1960's Gibson GA15RVT on the bench

[Cliff Schecht]

I'm working on a friends GA15RVT. Nothing too crazy so far, just a cap job, giving it a three prong plug and removing the "death cap". This is my first time being inside an old Gibson and the first thing that stood out at me with this amp was the chicken-wire they used everywhere in the amp and how sloppy it was. Not a huge deal, I've built worse before, but kinda shocking to see for the first time. Also noticed a bunch of Orange Drops in there, not so sure if those are original but they don't look like they were jigged in either. If somebody did change out the caps for OD's, they sure didn't think to make the amp safe!

Anywho, onto the questions. Aside from the obvious fixes the amp needs, is there anything else I should replace or at least check while I'm in there? The board is pretty badly flexed in the middle and some of the cathode caps are OLD 2uF 20V parts that I'm a bit weary about. I don't want to tweak the circuit before I get the thing up and running in stock form, but I also don't want to mess anything up because another part was bad that I wasn't suspecting. I guess checking the power supply CC's wouldn't hurt, but what about those goofy looking ancient silicon rectifiers? Maybe I'll take some pictures, the amp is in good shape cosmetically and just some needs some TLC on the inner bits. Any advice from you more experienced repair guys is much appreciated!

[selloutrr]

a couple routes you can go with this. from personal experience and the nature of adding new good parts , can often lead to other old parts failing after being pushed, where as the old part might have been holding voltage back from the now failing part.

so you can go threw and pick and choice which parts to replace. or treat it like a restoration and test any part in question.
while you are in the amp test all the tubes, the speakers for impedance, and function free of rubbing, etc. make sure all the connections are solid as well as the grounds. if the amp wasn't operational when given to you you can also test the transformers and test continuity threw the circuit. you said it looked to have a possible tone cap change. it might be good to also trace the circuit and make sure it's still stock and not modified.

the fiber board can hold moisture I would also meter it for continuity and make sure it's not arching.

[Phil_S]

Cliff,
I've had quite a bit of experience problem solving with my GA20-RVT. In a way, you might say it is the big brother of the GA15-RVT. While there are clear differences in the actual circuit, some rather subtle, the style of construction is very much the same. Wires all over the place and that crappy cup board make it really tough to figure out what's going on.

My GA20-RVT, acquired around 2002, came with a bunch of OD and brown turd caps and they appeared to be original. I'd leave those alone, absent a compelling reason to "fix" them.

I'd agree, the cap job and 3 prong are way overdue. The rest, I'd say, just a routine exam, and replace anything that is well beyond spec.

Does the customer have any specific requests? IMO, these amps tend to be very bright and a common complaint is ice pick tone. In part, that is due to the notch filters you see at the very top of the schematic, look for C3 and C6, a couple of 220K, and another cap between the pair of 220K.

Another common problem is the interstage tranny going open. If you hear a static-like popping sound, that's a bad sign. Mercury Magnetics makes a replacement, but you'll pay dearly for it. This is the same part for both the 20 and the 15.

I've attached a picture sent to me by a friend from another forum, who did quite a bit of tinkering with his GA15-RVT. I've also got his notes - let me know if you want them. I don't think the turquoise cap at the lower left is original.

[labb]

Now Phil that is really ugly. Looks like something I would do..You know you look inside these old amps and one wonders how they ever played..For sure when you look at the lead dressing and the grounding scheme..Most of them had ground points just where ever it was convent, so it would appear.

[eddie25]

Cliff,

Does the customer have any specific requests? IMO, these amps tend to be very bright and a common complaint is ice pick tone. In part, that is due to the notch filters you see at the very top of the schematic, look for C3 and C6, a couple of 220K, and another cap between the pair of 220K.

Good call. I just fixed up a GA-45RVT, which had about 4 or 5 RC filters like this and removing these in place of just resistors opened up the amp tenfold. The OD's where original in mine.

