Gibson GA42RVT repair issues

[Wunderbar]

This is my first post. I recently acquired a used GA42RVT for what I thought was a good price. However, it appears to have some problems.

1. It seems heavily tilted toward the bass.

2. Reverb is VERY weak, and neither Reverb nor Tremolo appear to work as described in the owner's manual. Both are present with or without the foot switch, and both are present on both channel 1 and channel 2 to some extent, instead of just on channel 2 or in mix mode.

3. Low volumes aren't really possible. The transition from no volume to volume is very steep.

Problems are present in both the Pentode mode and the Triode mode, but may be worse in Triode mode. Problems are definitely worse when guitar output is high.

I've searched on this forum, and found some threads related to this amp. There was a schematic posted, and then further posts indicating that the schematic was wrong, along with hand drawn corrections for some sections.

I took the amp to a repair shop today. In a few minutes of playing an old telecaster through the amp, the technician recognized what he said was a microphonic 12AX7, and a runaway 6L6. He seemed to be knowledgeable, although he had not worked on one of these before.

He asked me if I had a schematic, and I told him about what I'd read here.

Does anyone have any further information about this amp and its problems, or a final version of the schematic that could be shared? I am in the mode where, if it could be fixed, I would keep it. If its a lost cause, then I would return it to the seller.

Any insight or advice? Thanks in advance.

[1971SuperLead]

Hello, I have a 1965 Gibson Minuteman. Anyways...
1. Have a technician put a smaller value cap on the tone stack.
2. Gibsons are notorious for weak reverbs. Mine sucks too. Gerald Weber talks about this in one of his books. I removed my reverb. It simply unplugs in the back.
3. It sounds like your volume pot is shot or very dirty. Clean it or replace it.

Replace those faulty tubes and if it sounds good....keep it! That bass can be toned down with a cap replacement. My Minuteman sounds like one of Eric Johnson amps.

It sounds to me you need a better tech. He should have been able to repair and mod all this stuff easily.

Good Luck

[tubeswell]

As for the trem circuit, a new tube will probably help a lot. If it doesn't, then for the price of a few beers, its worth just going through the trem circuit and replacing every cap (and probably even every resistor - particularly if they are old CC that may have drifted quite a bit) with new ones. Your typical LFO circuit is somewhat finikity (within a ballpark) about the tolerance of the RC network, which it relies upon to complete the 're-inversion' of the oscillator wave.

While I'm on the topic, new reverb, trem and output tubes all round are probably warranted. Power/output tubes wear down quickly in geetar amps and a new set is probably needed every 12months to 2 years if you use it a lot. Reverb (driver) and trem tubes tend to wear out faster than other pre-amp tubes due to the operating conditions. You need to carry a spare set of tubes anyway, so go ahead and splash out on them. (Who knows how much them old tubes have been used?)

Also being an old amp, the filter caps (and the other electrolytics in it) are probably overdue for replacement (particularly if they are original).

[gomez63]

There is a fix for these amps that I had done to a ton of the Gibson GA20RVT Reissue amps to cure ticking in the tremolo and weak reverb. You can give them a shot, their really not hard..

*Replace the trem "bug" with a Perkin Elmer VTL5C1 optoisolator

*Install a 0.01/mfd/630v cap at the resistor junction coming off of the plate of the "trem tube" and connect the other end to ground. In the case of the GA20RVT, this is at the R38/C28/Pin 6 junction...and I ground the cap at C33 ground point.

*Adjust lead dress...use a chop stick to move the wires around with the amp in operation.

*Use (shielded) RG-174 wire with the shield grounded on one side only coming off of the grid (pin 7 going to the C30/R41 junction on the GA20RVT).

[EFK]

It's the circuit itself which is incredibly bass-heavy. That's the way it was designed. You can make some cap changes to reduce the bass, or you can run your bass knob on channel 1 no higher than @ 3 and on channel 2 (even bassier) at 1 or 0. Just like a JTM 45.

Reverb is just plain 'average,' period. It's not the type that is going to bring on the surf.

This circuit went through a number of revisions and there are different versions of this same amp out there with no way to distinguish between them without removing the chassis. The latest variations have reverb and trem on *both* channels, regardless of what the manual says.

Inside is filled with cheap Chinese filter caps and cheap coupling caps. Something good like Mallorys can make a world of difference here.

It is a VERY loud 30W amp - be ready for that. It's not a bedroom amp.

Triode mode on these exaggerates the excess bass problem and it sounds like a**. Might as well tape the switch on the pentode setting because as a volume reduction switch it's mostly useless - should read, "Pentode/Muffled."

