Gibson GA-15 RVT

[hossymandias]

At the encouragement of Phil_S I thought I would post some work I've done on a Gibson Whiteface GA-15 RVT. I have had both success and frustration so maybe this will help others and others will help me.

When I bought the amp it turned on, passed signal, and produced crazy reverb (really why I bought it). It also smelled REALLY strongly, a lot like my dad's '57 Chevy Nomad. Is that mildew or wood rot or what? Anyway, the amp was in really good physical condition.

It sounded like ice pick city though, which I found is generally the consensus. Thankfully, many people on this site have posted solutions to this so I began my modifications using their info.

I replaced both filter caps, the undocumented input capacitor, and did the notch filter bright fix found HERE.

I fired her up, and wasn't very impressed. My modifications didn't seem to improve things as much as I thought they would. Well, onto the footswitch.

When I bought the amp, the reverb worked, but no tremolo. I figured it probably needed the footswitch to activate the trem. Unfortunately, the previous owner only had a 4-pin footswitch whereas the amp wants a 5-pin. Instead of guessing at the right part on Mouser I decided to just solder the conductors directly into the amp. Sure, I won't be able to the remove the footswitch...but why would I?

I drilled out the center of the footswitch socket so I could run the four conductors up through and solder them to the tabs on the inside of the chassis. When I tested everything...lo and behold the tremolo kicked on! It sounds pretty good, too!

But...no reverb! At first I thought I had the polarity switched on the reverb transformer (the tank is fed by a transformer with two tabs. A wire with two crimps on one side connects to the tabs and on the other side has an RCA jack that feeds into the input of the reverb tank). Switching the polarity of the transformer crimps didn't work. I checked the reverb potentiometer and it works fine. I checked all the resistor values inside and they check out. I put the tubes in my tester and they seem okay. Also, the reverb comes through VERY faintly if I play at times, but the reverb pot doesn't change the level of the signal. I think this was due to the vibration of the speaker, though, because when I tap the tray, I get a nice, loud reverb.

Any advice?

[Stevem]

Go back to the way the wiring was before you got the tremolo working and you will likely find that now the foot switch for the verb has a constant short across it!

[Phil_S]

I read somewhere, someone's guess this amp must have been designed with jazz players in mind as it doesn't easily overdrive and our idea of good tone isn't what you get from this amp. I continue to believe, with enough minor mods you can get this amp to produce tone you like.

Have another look at the footswtich wiring. You may have miswired it when you removed the 5-pin connector. I'm thinking the problem is that reverb simply isn't turning on.

The diagram says there are 3 wires from the f/s to the connector plus a shield on the reverb wire, which is grounded at pin 2. Pin 4 is the common ground that is used to turn both reverb and tremolo on and off. The reverb wire should couple to C15 which is connected to the reverb recovery tube.

Pin 4 is grounded to the chassis and common to both reverb and tremolo. The "on" position of the reverb switch lifts the ground. (I'm not all that comfortable with switch wiring. I seem to have a mental block switches.) If I read the schematic correctly, I think you should remove the hard wired connection for the reverb and see what happens.

It looks to me like the opposite is true for the tremolo. Grounding it turns it on.

[Stevem]

I agree, I think the amp was made with the use of a neck pickup only Jazz box guitar in mind!

[unbiased]

I believe I have the same amp and I have only found one schematic for the model number which matches the hardware that I have. As often noted, Gibson could have several different versions of amps with the same model number. For the GA-15RVT, most schematics I found don't show the reverb as being transformer driven and mine is. I have posted the correct one for my amp on another thread in this forum and will post again if there is interest.

I agree that the first suspect would be the footswitch wiring. Mercury does make a replacement transformer for it at about 4 times the cost of a typically Fender so you are better off preserving it as best you can. The slip-on tab connectors are not attached as well as they could be and can be pulled out of the windings if you're not careful around it. Also, the heaters are grounded on one side and although the amp is quiet enough, I would change it to the virtual center tap with 100 or 220 Ohm resistors and add the second wire, twisted with the original. I like the amp and with minor modification, it's a great amp. It's not terribly loud but most of us can find a place for it in our amp garage. Good luck with it.

[gui_tarzan]

As far as the circuit goes, I repaired one of those last summer and it was a '65 with a '62/63 circuit board and components. Check a couple years back and a couple forward and see if you find the right schematic.

[hossymandias]

Thanks everybody.

unbiased, thank you! I saw that schematic you posted this morning and rejoiced! Finally, that is my amp. The other schematics I was looking at had me thinking I had lost my mind (or was worse at reading schematics than I know I am).

To everyone, I totally agree this is a great amp. I forgot to add that once I got the tremolo working, I noticed a HUGE improvement in the tone of the amp. I attribute this to the mods and the other big thing I left out: the speaker it came with was a Realistic Radio Shack (yes...Radio Shack) 12" speaker. After replacing it with a new Jensen C12Q and with the new trem-working, reverb not-working, the tone is more than acceptable. I'm really looking for a surf machine though, so I need to get this reverb going!

I'm pretty sure I have the footswitch wired correctly, but check me if I'm wrong. I used the wiring from the schematic:

https://tubeamparchive.com/download/file.php?id=24010

and here is the inside of my footswitch:

https://drive.google.com/open?id=0B2FHL ... authuser=1

On the inside I have:

pin 1: open
pin 2: braided shield (rev)
pin 3: white (rev)
pin 4: black (trem)
pin 5: red (trem)

[/img]

[Phil_S]

I'm not sure I see this correctly. The reverb switch looks like it has a lug at the top and another lug at the bottom. This is different from the tremolo switch with two lugs at the top.

