Gibson GA15RVT reverb transformer

[cminor9]

Got a Gibson GA15RVT amp on my bench. It wasn't working, I got it working now and sounding good, except for the reverb. The reverb transformer's secondary has to leads that go into the chassis and onto the board. The secondary, though, is two tabs right there on the coil itself, and then two leads go out to the pan. One of these tabs is messed up and when I jiggle it the reverb seems to cut in. I tried to solder some leads to the transformer to salvage it, but no dice. So I guess I need a new one.

I cannot reference this transformer anywhere, so I guess I have to find a substitute. I assume I can find a transformer I can sub in, but I don't know how to figure out what to use.

The reverb transformer has "E6400" written on it. The primary impedance is 1.4k and the reverb pan input's impedance measures 172 ohms. I cannot measure the transformer's secondary impedance, since the leads are messed up.

This thread seems to indicate that a standard fender transformer would work: http://music-electronics-forum.com/t27697/ But how do I know that will work? How on earth do I figure out what to use here? What do I measure here to figure it out?

Thanks folks!

[g1]

Any chance one of the wires is broken near the tab, and wiggling the tab is making the connection? Is it an open type where you could unwind a turn? Also, sometimes you have to sand off the enamel to get the solder to take.

[cminor9]

yeah, I tried burning the enamel off with a match but I guess I could try sandpaper. That's a good idea, so thanks!

I tore the tape and cardboard off, but the lead is coming from the center of the coil. So unwinding it doesn't appear that it'll be an option here.

I'd so much rather salvage this thing. Would make life easier for sure!

[cminor9]

The leads coming out of the coil snapped and I just can't get to them. Tried to salvage it though. So I need a new one. How do I know what kind of transformer to use?

[Leo_Gnardo]

That suggestion about using a Fender type reverb drive transformer is good, IF you also change the reverb pan to a low impedance input type. Unlike any other reverb transformer I've ever seen, the ones in GA15RVT's have a very high output impedance. I've measured the DC resistance at @ 450 ohms so imagine what the impedance is = higher yet. Fender's secondary typically measures about an ohm and is happy driving the nominal 8 ohm input reverb pans. (Which also measure about 1 ohm DC R.)

Hi impedance reverb drive transformers are rare - can't say I've ever seen one on the market. So I'd say swap out the transformer AND tank. Store the current tank for use on another repair. Ampeg 60's amps use those hi impedance input tanks. And sand-driven reverbs on modern amps like the Fender Hot Rod series & similar.

Alternate solution: you could modify the Gibson drive circuit to work like the old Ampegs. Swap a 10W resistor of 5K to 10K ohms for the transformer primary. Pass the signal off the plate of the drive tube, block the DC with a sturdy film cap 400VDC or higher, say 0.1 to 0.47 uF, and that will likely drive the tank you have just fine.

[g1]

Agree with Leo's suggestion about the cap coupled reverb circuit without transformer. Looks like Gibson even had a version of GA15RVT like that:
http://www.sophtamps.ca/images/ga15rvt/ga15rvt1.jpg

[cminor9]

I am doing this for a friend. He has already laid out for tubes, filter caps, new cord, a little turretboard for the filter caps, and now a reverb transformer. If I can save him the cash and make the old pan work I'd like to.

So I'll try that suggestion, plus I think I have the stuff I need laying around. So, let me see if I have this right:

This is a snippet of the schematic, and I have verified that this matches the actual circuit in the amp:



So from the plate of v3b, I run a big 10W 5-10Kohm resistor and a .1uf 400V cap, like so:



So what is this doing? Is it increasing the input impedance therefore increasing the output impedance to match the reverb pan?

[Leo_Gnardo]

I am doing this for a friend. He has already laid out for tubes, filter caps, new cord, a little turretboard for the filter caps, and now a reverb transformer. If I can save him the cash and make the old pan work I'd like to.

So I'll try that suggestion, plus I think I have the stuff I need laying around. So, let me see if I have this right:

So what is this doing? Is it increasing the input impedance therefore increasing the output impedance to match the reverb pan?

It's an "either - or" not both. The Ampeg method has no transformer to drive the reverb pan, however you must use a hi-impedance input pan (typically measures @ 200R on the input, you have one from the Gibson.) Admittedly it's not a terribly good impedance match, OTOH it doesn't take a whole lot of signal to shake the springs in the reverb pan.

You're kinda stuck - can either change the transformer AND tank (keeping in mind should the tank need replacement eventually you'll be able to find one with proper input impedance), or use the Ampeg setup with the existing tank.

There's a little safety alert on the Ampeg setup. If the capacitor isn't attached to the tank, it will develop a charge which can give you a nip of a shock. I keep getting reminded the hard way... Plus it's a good idea to make sure that cap is discharged before attaching the wiring to the tank. If it discharges thru the tank's drive coil, could fuse the skinny wire in there.

