new Gibson GA40RVT??

[EETStudent]

Anybody have ANY knowledge of these? They seem very interesting, but very little info on these other than the generic specs.

[EFK]

I have the slightly bigger brother, ga42rvt, which is the same thing w/ 2 speakers. It's a very nice amp and I love it albeit a little pricey. If you can get a good deal on it - say $750 + or - a little - it's definitely worth it. Then go buy new tubes. They advertise it as class A however I believe it is only A in the lower power setting. Personally, I don't trust those half-power switches and just use it at standard full-power. It's one seriously LOUD mother! It's also solid state rectification: sometimes they advertise this and sometimes not. Stock are (5) Sovtek WA 12ax7 which could use some replacing for best sound and chinese 6L6GC which also could stand to be replaced. The speakers are Eminence Legend modeling-128, which are very 'neutral' and were designed to impart no tonal color. Eventually, I may want to add some tonal color and go with something else. Reverb is a type 4 spring unit and is very nice but not over the top, which is perfect for me as I don't use a lot anyway. One odd thing - Gibson states that channel one is more aggressive (starts to break up @ 2 or 3 if you pick hard) which it is, but that the cleaner channel (chan. 2) is the channel w/ the reverb and tremolo. Well, I've got reverb and tremolo in both channels without using the mix (the amp has a third input which mixes the two channels). ??? I'm not complaining! I had a hard time 'pegging' this amp as to where it fits tonally. It's not as screaming bright as a Fender, although you can get it nearly there, but you can also take it down to a good solid chunky tone. I think it's really quite unique. I'm using it w/ a SG. Have not tried it w/ single coils. The mix of the two channels is my favorite: can get a great Mayall/Cream/early AC/DC by running channel one at @ 4-5, and then running channel 2 slightly less volume to 'fill it out' and make use of the added tone shaping. I don't use pedals save a wah. This is NOT a master volume amp so keep that in mind! I have no neighbors but I sometimes use an attenuator anyway just to save my hearing. If you are not planning on using an attenuator and wan't dirt, you better buy either a loooooong cable and stand in the next room (and plan on...no, absolutely depend upon, an impending police visit) or use a OD pedal.

Few points: this is an unbelievably solid amp. It is indeed point/point hand wired. Very sturdy and well-made, classy, a great choice for dinosaur music but for anything post-70s you're probably going to want something else. I can't speak as to how it would perform w/ a lot of pedals, preamps or high power pickups. It's probably best with pafs, patent numbers (mine) or the like, or p90s/fender singles.

What I don't like: the tubes are located straight out of the back of the chassis, so w/ knobs at front of amp, tubes are sticking straight towards the back. In my opinion, they are too close to the top of the cab. There is heat shielding there, and a metal vent, but I'd like to see more room. There is very little room. I'd like to try KT66 in place of the 6L6 (haven't figured out yet whether this is do-able w/ the existing circuitry) but I doubt there's room for them. Also, the amp has a semi-closed back design which is also too close to the tubes and worse does not IMHO allow much ventilation. I took it off the first day I had it and when I get around to it I plan on getting a good piece of birch or hardwood ply and making a better back which will allow a lot more breathing room for the tubes/transformers. OR I might just modify the existing back - just don't want to bugger up the beautiful tolex which is really nice. A screwed-on back of some form does add stability to the overall cab, though, so if you take off the back don't go moving it around all over the place. Someone at Gibson should have caught this and built the cab w/ a spacer, even 1" would have been fine, b/t the chassis and top of cab.

I think overall I'd rate it at about 7 3/4 or 8 out of 10. Nothing negative related to sound, really my only issues are the stock use of cheap crappy tubes on a well-made amp, the questionable choice of modeling speakers (again - doesn't sound bad at all, but I think it could sound BETTER w/ a different eminence choice - redcoat or patriot line, depending on your tastes) and the lack of space around the tubes. I suspect I would find things like this to bitch about in most amps outside of a custom shop.

BTW - I pestered Gibson into sending me a schematic. I have no idea how to read it however. I'd love to know what preamp tube is driving what and I'd love to know if the schematic is derived from any older existing amp, say a particular Fender circuit or older Gibson circuit. Anyone up for it? I can email as attachment.

[EETStudent]

Yeh I got mine in about 2 weeks ago. At first I would really floored with the amp. The tone was great and the construction quality seemed excellent. It defintely looks beautiful. However, as other reviews as mentioned, Im having some problems with it. Tonally, there is a subtle buzz and mild fizz(not pleasant overdrive) on most lower notes that I pick. Its only present if Im picking and listening critically, but still its irritating. Also, in Pentode mode I get an annoying hiss as I raise the volume. I cannot stand this, especially when Im playing 'pretty' stuff. When Im rockin out its not so bad. There is a consistent low hum, but Im pretty positive thats due to Class A bias. Its a very hot amp. All in all, it has good tone.