I think I see the white wire and the braided shield both connected to the top lug on the switch. If that's the case, it would be impossible to turn the reverb on. To turn the reverb on, you need to lift the ground. If there is indeed a second lug at the bottom side of the reverb switch, move the white wire to the other lug. (In other words, the off or open position on the switch causes the reverb to be on; the on or closed position grounds the reverb and shuts it off.)

You can easily test this by disconnecting the f/s from the reverb circuit. That should allow it to turn on.

BTW, since you only need 4 connectors, I think I'd buy a new 4 pin plug/receptacle to replace the one you drilled out. Somehow, I don't think I like the idea of direct wiring the switch, but I don't know why. I imagine you can use something like the Conn SD-40 with SD40SN.

[hossymandias]

Sorry about the photo, Phil_S, it is misleading. There are actually THREE lugs on the reverb footswitch for some reason: two at the top and one at the bottom. The bottom one never connects to either lug (as tested with my DMM). The photo doesn't show it well, but the white is connected to one lug and the braid is connected to the other, which is in turn connected to the ground(?) of the switch. Am I right that the blob of solder on the bottom of the switch is the ground? The other switch (trem) had a blob at some point, but you can see where it fell out.

As far as getting a replacement socket and plug, I will reluctantly take your advice. You are right that my method is a bit janky. But before I order that stuff, I'd like to get it working right. I'll try just disconnecting when I get home.

Thanks, as always, for your help!

[Phil_S]

I'd get it working before I ordered parts, too. I'm not sure I follow completely about the switch -- the blob thing. I think you are saying is it like the one next to it. Have you disconnected the wiring at the switch and tested the switch to see if it actually turns off (goes open)? Maybe the switch is bad?

The shield should only be grounded at the amp chassis, IMO. Although, there really is no ground present in the switch box, so I guess it couldn't hurt. I just don't see how it helps, either.

Good luck with this.

[hossymandias]

I'm more and more convinced that there is something wrong with the drive section of the reverb. pins 2+3 are being shorted and unshorted (?) by the switch so I believe the switch is working correctly.

I also disconnected the braid and the white wire from the plug to go back to the way it was before I wired up the footswitch and no deal.

I noticed just now that as I turn up the reverb pot, there is a noticeable but small amount of unpleasant distortion added to the signal. Any thoughts on this?

Do you have any advice on testing reverb driver or the tiny input transformer of the reverb tank itself?

[Phil_S]

Shhh! Don't tell anyone I've got a spare reverb transformer because it isn't available. This is p/n TF 1000 R. Blue-Red meters ~24Ω. The side with the lugs (should have two brown wires, one light, one dark) meters ~98Ω.

The tank? No. That's above my clearance. I haven't been read in on how to do that. Sorry. According to email I have from Accutronics dated 10/20/11, the correct tank is 4FB2A1B. You are likely to pay shipping equal to the purchase price and I have read these new tanks are not as good.

[hossymandias]

Thanks Phil_S! I'll pm you and try to convince you to part with the non-existent transformer, IF I need it. Hopefull,y I won't need it. I found an old thread in which someone ran their record player through the input of the tank to test the transducer. Let me tell you, I'd much rather replace a tank than that driver. Not only is it riveted to the chassis but it's also under that weird circuit board thing that I want as little to with as possible.

I also just realized that mine is strange in that it has a 12" speaker. Right? Someone must have modded it?

[Phil_S]

Seriously, the transformer isn't available. Sorry. I think you'll have to go to Mercury Magnetics. I promise, if you bought it from me, it wouldn't be less. MM has a proper clone, as I understand it.

The GA15-RVT came with a CTS 10". I think it was stamped S-2009 or something like that. I have a GA20-RVT and that came with a 12" CTS. There is very little difference between the two amps, except the 20 is a 2 channel amp with a 12". In your case, with a tx driven reverb, the circuit is very much the same.

Some of your model have a cap driven input the reverb. If your tx is blown, you might convert to the cap driven version.

I can check the turns ratio for you. You might be able to find something that is a reasonable replacement. Do you want me to do that? I won't get to that until the weekend.

Yes, if you need to pull the board, it is a real hassle. I know that for a fact. You must unsolder all the wires on one side to tilt it up to drill out the rivets. I know this because I replaced the interstage transformer, which is also under the board. That one came out of the full set of iron I was lucky enough to acquire on eBay of all places.

[Pierre]

I repaired one of these for my son's guitar teacher a few years back. His amp was missing the foot switch. I replaced the two rectifier diodes, electrolytic caps, and repaired the reverb transformer. Since the reverb was previously working for you, I would look again at the work that you did on the foot switch jack. The reverb on the amp that I worked on was switched on and off by grounding the signal between the reverb recovery stage 6eu7 and the 12au7 that feeds the PI transformer. The model that I worked on had a choke in the power supply. The preamp tube lineup included two 6eu7s and one 12au7. My impression of the amp was that it had a good tone, but was very tame. I would have tried a 12dw7 in place of the 12au7 if I had one. I suspect that it probably would have been an improvement. I traced out the wiring on that amp and drew a schematic, but I can't get my hands on it to post it. I hope this info. helps. Good luck.