will never match a Fender, but they do have a nice vibe. I have never before or since seen a reverb that is speaker driven. When I got the amp it was all unhooked, but once I got it set up right, it works dandy.

Yeah, it also lacks the fat bottom a fender has, but it's got a bright, chimey thing going which I kinda dig. Might sit really well in the mix for recording.

[Leo_Gnardo]

There is no AC signal coming out of the plate on v3b (a 12au7). Shouldn't there be? I mean, that's what we are trying to let pass through with the cap, right?

Kee-rect.

Is there any signal at the grid? If memory serves, in the '15 the send to reverb drive tube is independent of the volume control setting (goofy yes I know but the "idea guys" must have thought some players wanted an all-reverb amp. go figure.)

Something for everyone to think about, Gibson never built any amps. How about that. The suits thought up what they wanted and jobbed them out to a number of shops. Where the local techs made modifications of their own. Explains why there are so many instances where the amps don't match the schematics.

Afer you confirm signal at the grid, and if you don't trust the 12AU7 that's in there, & have no spares you could slug a 12AT7 in for a quick check.

[cminor9]

Kee-rect.

Is there any signal at the grid? If memory serves, in the '15 the send to reverb drive tube is independent of the volume control setting (goofy yes I know but the "idea guys" must have thought some players wanted an all-reverb amp. go figure.)

Something for everyone to think about, Gibson never built any amps. How about that. The suits thought up what they wanted and jobbed them out to a number of shops. Where the local techs made modifications of their own. Explains why there are so many instances where the amps don't match the schematics.

Afer you confirm signal at the grid, and if you don't trust the 12AU7 that's in there, & have no spares you could slug a 12AT7 in for a quick check.

My Hammond has a 12AU7 that I know works. So I put it in. Same thing, no reverb.

I tried the reverb circuit the way it on a schematic posted here. I made a picture from the schematic resonator guy attached. It is closer to this amp that any schematic I have seen:

That's now two known or at least assumed good circuits that have not worked (ampeg, and the one from the schematic).

When I get back to it later, I'll first check any signal at the grid. If that doesn't work, I'll trace the circuit back a bit farther and see if anything odd turns up.

Either way this is a good learning experience. I am gonna get reverb working on this amp.

[g1]

This was the strangest reverb I have ever worked on and the amp didn't need anything but the leads put on correctly to make it function, so I am not intimately familiar with it. I last opened it up about 5 years ago. The smallest right hand transformer has two terminals on it but they have never been used, to my eye.

I'm fairly certain the reverb would have come from the factory driven by that transformer rather than the speaker. Perhaps that transformer was replaced and never connected. Or perhaps you just can't see any marks on the lugs anymore. I don't believe there are schematics for this model with speaker driven reverb, also I think if you connected the tank to the reverb transformer (rather than the speaker) it would work properly.
Sorry to go a little off topic, just can't see any reason they would load up the amp with unused tube and transformer. (the transformer is connected to a tube, yes?)

[Resonator Guy]

I looked back a couple of years and can't find much reference to this issue, but everything looks factory and it works pretty well. I didn't even have to change the caps on this, I discovered. Those lugs look absolutely unused and if some one did "mod" it they did a great job. Two very factory looking leads from OT to speaker and two factory looking leads from speaker to reverb tank. These leads are tipped with those slide on receptacles. I wonder what would happen if I slid them on to the transformer lugs, hence no soldering and more ease of breakage like cm9 's workbench item.

[Resonator Guy]

g-one, thanks for that suggestion. I had never noticed those lugs prior to this thread, and the leads do look just the same as speaker hookups. I put them on the transformer and the reverb was, well, transformed. It is still reserved and polite, like a Canadian, but a good bit louder and thicker. I won't say who that reminds me of. So cm9, now that I know a little better, if I offer some assistance, I will try.

[g1]

Cm9: In light of the above, why not try hooking up the tank input directly to the speaker. Then try reversing the connections at one end to see if the polarity makes a difference. You may get better results one way compared to the other, or you may get squealing one way and not the other.

[cminor9]

Haven't had time to mess it with it the last couple days but I will try hooking it up to the speaker. Good idea! Just right there on the speaker jacks, huh?