Gibson GA-15 RVT

[hossymandias]

Phil_S, lol I thought I had gotten lucky. Don't go through the trouble of checking the turns ratio until I make sure it's necessary to replace it. You are generous enough with your knowledge and advice.

Pierre, thank you for the info, I will look at the circuit again this weekend. How did you go about repairing the transformer?

I have noticed that the coil or core (forgive my ignorance of correct terminology) of the transformer is loose and slides up and down on its mount. Is that a problem? As far as I can tell, the two tabs that attach to the RCA and the taps are intact.

Thanks everyone!

[Phil_S]

That's odd. The core should be tight in the window. I'm not quite sure if it matters. The middle tongue of the E shaped lams is in the middle of the core. It's not going to go anywhere. I might think about a little goop of some kind to make it stay put, but I'd be careful about what type of goop. I'm thinking it needs to be resin-like, something that dries hard, not silicon. In any case, I wouldn't fiddle with it until I isolated the problem.

I'll give some thought to how to trace the problem. Meanwhile it would be really good if one of the ace trouble shooters here at TAG would weigh in on this.

[Pierre]

If I remember correctly, the reverb transformer had a couple of spade type connectors which were loose and wobbly and tested open on a meter. I decided that I had nothing to lose, so I carefully cut the brown paper covering away until I found both ends of the windings. The connectors were detached from the windings so I did away with them and soldered a couple of leads to the windings (which are hair thin). I then hot glued the new leads to the transformer core and wrapped it up with brown paper. If you put some super glue on the brown paper, it gives it a little patina and the repair is almost invisible. And after all that, it worked beautifully! Sometimes you do get lucky!

[hossymandias]

Thanks for the details, Pierre. I'm thinking I'm at the nothing-to-lose point now, too.

I hooked up my record player (unamplified signal straight from the RCA out) to the input of the reverb tank, and with the amp on, turned up the volume and the reverb. All I got was an irregular, scratchy distorted sound. I couldn't quite tell if it was related to the sound coming from the record player, but it certainly wasn't reverberated Ike and Tina.

I measured the resistance between secondary tabs on the reverb driver and got 44 ohms. The resistance between primaries was 1k3. Please tell me that means my driver is okay. lol. Or does anyone have a better way to test my driver? I really don't want to have to replace it.

Honestly, I'm lost. I can't figure out what the problem is. Voltages in the amp check out. They're a little higher than indicated on the schematic, but I attribute that to the new caps.

[hossymandias]

I'm considering losing the transformer and running the reverb straight from the tube. What do you all think? I was reading here:

http://music-electronics-forum.com/t36664/

Leo Gnardo says: 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.[/url]

[Pierre]

I'm a complete amatueur, but those readings sound good on the transformer. You had reverb before you worked on the foot switch jack, so I think that's where your problem is. If your amp is the same as the one that I worked on, the first preamp tube should be a 6eu7. From pin 7 there is a .0047 coupling cap that then goes to a 470K resistor and then to pin 3 of a 12au7 which would be the third preamp tube in the lineup. The junction of the cap and resistor should have a wire that went to your foot switch jack. Check that wire and see whether or not the footswitch switches it to ground or not. I think that the footswitch shunts the signal to ground to kill the reverb. Also, test your rca cables.

[Phil_S]

Can we clarify word usage please? By "driver" do you mean the reverb tank, sometimes called a reverb pan, the thing with the springs inside? I not sure if you mean the reverb transformer or the tank when you say "driver" though your ohm readings certainly suggest the tank. I'd say the tank is OK by the ohm readings, though I'm not exactly sure what they should be. My amp is in a closet that isn't easy to access and it would be some effort to get the pan/tank and measure mine. I'll do it if you need it done. Please let me know.

If you want a transformerless reverb, just follow the "other" GA15-RVT schematic where it shows cap input. If you need that let me know and I'll post it.

[hossymandias]

Phil_S, when I wrote reverb driver I meant the transformer, not the tank. The tank measures 180 ohms across the input and 162-170 ohms across the output.

Pierre, I'm fairly certain the footswitch wiring was okay. I tested to make sure that it shorted pins 2-3 to ground and kill reverb. Now, however I have just temporarily disconnected the footswitch to simulate the condition I got it in. Something else has changed in the amp.

I measured the secondary tabs on the reverb transformer again and got 180 ohms across them. The primary has 1k3.

[hossymandias]

Also, if I went the transformerless route, what wattage should the added resistor and capacitor be?

Thanks!

[Phil_S]

Phil_S, when I wrote reverb driver I meant the transformer, not the tank. The tank measures 180 ohms across the input and 162-170 ohms across the output.
<snip for brevity>
I measured the secondary tabs on the reverb transformer again and got 180 ohms across them. The primary has 1k3.

The primary is wired blue and red? That's what I have. Is the part number the same as mine (posted earlier)? The secondary is a pair of push-on lugs.

I don't see how the primary can be 1,300 &#937;. It is just too many ohms for a transformer that size. The DCR is a function of the length of the wire used to make the coil and the wire size. There just isn't enough room in the "window" for that much wire. It is far from what mine measures, which is 24&#937;. Your secondary is not so far from the 98&#937; that I measure on mine.

It is possible you have a totally different setup from mine, with a different transformer and a different tank. Still, 1K3&#937; primary still seems wrong. I'm suspecting a problem with it. Assuming similar size wire to the one I've got (and I don't know what it is because I haven't opened it up!) you'd need about 50X the wire that mine has. So, I think there must be another explanation that says your primary is blown.

[hossymandias]

Okay, sorry about the long delay. I gave my workbench a makeover and it turned into a huge project. I'm ready to solve this reverb thing finally.

So I'm going to accept that my reverb transformer primary is blown and rather than get a replacement I'm going to try to bypass it and go with the transformerless reverb option.

The schematic that matches my amp is here:

http://ampgarage.com/forum/files/ga_15_rvt_1965_492.pdf

The one with transformerless reverb is here:
http://www.sophtamps.ca/images/ga15rvt/ga15rvt1.jpg

What exactly do I need to modify? I'm assuming I need to remove the 1k from pin 8 of the 12AU7, add the 3.3k and 10k, attach the 220k, and then run the 10uf cap from the 3.3k into the reverb tank.

Do I need to change anything else?

Thanks!

[Phil_S]

Look carefully. The driver tube changes from plate driven to cathode driven. When you remove the reverb transformer, pin 6 (plate) will need to reconnect at the 20uf cap on the upstream side of the 4.7K dropping resistor (either R32 or R24 depending on which schematic.) Connect the plate to C17B/C12B. Remove the 1K on the cathode (R18). Go to the other schematic and add 3.3K+10K (R22, R23). Attach the 10uf cap to the junction of the 3.3K and 10K. + side goes towards the tube, - side goes to the reverb tank. The 220K is already present as the grid reference. Iift it from ground and attach it to the same point as the + side of the 10uf cap.

Good luck with this.

My bench needs a makeover, too. When you're finished, please come on over.

[hossymandias]

Woohoo! Success!

I bypassed the transformer like you said (and it's a good thing you replied because I would have forgotten to reconnect pin 6) and there is tons of spacey, weird reverb that actually doesn't sound ridiculous dialed back to 4-5.

Also, the footswitch now works as intended.

Now I just need to replace the hardwired switch with a new plug and this will be a decent amp!

I think I owe you a brand new work bench, Phil_S and Pierre both for your patience and help. Please excuse me, it's time to go surfing in outer space.

[Phil_S]

Hey! That's great news!