Gibson GA-15RVT Overhaul

[emulator]

So I spent some time with my GA-35RVT, changing things one at a time and listening after *every* change (something I do not usually have the patience to do).

I ended up liking the following mods, which are all easily reversible:

- Bypass .0047uf input cap with .01uf in parallel on both channels
- Change cathode bypass caps to 22uf 25V in first gain stage of both channels
- Short out notch filter in 'normal' channel with jumper across 330pf cap, C6 in the GA-35RVT
- Bypass notch filer in Rev/Trem channel with .022uf cap across 330pf cap C18 in the GA-35RVT

I did try a few things with the remaining notch filter, which both channels pass through on their way to the PI driver, but bypassing with a jumper or a larger cap lead to undesirable distortion at higher volumes so I just left that one as-is.
The normal channel is a little beefier than the Rev/Trem channel, both still have a good clean sound and now have enough drive to create a pleasing distortion at higher levels.

I should also note that this amp has had a full cap job with stock values, and is driving a professionally re-coned modern Jensen C12N that I had hanging around. I'm going to try an old Canadian made Univox 12" once I obtain a new gasket for it.

Not sure if this should be re-posted to a topic on the GA-35RVT, just wanted to share my results for the archives...

[Phil_S]

I think this fits into this thread just fine. It is no more than a minor diversion at worst and certainly not a hijack. The results apply to all the amps in this series, as they all have some iteration of the notch filter. Curiously my GA20-RVT is weaker on the normal channel and stronger on the reverb channel. Actually, the amp is best when there is a jumper in the normal and reverb inputs. In any case, it is good to get some feedback. Thanks for posting it.

[astark117]

So I replaced the filter caps, replaced the power cord, and replaced the 12ax7s with 6eu7s. Now the amp is extremely quiet. When i have the volume dialed it's slightly audible. Any probable causes? Its clear and sounds good, just barely audible.

[Phil_S]

The fact that it passes signal is good. That it is far to quiet isn't so good. So, let's see if we can figure out where/what is eating the signal. This is easier with an oscilloscope (with which I have very little experience), but it can be done adequately with a DMM.

First things first, let's get a full set of DC voltage readings: at each tube pin (but not the heaters) and at each filter cap.

While you are doing this, monitor for the "pop test", which is simply listening for a pop at the speaker when you touch the red lead to the socket pin -- you should hear static or a pop. On this one, start with the power tubes. If you are getting a good pop at the power tubes, then we can be reasonably sure the problem lies either with the interstage transformer or in the preamp section, and this is what I suspect.

Also, give us an AC voltage reading from the high voltage secondary on the power transformer. The rectifier diodes should be on a terminal strip, providing an easy place to put your test leads.

I like to use clips to assure a stable voltage reading and to avoid slipping.

This could be as simple as a cold solder joint or a missed ground. Check for this by visual inspection and by chopsticking (use of non-conductive probe) the wires and tube pins while the amp is live.

Do you have the footswitch and does it work? It might be helpful to determine if the weakness is related to the reverb or tremolo being on or off, but I am thinking this is not likely to be the problem.

[astark117]

Thanks, I'll check and post those voltages when I get off work. The amp came with the 5 pin cable, but no footswitch, so I rigged one up with 2 SPST switches and an enclosure. The amp is too quiet to really tell if it's actually activating the reverb or tremolo though. There's no change in volume when I turn them on or off.

[Phil_S]

If you got the cable, that's huge because I don't think they make that connector any more. Provided you get the amp running right, I'd say to buy a 2 button footswitch on eBay (max $20 with shipping) and adapt it to your cable.

[unbiased]

By the way, the schematic posted earlier was said to be the correct one for the amp in question however, the reverb in the schematic is not transformer driven so it is likely not the right schematic. I have the same amp, transformer driven reverb, and have found several schematics, all of which were not the same as my amp. Here is the correct one for my amp and the only one I found to have a transformer driven tank. Hopefully, it will be a better match for yours. I do like the amp, my only complaint being that it isn't quite loud enough for my tastes but it works well in quieter realms. Enjoy. With a little attention and restoration it has excellent tone.