Gibson GA15RVT reverb transformer

[g1]

I was thinking of the actual lugs on the speaker itself, but it may be easier at the spkr jacks, depending on the layout. Whichever way you find easier.

[cminor9]

I was thinking of the actual lugs on the speaker itself, but it may be easier at the spkr jacks, depending on the layout. Whichever way you find easier.

Haha, yeah, I just used the wrong word. I don't English real well today apparently Thanks

[cminor9]

well, I ran the reverb off the speaker. The amp still worked, but when the reverb was turned on it would emit a high pitched squeal. Not super loud, and you could still hear the guitar over it, but it was very noticeable and high pitched.

I wonder if this reverb tank is actually bad. I hear the springs crash if I jar the amp with the reverb turned on, but does that necessarily mean the tank is all good?

[g1]

Did you try reversing the connections at one end to see if it would affect the squeal?

[Resonator Guy]

I found if I messed up the reverb leads on the speaker it wouldn't work at all. And I sold the amp along with a GA-5 Medallion Skylark Friday. I now have room for more.

[cminor9]

Did you try reversing the connections at one end to see if it would affect the squeal?

Yeah, I did that. I think I am going to have to trace this circuit and just see what is happening upstream. I will post back what I find. Thanks as always for your help thus far!

[cminor9]

I now have room for more.

Lies! There's always room for more.

[cminor9]

I know this is a bit of a zombie thread, but I hate to leave these things hanging around without a resolution. It has been a busy few months for me. I tried several iterations of circuits recommended by others here, no success.

So I traced the circuit all the way through and compared it with the schematic. It was, indeed, a bit different from the schematic. So I made it match the schematic with a few changes (interestingly, many of the components were the same values, just rearranged a bit). Still nothing. It's the craziest thing, though. No matter how I slice it I get no signal coming out of the cathode of V3B, which then goes through a resistor into the pan. Tried several tubes. Getting voltage to the tube. Just nothing from it.

As much as I never like to concede defeat, there's only so long I am gonna spend re-arranging a circuit. I don't even know if the pan is any good anyway.

My friend has been patient, and I want to get the amp back to him. I told him I could just build a Fender reverb circuit with a transformer and a new pan. We'll see what he decides.

[The Dude]

Do you have signal on the 2nd grid of V3?

[lowell]

I have the same amp on my bench, do you guys have the correct schematic? i don't see one posted in this thread. Mine has a reverb transformer AND a choke. Neither of which are in the only schematic I can find online. It's a Gibson Explorer GA15RVT... or so it says.

[Resonator Guy]

Here is the one from the Gibson Service Manual from way back in the thread.IMG.pdf

[Commodore 64]

I know this is a necro-thread. But I feel like this is important, in that I have the exact same amp as Resonator Guy, but it's an Epiphone EA-32RVT! Mine isn't working right now though. And when it did, it wasn't very compelling, which is why I opened it up to put a Hammond 1750PA OT in, and maybe make some mods.

I have 3 schematics: EA32 RVT, GA15RVT, GA20RVT. My amp says EA-32RVT, Epiphone Comet, made in Kalamazoo, Michigan on the back. I've traced pins from tubes, etc and I have identified parts of EA32, GA15, and GA20 circuits.

1. It has a reverb transformer, like the GA20RVT. Some of the preamp tubes are wired like the GA20RVT, and this is consistent with Resonator Guy's amp. The reverb transformer only has 2 lugs: one is to the 12AU7 plate, the other is to B+ node. This connection would appear to provide voltage to the 12AU7 plate, and instead of a plate resistor, the transformer is wired in series. (WTF?)

2. It has a choke, like the GA20RVT Schematic.

3. Again like Resonator Guy's amp, the reverb pan is SPEAKER Driven, as anecdotally reported by the owner of that amp. Speaker driven reverb is not on any of the 3 schematics I have (EA32RVT, GA20RVT, GA15RVT). The output of the reverb pan has 2 wires, which do directly to the speaker lugs. SO the speaker is connected to both the OT and the reverb pan. (WTF?)

-notice my emphasis on another amp that is not mine (In fact, that one is a GIBSON). But it is wired the same way, it even has the wonky cap coupling the input jacks to the grid of V1. This to me suggests that this was wired this way on purpose, at the factory. But there is no schematic that I can locate. My theory is that this is some F'd up amp that started life as GA20RVT. Then Epi/Gibson was using up some parts (a la Fender) to sell some inventory.

4. Yes, work has been done to the amp before I got in there. I believe it is limited to replacing the bypass cap on the power tubes (cathode bias), and the electrolytic filter caps, and installing a proper 3 prong power cord.

At this juncture, it seems that I would be best served by overhauling it quite bit. Perhaps a circuit that keeps the transformer based PI, and tube complement. Maybe even uses the same knob controls. But with a proper transformer driven reverb?