Have you had problems with the sound fading in and out? Prepare for that at some point - most of these seem to have problems with an inadequate bridge rectifier which converts AC to DC for the heaters. Easy fix with a heftier rectifier but you'll have to dig around inside.

[Steve Conner]

Are we talking about an old Gibson here, or a reissue? I suspect it's the Made-In-China reissue, so 1971SuperLead and tubeswell's comments may not apply.

Buy Gibson GA42RVT 30W 2x12 Tube Guitar Combo Amp | Tube Combo Amps | Musician's Friend

Many amps seem to have a very steep transition on the volume control, from no signal at all to super loud, if you try to play them at bedroom levels. Some amp makers make matters worse by fitting a linear taper volume pot. It impresses prospective buyers when they try it in Guitar Center and it screams like a banshee even on "2".

[Wunderbar]

These amps are Chinese chassis amps, but not re-issues in the true sense

I appreciate all the input. Thank you.

Gibson Customer Service sent me a schematic, although it is not nearly so detailed as the schematics you can download for Fender amps on the Fender web site. They also offered to issue a return authorization for warranty repair, including shipping costs, but from my point of view, the less shipping an amp like this has to endure, the better. One of the Gibson authorized warranty repair centers (about 300 miles from my house) told me "We've never seen one, and we're stacked up for six weeks. You ought to just send it back to the seller." This is looking more and more like "good advice."

The amp is from Gibson's Hand Built series. It is advertised with the implication of high quality. It is not a re-issue, but appears to be based on previously unreleased Gibson designs. They started selling these about 5 years ago.

I talked to the folks at a Gibson Factory Retail Store, and was steered toward the Epiphone amp products. I was told "The GA 40 and GA 42 have been discontinued. We are steering customers to the Epiphone amps."

It does appear that this amplifier and its sibling the GA-40RVT, are both really made in China as far as their performance parts (chassis and electronics) are concerned. The amp is marked with notice that it includes some components of foreign origin, but I assumed this would be the Chinese or Russian tubes, not the whole chassis. I was wrong.

The skilled craftsmen in the USA do the cabinet building and the upholstery. Sigh....Its come to this.

FWIW, I have confidence in the fellow who is doing the troubleshooting. He has done other work for me with an excellent result, and he is very well thought of in the Music Industry. Unfortunately, last I checked, he had not had a chance to get to it. I am hoping he will tell me the same thing today so that I can just go and get it and begin the process of the return.

Regarding the observation that the amp is bass heavy, I think that if this is true and you cannot run it above the lowest bass setting, that would be a serious design defect. Similarly, a volume pot that won't give you any settings but on or off is typical of cheap junk components. There is virtually NO reverb. Its not just "weak." If this thing is functioning on the intended design level, then the folks who designed it were amateurs. I think there are one or more things wrong, possibly including some bad tubes and crummy caps.

[Wunderbar]

Best service yet

I called the repair technician, and he hadn't started working on it yet. So I got it back with no expense, and called the vendor who sold me the item. Not only did the vendor authorize the return, the vendor also sent me a UPS label so that the shipping for the return would be charged to them instead of to me.

I know this doesn't contribute anything to the forum regarding the maintenance, troubleshooting, or repair of the amp, but I had to share that the vendor did stand behind the product. For those who are curious, the vendor was Musician's Friend.

Thanks for all the suggestions and information that you folks shared.

[gomez63]

These units were made hand wired in California. The circuit design was finalized by techs here in the U.S. and in China.

Follow my advice and you will see significant changes.

[Wunderbar]

I thought your ideas were good

I thought you had good advice on this, and I shared it with the fellow who was going to do the troubleshooting. I honestly think that if I were willing to fool around with it, I could have taken Gibson up on their offer to stick it into their system and hope for a repair. Gibson has been very fair with me on instrument issues. And although I've owned (and own) guitars from Gibson, Martin, Taylor, Fender, Collings, Yairi, and even Takamine, my first guitar was a Gibson, and the company has a permanent place in my heart.

However, since it was a new amp, and since the seller was willing to take it back without any charge (and even pay the shipping cost), and since there was so much foreign content in the amp, and since the schematic and state of the actual design of my particular unit were uncertain, I'm pretty happy with the decision to send it back.

I am thinking maybe a Rivera would be a good choice instead. They seem to offer the sound I'm looking for, and they appear to be actually made in the US (although I suppose the tubes must be foreign sourced). I know this isn't the place for that discussion, so ....

I'll sign off. Thanks for all the help.