[billyz]

Anywho, onto the questions. Aside from the obvious fixes the amp needs, is there anything else I should replace or at least check while I'm in there?

but what about those goofy looking ancient silicon rectifiers?

If they are Selenium Rectifiers it would be a good idea to replace them with silicon types like 1n4007. Seleniums leak over time, unless they used very large ones, which they usually did not.

[Phil_S]

Now Phil that is really ugly. Looks like something I would do..You know you look inside these old amps and one wonders how they ever played..For sure when you look at the lead dressing and the grounding scheme..Most of them had ground points just where ever it was convent, so it would appear.

Now, you are forcing me to show off. Here is the board from my GA20-RVT. It makes the GA15-RVT look like a walk in the park.

[Cliff Schecht]

Wow guys, thanks for the great (and prompt) responses! I finished the cap job last night, albeit I sorta jigged them around the stripboard pretty tight. I may go in and change this later but for now, I need to see what needs fixing in the amps audio circuitry. The death cap was already removed thankfully so fitting on a three prong plug wasn't a huge deal (I HATE those little black power cord holders!).

This amp was purchased to play, so I was really told to just make the amp kick ass. If that involves fiddling with the circuit, I have no problems doing so. I've seen a set of recommended mods already but I'd love to hear what you guys have to say about it. I'll ask about the sound he's looking for, but I know what the guy plays and what he'd want from the amp (some balls, but not necessarily from a lot of distortion).

[Cliff Schecht]

Something else I noticed was that my guy has a choke installed after the rectifiers as opposed to a 270 Ohm resistor. Working around the choke is what forced me to cram in the capacitors like I did. Now that I did this though, I'm thinking the power section deserves a complete rebuild so that the B+ dropping resistors aren't surrounded by electrolytics (the heat build-up is what worries me).

Is it common to replace the rectifiers in these amps?

[BTF]

You know, on the subject of those old Orange Drops, years ago I was given an old Silvertone Twin Twelve to scavenge after the trannies had shorted. Some of these amps were, I believe, made at Chicago Electric and used those polyester ODs. I rewired my Fender PA100 amp into a Twin/Showman-type amp a few years ago and used these surplus caps after trying some old blue ITWs and recent-make polyprop ODs. I thought those old ODs sounded fantastic! There's often arguments over whether you can actually hear the differences in caps, but I'd love to have a boat load of that series and vintage. Maybe it was aural voodoo, but those caps were so warm and full in my build.

Regards, BTF.

[Phil_S]

Something else I noticed was that my guy has a choke installed after the rectifiers as opposed to a 270 Ohm resistor. Working around the choke is what forced me to cram in the capacitors like I did. Now that I did this though, I'm thinking the power section deserves a complete rebuild so that the B+ dropping resistors aren't surrounded by electrolytics (the heat build-up is what worries me).

Is it common to replace the rectifiers in these amps?

You'll need to use your best judgement on the power supply components. I think, as long as they are not touching, you should be OK.

I've gathered that Gibson had a habit of making the amp du jour based on what was on the shelf. This caused numerous iterations of the same model and were rarely documented in the schematic. If that choke bears a period correct part number, it is likely original. Here is the choke in my GA20-RVT, with the p/n and EIA clearly visible.

You might try building a jumper with a resistor in the middle to see about bypassing those notch filters. You'll want to experiment with the value. Sorry, I don't have a suggestion. You may find that it is beneficial to eliminate C3 and or C6 and that's easy enough to test with a simple jumper wire.

Other useful things to consider or experiment with:
-preamp cathode bypass caps of 10u or 20u
-increase C1 .0047u to .01 or as high at .1u or .2u
-are the input jacks fed to an undocumented .0047 coupling cap? Try .015.

The cold truth is that these amps were not tone monsters. I think they were intended to be low volume practice amps for jazz players.