[Resonator Guy]

cm9, it's kind of early in the morning for me. If you can carefully explain what you are trying to measure and where, I will take some readings from mine.

[cminor9]

Resonator guy, thanks! All I need is the secondary impedance for the reverb transformer. The reverb transformer is under the chassis and has two leads coming off it that go to the reverb tank. I'd like to know the impedance across those two tabs. Be careful handling them. On the one I am working on they were wiggly, which is bad news.

EDIT: if it's not too late and it is not too much trouble, I wonder what the voltage is across those two leads when the amp is on.

Thank you!

[cminor9]

It's an "either - or" not both. The Ampeg method has no transformer to drive the reverb pan, however you must use a hi-impedance input pan (typically measures @ 200R on the input, you have one from the Gibson.) Admittedly it's not a terribly good impedance match, OTOH it doesn't take a whole lot of signal to shake the springs in the reverb pan.

You're kinda stuck - can either change the transformer AND tank (keeping in mind should the tank need replacement eventually you'll be able to find one with proper input impedance), or use the Ampeg setup with the existing tank.

There's a little safety alert on the Ampeg setup. If the capacitor isn't attached to the tank, it will develop a charge which can give you a nip of a shock. I keep getting reminded the hard way... Plus it's a good idea to make sure that cap is discharged before attaching the wiring to the tank. If it discharges thru the tank's drive coil, could fuse the skinny wire in there.

Alright Leo, I think I got you now. The picture helped I think I can do that with what I have laying around, and if he ever messes up the pan we can just replace with a more fender-y setup.

So one more question: in your picture the resistor kind of takes the place of the transformer (since the primary has one end connected to the B+ and the other end connected to the plate). The need for the big power wound resistor indicates I need to drop a lot of voltage here. What voltage am I aiming for to send to the pan?

[cminor9]

So I went downstairs and soldered away.

From the B+ where the xformer was hooked up, I now have a 10k power resistor feeding the plate. From the plate I have a .1uF orange drop cap going to the tip of a jack. The ring of the jack is of course grounded. This jack now feeds the reverb pan.

Turn the amp on, and it works. But no reverb regardless of the footswitch and/or reverb dial. Everything else works as it always did.

So here is what I am wondering: the old transformer had a primary impedance of 1.4K and fed the plate in the exact same manner as the resistor is now. The old transformer provided isolation as well as prevented DC from passing through, which the cap should be taking care of. (is all of this right? Please correct me where I am wrong).

So it seems like I could do this: put a 1.2k or 1.5k resistor (since I don't have a 1.4k laying around) in place of the 10k to feed the plate. From the plate, I'd want a cap to block DC (like the xformer did). Then a resistor to drop some voltage (like the xformer did), and I assume I want to drop a good amount. So it seems like this is where the 10K resistor should go.

What do you think about that? Looking back at your post, it seems like that's actually what you are describing. The drawing seems a little different though. I followed the drawing.

[cminor9]

update:



I made the changes above. No reverb. But now I get spring noise if I jar the amp so that's progress I guess.

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.

But after the .1 cap, I have 0V AC (or DC as expected). Nothing. This is regardless of the reverb footswitch setting, the reverb dial, and whether or not there is signal coming through.

I would expect *something* coming through the cap, right? In my mind the resistor/cap combo should approximate the transformer: it steps voltage down and removed DC. I have the cap before the resistor, but that shouldn't matter, right?

The old reverb pan had both input leads connected to both secondaries. Now I have one connected to what should be the equivalent and the other to ground. That actually seems wrong to me. It seems like I should have this



To approximate what the transformer was doing before. The only question is what the impedance should be for the resistor in red. Should this match the impedance of the reverb pan? 200 ohm?

Please bear with my questions. I am trying to learn and understand. Thanks again!

[Leo_Gnardo]

Mmm, no that ain't gonna work. No resistor in series with the tank drive connection. And put your 10K 6W resistor in the plate feed.

The transformer was 1) blocking DC. 2) gearing AC voltage down but not much. and 3) output winding was "floating" that is not connected to ground or anything else.

Don't worry about matching impedances on the drive side. As I mentioned it's far from an ideal match but if you can get a signal of say 1/4 watt to drive the tank that would be plenty. It worked for Ampeg, it'll work for you.

Getting some clang from shaking the springs - OK good at least that end works.

With the transformer primary on the plate of the drive tube the voltage was even higher no doubt. It will drop some when you put your 10K R there but no worry.

I added a line on my schemo to make sure the tank's ground connection is brought to chassis ground, no longer floating.

[Resonator Guy]

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.