Gibson GA-15RVT Overhaul

[astark117]

Hi, I recently bought an 1966 Epiphone Comet (Gibson GA-15RVT) on ebay. The seller described it as having a hum, and probably needing servicing (hasn't seen service in 48 years).

I've built a few amp clones, but have never restored one before. I was thinking of replacing the two prong power cord and removing the death cap, replacing the filter caps, and looking for loose ground connections and burnt resistors. I also want to put a fresh set of NOS tubes in and bias it.

Is there anything else I'm missing? Also, I have a bunch of NOS 6AQ5A tubes, if I rebias the amp for them can I use them instead of the 6BQ5 tubes? Also I was thinking of converting the 6EU7 preamp tube sockets to 12AX7, so I can use some tubes I already have. I've heard the stock speaker kinda sucks, so I was wondering what speaker suggestions you have, preferably under $100 speakers.

I'll post some pics when I get the amp. Anyone have any experience working on this amp

[Phil_S]

I have a GA20-RVT that I worked on extensively. It is a different amp but there is a great deal in common with the GA15-RVT circuit and I think they use the same iron. The biggest difference is that the GA20-RVT is a 2 channel amp. The GA15-RVT is nearly identical to the GA20-RVT reverb+tremolo channel only. (The other channel in my amp is a dry channel.)

If you haven't opened it up yet, you are in for a big treat. This series of amps used a rat's nest style of building with a funky 3-row board. It is actually not eyelet. Those little eyelet like things are actually cones that are slit crosswise from the bottom. The good news is there is plenty of real estate inside if you have to go off the board for some reason.

Mine had a serious hum and it took me forever to figure out that it was a lead dress problem, primarily related to the interstage transformer. I believe my amp hummed from day one. I worked so long and hard on mine, I practically have the schematic etched on my brain!

To address your question of using 6AQ5's, this is a 7 pin tube, whereas the 6BQ5 (EL84) is a 9 pin tube. I would not change the sockets, which is what is required. You might consider building socket adapters if you are really bent on using them. This is a cathode biased amp. A pair of 6BQ5s uses Rk=130. A pair of 6AQ5 (same electronically as 6V6) needs about double that. You should find that 130r wirewound sandblock on the terminal strip near where the power cord enters the chassis.

You should not hesitate to remove the death cap and install a 3-prong cord. Actually, this should be your first order of business. If you need pointers on how to do this properly, please ask. BTW, that death cap should be a Sprague black beauty .022uf 600V and can be sold to cork sniffers on eBait, so try to preserve lead length. Or you may want to keep it and use it.

The amp only uses two 6EU7's. This tube is electronically equivalent to 12AX7. The reason to leave it alone is the filament wiring. These amps used a lazy loose filament dress that doesn't hum. I would be unwilling to disturb this lest it cause 60Hz hum. You can buy new production 6EU7's or you can roll the dice on eBay for old stock tubes (probably cheaper than NOS USA 12AX7) and you'll be fine. Are you sure these tubes need to be replaced? They should be good for thousands of hours of use. Don't jump in with both feet before you are sure there's a need to think about replacement.

The speaker...a definite weak spot. This amp needs a 10". I think it would be hard to go wrong. I'd check out Celestion, WGS, Weber, Eminance and any of the other "usual" suspects. I think for ~$50 you'll get something very nice. Don't overlook the used speaker market on eBay. I once got a Celestion G10L-30 for about $20, which, incidentally, I think would be a good choice here. That one went into a Gibson GA17-RVT to replace that same crappy speaker you've got. BTW, the 17 uses a pair of 6AQ5's and is a great sounding amp in stock configuration.

In addition to possible lead dress issues (which you will get to later), and after the 3-prong cord is installed, the next order of business is a cap job. A cap job may completely eliminate the hum, in which case you don't have lead dress issues. Due to the layout of this particular amp, I recommend avoiding the use of single caps, and believe me, I'm not usually a fan of multi-section can caps. In this case, I am recommending can caps. Others may disagree on this point. I think (please verify) this amp needs two 20u+20u 450V can caps. JJ makes a reasonably priced (under $10) 32u+32u 500V that you can use instead, or F&T makes a 16u+16u 450V (also under $10) which is probably enough filtering. I would not waste my money on the $30+ can caps. I might use the 32+32 for the first and second caps, and the 16+16 for the 3rd and 4th caps. the increased filtering for caps 1 and 2 will tighten up the bottom end, IMO, an improvement and what I've got in mine. Remember to buy clamps for the can caps.

