1960's Gibson GA15RVT on the bench

[Cliff Schecht]

I stared at the transformer for a good 10 minutes but could not find any tabs or even evidence of solder lugs being on that transformer. I completely agree that a choke there is all wrong, I didn't say it was right, it just doesn't hurt to leave it in the circuit. I essentially did a modified form of what the schematic does (driving the reverb off the cathode, I just left out the voltage divider). Not the smoothest trick in the book but it works. The reverb doesn't sound all that good anyways, I think that is the general consensus with Gibson amps. The guy is really interested in the tremolo sound with this amp. I could always go in and modify the circuit later if needed.

The speaker that is in there probably isn't stock. It sounds fine though.

[Cliff Schecht]

Pulled the amp apart, cut open the "choke" and found two wires that make up a secondary side. Not sure if it was broken and then rigged up by someone or what but I'm going to repair the taps and use this part for the reverb. I'll bet the reverb works 10x better with a proper drive circuit.

[Cliff Schecht]

Well, nevermind, the reverb does work with the repaired transformer but it's still weak as crap! Maybe I'll just leave it alone for now..

[Phil_S]

I'm glad to hear my instinct on this was good. Sorry to hear the reverb isn't all that good. I had expected something similar to my GA20-RVT, which has a very nice reverb. It got me to thinking and I just compared the two circuits. The 20 is driven from the plate and I think it sounds great. I realize that you may have run out of steam on this particular amp, but I suggest it's worth taking a look at the schematic. Probably, moving just a few components, you can modify the circuit into one that actually works well. YMMV. Good luck and thanks for sharing.

[Cliff Schecht]

After the transformer repair, I started studying the GA20RVT schematic. It's close to the GA15RVT in a lot of ways and is drawn better. So what I ended up trying last night was reducing the size of the 470k resistor that feeds into the signal path to give reverb. I purposely placed in a value that was going to be too small in order to see where the limit was. This brought the reverb circuit to life and, as I expected, was too dramatic of a change. It does sound a LOT stronger now though and does have a good sound, especially before the whole circuit gets saturated (about 5 on the knob right now) and goes into a purely delayed signal (still kinda cool sounding though). I'm going to tweak this value a bit tonight to find a good trade-off between knob rotation and reverb amount.

[Phil_S]

Excellent news on the reverb. You'll get it tweaked.

It was just killing me when you said it wasn't good reverb. If you're gonna fix it, I figure it's worth fixing it right. If you ask me Gibson F'd up on that particular design big time.

[Cliff Schecht]

The transformer leads failed quickly, even after I secured them pretty well. There really just isn't an easy way for me to repair this transformer without drilling out the rivets. That's very inconvenient because this transformer is mounted under the flexing board that is covered in chicken wire.

Long story short, I took the transformer out of the circuit and just wired the stage as a cathode follower with about 15k on the cathode and the 2uF cap to ground (DC blocked with a 10uF cap and then right into the reverb tank). Hooked everything up, flipped on the amp and BAM! (the good bam), the reverb was alive and kickin'. The only other thing I touched was the 470k mixing resistor that goes from the reverb circuit to right after the tonestack. I dropped the value from 470k to two 470k's in parallel which really brought the reverb out at the max end of the pot rotation. Sounds great and is getting final testing, cleaning and shoved back in the snazzy case tomorrow. These vintage amps are a lot of work!

[Phil_S]

Way cool, Cliff!