[Enzo]

I have 3 schematics: EA32 RVT, GA15RVT, GA20RVT. My amp says EA-32RVT, Epiphone Comet, made in Kalamazoo, Michigan on the back. I've traced pins from tubes, etc and I have identified parts of EA32, GA15, and GA20 circuits.

This is extremely common in Gibson/Epiphone amps. I have one model I have SIX different schematics for. pretty much if the lineup of controls was right, it didn't much matter to them what circuit they put behind the panel. Roll wwith the punches.

The reverb transformer only has 2 lugs: one is to the 12AU7 plate, the other is to B+ node. This connection would appear to provide voltage to the 12AU7 plate, and instead of a plate resistor, the transformer is wired in series. (WTF?)

Unless I misunderstand you, this is how single ended amps are wired. Look at the output stage of a Champ for example. Or one of a zillion Fender reverb drives. The transformer impedance is the tube load, no resistor. But later you describe it as speaker driven, which means this may not be a reverb transformer. Only two wires sounds more like a B+ choke to me.

2. Again like Resonator Guy's amp, the reverb pan is SPEAKER Driven, as anecdotally reported by the owner of that amp. Speaker driven reverb is not on any of the 3 schematics I have (EA32RVT, GA20RVT, GA15RVT). The output of the reverb pan has 2 wires, which do directly to the speaker lugs. SO the speaker is connected to both the OT and the reverb pan. (WTF?)

I am just going to write this off as a clumsy sentence. If reverb is speaker driven, then there is no need for a drive transformer. Speaker driven means the signal at the speaker is ALSO used to drive the reverb pan. And that means the speaker is connected to the INPUT of the reverb pan, not the input. So the amp OT drives both speaker and pan. The OUTPUT of the pan is sent to the return circuit of the amp.

My theory is that this is some F'd up amp that started life as GA20RVT. Then Epi/Gibson was using up some parts (a la Fender) to sell some inventory.

Nah, no conspiracy theories. They did this all the time. It wasn't a couple guys in a basement creating new circuits to use up parts amp by amp. EPi and Gibson amps were from the same lines, no mystery or secrets. So GAxxx and EAyyy can be the same thing underneath the skin. These are not one-offs or modified amps. it is just an amp they made, they made them that way for some time, then moved on. Like Chevy made the 1989 IMpala, then they changed it for 1990, and again for 1991, etc.

But with a proper transformer driven reverb?

There is nothing improper about speaker driven reverb. It saves a tube and some circuitry. Hammond did it that way for years.

[Helmholtz]

1. It has a reverb transformer, like the GA20RVT. Some of the preamp tubes are wired like the GA20RVT, and this is consistent with Resonator Guy's amp. The reverb transformer only has 2 lugs: one is to the 12AU7 plate, the other is to B+ node. This connection would appear to provide voltage to the 12AU7 plate, and instead of a plate resistor, the transformer is wired in series. (WTF?)

If the reverb "transformer" only has 2 lugs it is no transformer but a choke/inductor. This reminds me of a reverb driver I found in a Hammond L100 organ amplifier.
The plate resistor of the reverb driver tube is replaced by a choke and the reverb input transducer is connected to the plate(s) via a large value (50µ) ecap. This has a couple of advantages:
- the inductor load about doubles available output voltage swing (like with a conventional reverb transformer)
- output impedance and voltage gain increase with frequency resulting in something like constant current drive. This in turn equalizes the pan's frequency response.

The coupling cap has to be large for sufficient bass drive as the reverb transducer's impedance is (very) low at low frequencies.

Of course it wouldn't make sense to have reverb driver but drive the reverb from the speakers.

[Commodore 64]

I, too, thought it was a choke!. But the Trafo says E6400 on it. It IS a gibson reverb transformer (GA20RVT schematic references a E6400 reverb transformer which led me to this thread in the first place!), but there's nothing hooked to the other 2 lugs. ON my am p the lugs are gone. On Resonator Guy's they are just unused. The secondary has no connection.


I'm self taught, with little electrical theory knowledge, just enough ohms law to calculate bias and spec resistors. I don't see the connection from the reverb pan to the speaker on any of the schematics (attached). So I assume this was an alternative way to run the 'verb? I hope I'm not coming across as arrogant or stupid, I'm just struggling to understand what I'm seeing on the schematic(s) and what is actually in the amp...and then figuring out how my amp should be wired.

Enzo and Helmholtz, I too am confused by the reverb trafo. Maybe it is functioning as a choke like Helmholtz described? If that's the case, it could drive the reverb pan via a large coupling cap (50uF?). But then why is pan connected to speaker?

Enzo, thanks for point by point reply. I believe I've mispoken several times. If the speaker is hooked to the output of the pan, then the reverb is NOT speaker driven. Is there a reason why the output of the pan would go directly to the speaker?