Get the basics done before you look at tube replacement and a speaker. Tonally, I believe (as do others) the amp suffers from the notch filters in the circuit. You'll see these across the top of the schematic at C2/C3 and C4. These notch filters typically contribute to what we describe as the ice pick effect. Mine has this problem -- and it is a 2 channel amp so it has it twice as much. Someone published a good mod for it (on my bucket list) that I'll share with you later if/when you want it. Right now, I'd call it a distraction and you already have your hands full.

Did you get the foot switch? If not, you'll have a challenge mating to the existing 5-pin plug because I believe it is obsolete. That will give you something to think about in terms of approach.

Don't be surprised if your amp doesn't conform to the schematic. This is very typical of Gibson amps (end Epi, too) of that era. I'd like to know if your reverb is transformer driven or cap driven.

Do be very careful not to disturb or cause the interstage transformer to fail. The only true to original replacement I know of is made by Mercury and it is pricey.

We can play this out for a while and see where it goes.

[Phil_S]

Oh, I see this is your first post. Welcome to TAG!

[Phil_S]

Here is the bright fix for the notch filter. I can't take credit. Someone else posted this somewhere, maybe here. It is on my list of things to do to my amp. If yours is too bright, try this.

I believe these amps were built with jazz players in mind. It is almost impossible to get any real overdrive going, but there's this and a few other easy things that can get you in to a more "modern" zone.

1nf = .001mf (or .001uf) in case you are like me an easily confused.

[Cliff Schecht]

We've discussed these amps thoroughly on these forums. Definitely do some searching, I've had to come back to threads I started years ago to find out exactly what I did to the first one I worked on to get it "right", and then figure which version I have. That's the trick with these, figuring out exactly which version you have. There are some with transformer based phase inverters, some with tube phase inverters, some with cathode driven reverb, some with transformer driven reverb, some with 100k/1.5k plates/cathodes, some with 270k/2.7k plates/cathodes.... They're all over the place depending on what parts were available that day, which can make them a nightmare to work on if you aren't up to snuff with all of the goofy variants. I actually rather enjoy bringing these back to life, they're always a different/unique challenge and can be nice clean amps when going. When paired with a good speaker and a good distortion or boost, they can really get after it too!

[eddie25]

The bridge T's I always find in those amps are two 220K resistors in series with a 500pf cap across those, and from the junction of the two resistors is a .03uF cap to ground. Sometimes they are in a single three legged unit, but I find just cutting that .03uF cap to ground makes a HUGE of difference. If it's too bright still then go ahead and remove the 500pf cap too. I've had one of those bridge T units cause some hum as well.

[xtian]

@Phil, I see you have a new avatar ("Phil of Darkness, Preventer of Information Services"), but you're still being very free with the information. Very awesome and contradictory.

[Phil_S]

No, it's Phil, the prince of insufficient light, carries a spoon, not a fork, and barely the stub of horns. Thank you for noticing. I hope the mods don't go nuts on me for IP infringement. BTW, I always thought the Bugle Boy tube looks like Dilbert's mother's twin.

[astark117]

Thanks for all the info, I just got it in the mail and tried to get some sound out of it. I got nothing except for a slight hum and occasional squeaking noise when i rolled up the volume pot. The tubes all lit up and got warm except for the 2 preamp tubes. Upon closer inspection I realized the PO had put 12ax7s in place of the 6eu7s. I checked the pinouts of the sockets and its definitely set up for 6eu7s, so progress halted until I get a pair of 6eu7s. I'm going to go ahead and replace the filter caps and do the 3 prong cord mod in the meantime though.

[astark117]

also the reverb is transformer driven

[Structo]

Is it this amp?

http://bmamps.com/Schematics/gibson/ga15rvt.pdf

[astark117]

yes, that's the one

[emulator]

@Phil_S - I have a GA-35 RVT here I'm trying to de-icepick ...Do you have any info on mods for that type of notch filter? It's the 2 x 220K/330pf bypass/.0047uf to ground version... I've been combing the archives and there's a couple different suggestions, looking for a definitive mod/fix if possible. Thanks!

[TUBEDUDE]

Is everything riveted rather than screwed? I had an GA 17 RVT and it needed a lot of help to get a good tone. But I really hated the rivets. Every damn thing was riveted down! Solid connection but a bad maintenance philosophy.

[Phil_S]

Emulator: The notch filter is nearly uniform across that whole line of amps. I've snipped and posted the GA35-RVT notch filters (under C6 and C30) from the upper channel. It looks about the same as the ones in my amp. Look for similar/same in the lower channel. If yours are different, it couldn't be much different. The mod I posted applies. Also, see what eddie25 says as he seems to be quite satisfied.