[gomez63]

No worries....let us know how it works out!

[VMR]

Hi guys, I need to wake up this thread. i too have one of these reissue GA-20RVTs and it appears to have developed the defective heater rectifier issue. EFK, if you are listening, can you suggest what you mean by "heftier" rectifier? It now has a BR101 rectifier, so do I just select one with higher voltage ratings, like a BR102, which doubles the voltage ratings? Should I go even higher? Also, is the 10 amp rating enough? I looked at 25 amp rated rectifiers and the housing is bigger, not sure if I could make it fit.

Also for the record on mods to these very nice amps (I think), here's what worked for me. I removed R11 and the rev/trem channel greatly improved, now very close to the other channel in overall volume. It was stupidly suppressed before (what were they thinking?). And the reverb was really transformed by removing a 2200pf cap that was in parallel to R30. It was just tacked on and appeared to be an after thought at the factory. Then I upped C26 from 2200pf to .003 uf and together it made it Surf City if you want it, and no longer muffled if you want the reverb more subtle. I plan to try the Gomez trem recommendations next and have the new optoisolator on order. It was all going so well until it started fading out as described by EFK above. I hope that's all it is.

Here's a look at the chassis before I tackled the reverb. You can see I cleaned up the sloppy tube socket wiring. You can also see where R11 used to be. The .022uf cap that was in series to it is to the left, but it is no longer in circuit.

[EFK]

The rectifier I used was a Fairchild GBPC1210W. It is considerably larger and able to handle a LOT more current. Here's a link:

GBPC1210W Fairchild Semiconductor Bridge Rectifiers

Because each of the 4 legs is soldered down into a turret, and there are also wires wrapped around those same turrets, the best thing to do is to (1) cut the old one out, leaving as much old leg as possible sticking out, then (2) heat each turret one at a time and yank the individual legs out, (3) use a solder sucker to clean out the turrets and (4) stick the new legs down in there. The higher off the board you can leave it the better, as it dissipates a lot of heat and it's good to get it away from the heater filter cap to ground right next to it. This rectifier is technically overkill, but once I stuck it in there, the heater fading issues faded into the sunset permanently.

[wildcatstrat]

It's the circuit itself which is incredibly bass-heavy. That's the way it was designed. You can make some cap changes to reduce the bass, or you can run your bass knob on channel 1 no higher than @ 3 and on channel 2 (even bassier) at 1 or 0. Just like a JTM 45.

Reverb is just plain 'average,' period. It's not the type that is going to bring on the surf.

This circuit went through a number of revisions and there are different versions of this same amp out there with no way to distinguish between them without removing the chassis. The latest variations have reverb and trem on *both* channels, regardless of what the manual says.

Inside is filled with cheap Chinese filter caps and cheap coupling caps. Something good like Mallorys can make a world of difference here.

It is a VERY loud 30W amp - be ready for that. It's not a bedroom amp.

Triode mode on these exaggerates the excess bass problem and it sounds like a**. Might as well tape the switch on the pentode setting because as a volume reduction switch it's mostly useless - should read, "Pentode/Muffled."

Have you had problems with the sound fading in and out? Prepare for that at some point - most of these seem to have problems with an inadequate bridge rectifier which converts AC to DC for the heaters. Easy fix with a heftier rectifier but you'll have to dig around inside.

Greetings! I purchased a GA-40RVT a few months ago from Guitar Center in Knoxville, TN, and up until a week or so ago, all was well. It appears that my amp is having the whole heater rectifier problem, so I am planning on ordering the one you suggested ASAP. I came across the thread you authored on the Telecaster forum as well, and it sounds like you have quite a bit of knowledge concerning how to get the most out of these amps. I am a novice in modifying electronics, so I was wondering if you might be able to pass along some advice for some basic mods for this amp. The heater rectifier fix sounds very simple, and after looking into the guts of this thing it seems like it wouldn't be very difficult to swap components out. The only problem is, I would have NO IDEA what components I would need or where to start! Mine is REV 20061110.

[wildcatstrat]

I suppose a better question would be, "Where would be a good starting point for me to learn how to do these kind of things?" As I said before, I am pretty proficient at using a soldering iron, but am just getting my feet wet with a lot of this stuff (the extent of my experience is "modding" my Boss SD-1 with a kit I bought off of Ebay and building a BYOC 2 Knob Compressor.) When I pulled the chassis out of this amp my eyes lit up like a Christmas tree, and I think I would have a ball tinkering with it. Of course I want to actually know what I'm doing. Any advice would be greatly appreciated!