[Cliff Schecht]

Went through the amp today and did the seemingly typical stuff like add bypass caps (22uF tantalums were all I had on hand) to the first two stages and increased the first few coupling caps (input and after first plate), these guys were just too small for my liking. All of these mods were tacked on and are easily removable later if someone gets a hard on to go back to stock (which sounds pretty bleh as others warned). I also forget yesterday to unground the heaters and give them resistors, so I did that today.

Some goofiness I noticed in the amp was the undocumented input coupling cap (.0047uF is too small here IMO) and 220k/2.7k resistor combo on the first stage instead of the standard 100k/1.5k, so I left the 220k and paralleled another CC 2.7k resistor on there (along with the bypass cap). Even with this, the amp wouldn't distort that much with my Tele. The overall tone of the amp improved a lot and I kinda like the wonky tone controls, pretty versatile for a 2 knober. I think with the right OD/distortion in front of it this would be a screaming amp. It's too noisy to setup as a screamer itself and the lazy wiring would not work well with high gain. It's got a good clean sound which I don't want to sacrifice for more gain. I think I'll stop where I'm at and do a more thorough test tomorrow.

[Phil_S]

Some goofiness I noticed in the amp was the undocumented input coupling cap (.0047uF is too small here IMO)

see above. Someone who tried a number of different caps in this spot settled on .015 as what he liked. Gibson amp du jour, undocumented!

Good move on the heaters. I don't know why you wouldn't rebias V1 with 100K plate load and maybe 1K - 2K Rk. Just my 2 cents, not worth all that much.

Sounds like you are doing well with this. Good luck with the rest.

Phil

[Cliff Schecht]

Amp is done. I didn't bother changing the plate resistors to 100k, instead I removed the extra 2.7k resistor I placed in parallel with the original 2.7k (left on the 22uF bypass cap). The amp gets NO distortion with my Tele but has a nice and somewhat unique clean sound. The tremolo sounds great too, very "Fortunate Son".

Some other more annoying goofiness I noticed was that the reverb drive was being taken directly off the speaker. This isn't how it was in the schematic and I didn't think that was right, so I did some digging. Turns out that the reverb driver in the schematic is a cathode follower. In the amp, however, there was no second RCA for the tank reverb input. The stage where this was supposed to be was quite weird as well. It was like the schematic up until the plate and cathode connections. The plate was connected to the B+ through a choke (not documented in the schematic but the transformer looking guy only had 2 leads!) and the cathode just had a 1k resistor to ground in parallel with a 2uF cap. So I put in a 10uF cap at the cathode connection and am now driving the reverb off of this. The reverb itself doesn't sound terrible but the reverb level and volume knobs are interactive as crap! With the "loudness" knob up all of the way you can't hear the reverb at all but with it turned down, the reverb starts to come in. Should I move this connection (the reverb drive) back to the speaker output? That just doesn't seem right to me, or at least I've never seen it done this way before. Plus it looked like this wasn't a factory job, somebody else had jigged the wires onto the speaker. The Jensen speaker code is 220521 which I assume means the 21st week of 1965 (if it's the original speaker).

[Phil_S]

Cliff,
As far as I know, the the GA15-RVT and GA20-RVT share the same identical iron all the way through. The reverb tranny is p/n TF-1000-R. It's a small one, red and blue wires on one side and tabs on the other. The tabs on mine are mated to clips, not soldered. I wonder if the tabs on yours are broken off. A small choke sounds all wrong. I'll take a picture of one if you like -- let me know. I've got a spare, but it is NFS. The standard Fender reverb tranny probably won't work with the Gibson tank, as I think it is the wrong impedance. The correct tank is Accutronics model 4FB2A1B. If you look this up, you will know the correct impedance if you need to try to find a transformer. You might peel back the paper and see if the connections can be recovered. If the part number is right, they are there. It's easy for me to say, but I don't think the owner should live with the mickey mouse setup you describe.

IMO, the Jensen is not the original speaker. That would have been a 10" CTS with a reddish/orange Gibson logo label. Anyway, that wasn't a great speaker and the Jensen is probably several steps up.