Thought I'd share my Experiences with a white-face gibson

[Leviman]

These amps are fairly notorious for being not great in the amp world. I of course bought one before I had heard this, for about $200. That was a fair deal for me! I wanted a cleaner all tube amp with some features (mainly reverb and tremolo) and I got one! The amp I bought Is a GA-15RVT. Of course after firing it up I realized it had issues. It was insanely bright and the reverb barely worked. Here's how I fixed it

-FIRST-
I replaced the cord with a three prong cord, this was for my safety.

-SECOND-
I replaced the power supply filter caps, these were old and sure enough, the amp ran quieter after their replacement.

-THIRD-
I swapped in a new speaker, in my case, an 8-ohm pignose G40V speaker I had laying around. Not a fancy expensive speaker, but better than the stock one. This didn't make tooo much of a difference.

-FOURTH-
Now is where I try and fix the tone of the amp. First I replaced all the tubes with newer ones. I swapped the original 6EU7s for 12AX7s which required re-wiring. This brightened the tone up a bit, but not in a bad way, it was just more clear. It did however increase noise, mainly because of my shitty lead dress (I didn't have any new wires to lengthen the leads so they're all crossed up and such).

-FIFTH-
Now I was longing for that broken reverb to work more and more every day and I decided I'd get to the bottom of the issue. I noticed if I shook the springs I got a nice loud reverb, this told me it was on the drive side of things. Next I took a battery and touched it for a split second onto the drive transducer while the amp was running (and the reverb tank output connected) this gave me a snap of reverb and told me the drive transducer was working. Next I looked in the circuit and saw a big black coupling cap which was polarized. AH-HA! Electrolytic! Sure enough, when I jumped it with a resistor for a few seconds the reverb jumped into life! I then replaced that cap (as well as all the other caps that were of the same style) and now I have the most insanely awesome reverb ever! And more gain!

-SIXTH (The bright fix)-
Now I took a look at the schematic and I drew up the tone stack in Pspice and played with some values to get more bass. La te da, I bought the parts to do the mod and then opened up the amp again. At this time I was struck with the ever famous Gibson circuit discrepancy. Apparently sometime between engineering and manufacturing someone decided that the tone stack could be built cheaper using a multi-section .001mfd cap to do the low-pass filtering as well as to couple the bass part of the tone stack to the treble! Now if you were to look at the amp it doesn't look like a coupling cap, but if you draw it out schematically you see this:

(Don't mind the pots, they are the correct 2meg as they are in the amp, the 1 and 0 are just position readings)
[IMG:725:574]http://i966.photobucket.com/albums/ae14 ... kschem.jpg[/img]

I replaced this bit of the cap with a .22mfd cap (just one I had laying around) and oh my god. What a difference, the amp has a nice full tone now, and when you crank it up you get a nice overdrive! The amp is very mid-rangey which is funny because before it was very mid scoopy (because the coupling cap was so damn small it was cutting out the mids!!!). I like the way the amp sounds now and I'm very happy with my purchase. An 18 year old kid, and some junk from radio shack solved (at least IMO) the plauge of the thin gibson. I bet a lot of the amps that have this similar issue have the same tone stack.

[Gibsonman63]

Nice work. Thanks for sharing.

[Phil_S]

Your "WTF" is a typical notch filter in many Gibson amps of the era. I have known for some time it is a problem, but did not know how to deal with it. I am very grateful to learn about your solution. I am going to try this on my Minuteman when I have an opportunity. Thanks very much for sharing.

[azatplayer]

Yeah great post dude. Welcome!

[xtian]

Photos, please!

[martin manning]

Nice bit of work, Leviman! Looking at the schematic, there are two of those notch filters in series in that section of the GA-15RVT. Replacing the 1n (or 500p) with a 220n essentially takes one of them out of the circuit, so the resulting mid scoop (at ~500 Hz) is only about half as deep, and the bottom-end comes up too. In fact, 220n is so big you’ll get the same effect by just shorting the 1n cap.

[Leviman]

Thanks guys! Yeah I should note my bit of schematic is what I've deduced the schem to be from the real amp. The as written schamatic is here: http://www.gibson.com/Files/schematics/GA-15RVT.pdf and as you can see, the tone stack is quite different! also those two one n's as they appear are actually that multisection cap I was talking about. I believe there might be a treble bleed circuit on the volume knob as well, there was another one of those 1nf multi section caps hanging out around there, but I haven't taken a look because I've been to busy playing it as is Maybe I'll crack into it again and make a youtube video or something, and show how it sounds!

[martin manning]

My comments were made based on the Gibson schematic. I noticed you show a 47n to ground where Gibson shows 4n7 (?). It's also possible that the circuit was not original when you got it. It would be useful if you could trace the schematic all the way from V1a to V2a so folks like Phil who own one of these can see it in its entirety.

[Leviman]

Oh! You are right that is a 4n7. Also, I'm almost 100% sure it was stock. This thing hadn't been screwed with at all, one of the screws on the chassis was rusted in. Anyhow, I made a video of how it sounds, I'll try and get one of the guts tomorrow.

http://www.youtube.com/watch?v=vg9T2sAqzxc

[azatplayer]

SOunds real nice man. Has lovely chime.
Ive been looking at the old ampeg schemos lately looking for something a little lower powered and unique. Looks like you found just that.
Pretty cool!

[Nephilyn]

Thanks for sharing, cool post!

[Leviman]

Stay tuned for more information about the technical side of things when I get around to making a gut shot video. I also put in a bid for a slightly broken tube radio for $10 and it ends in a few hours... maybe the next project . Now that I've experienced the whole dimed amp thing I've gotta do it quieter (and yes I'm aware of the iso transformer, allegedly not good tone, etc of radio conversions). And damn if I do that I need an A/B box too... what did I get myself into haha

[Structo]

sounds good!

I really liked the overdrive tone, that sucka wails!

Good job of getting it going.

[Leviman]

Alright I actually take back what I said about getting gut shots, I'm visiting home this weekend and have to pack. Sorry! But eventually, eventually. Oh and you all should subscribe to my youtube Oh and I won that radio for $9 plus about $15 shipping

[statorvane]

That is an excellent save. A friend of mine has one of those amps - doesn't know what to do with it. He mentioned it seemed overly bright. I'll forward him this forum link and your video. Nice playing BTW.

Thanks again.