The construction has taken a step backwards. The plastic thing that you plug the mains power cord into has come completely loose and I can nearly pull it out, wires attached. And then a piece chipped off. This is annoying too.

My girlfriend bought this amp for me after I graduated college, so I could never bring myself to selling it or exchanging it. I want to send it to a professional tech to really go over it.

That schematic you posted on your other post isnt even close to whats inside these amps!

[EFK]

It sure is hot isn't it? Could grill hot dogs on that metal vent. It really needs a better-designed back. By removing it, heat dissipation was improved exponentially. I'm going to just modify the original back (I like this amp and won't be returning it) and drop the upper edge of the back down a few inches to allow the tubes and transformers some breathing room. It looks like Gibson couldn't decide whether they wanted an open or closed-back effect so they got drunk and hit the blackboard.

I haven't had any fiz at all since I ripped out the Sovtek WA preamp tubes and got some TungSol (reflektor), JJ and Penta labs stuff from Doug Preston (great service by the way) to play around with as well as TAD str power tubes. Try retubing it - it will make a dramtic difference you will definitely hear.

[EETStudent]

Well, I did retube it. I bought 1 old Sylvania 5751, and 2 RCA 5751's. Electrically these tubes tested pretty solid, and they definitely sound better. But still, I got the fizziness going on. Could be the overly biased/low quality power tubes. Ive got a pair of NOS JAN 6L6-GC,that I tried out,but they could not handle it. Im raising that cathode resistor to 500 ohms to bring the bias back down to Class AB. May be a circuit issue though, more specifically a phase inverter issue seeing as both channels produce the same fizziness. I also did a test for noise/hiss by pulling all of the preamp tubes(including the PI tube) and the noise/hiss was gone and only a little power amp hum was present. I put the PI tube only back in and the noise and hiss I had been getting was present. I would like to devote a lot of time reworking it, but Ive gotta focus on school right now.

[Steve A.]

BTW - I pestered Gibson into sending me a schematic. I have no idea how to read it however. I'd love to know what preamp tube is driving what and I'd love to know if the schematic is derived from any older existing amp, say a particular Fender circuit or older Gibson circuit. Anyone up for it? I can email as attachment.

Sounds great! I wanted to send you a PM with my email address but could not figure out how to do it... (I think that you have PMs turned off in your User Profile)

Steve Ahola

P.S. These forums are indexed by Google so people might want to refrain from posting their email addresses in the context of a post here.

[Enzo]

You can send an email through the forum software and it won't reveal your address unless you want it to. Google won't find that. Is his email turned on in the profile?

But you are right that if you enter your email address it will last forever. I used to write out my email address in ways that robots couldn't detect - such as joe blow at msn dot com instead of joeblow@msn.com. But anyone pulling up the thread through google would see that.

I now post shematics on Ampix instead. Or when someone emails me.


DOn't we already have this schematic posted on another GA40 thread here?

I have seen this question now asked the same way three or four times now. WHy would we think this amp was "based" on some other amp? Most amps are just designed. It is not like the technology is new. A competent designer doesn't need to copy someone else's design.

[EFK]

Steve - thanks for the interest. I posted the schematic that Gibson sent to me above on the 'theory and design' sub-forum. Insofar as why I am comparing it to pre-existing circuit: Enzo I am sure you are correct and make a very valid point, although for someone like me (electrically-challenged...) it is easier to think of it comparatively. I have spoken to a few people who have compared it to a vague Fender circuit, and frankly it reminds me very much of one of the Fender amps I had back in the 80s (which spent more time in warranty service than being played) but I just can't recall what it was. Getting old!

Hey EETStudent - I've read through some of the other posts which I assume, maybe incorrectly, refer to electrical components and characteristics of this amp. I understand the sentimental value, and it seems like you have reservations about this amp. I don't mean this in a negative way at all - everybody likes different things right? Here's an idea from a happily married guy (going on 16 years now): you've got a solid cab, a decent speaker and a chassis. You seem to know a heck of a lot more about the guts of a tube amp than I do. How hard of a job would it be to rip apart the chassis and either build, or have built, a clone of a ciruit you like and retrofit it to the Gibson amp? It'd be a sleeper and still maintain sentimental value. And your girlfriend would probably never be the wiser! Ha!

[Enzo]

You will find many amps resemble other amps, they are all pretty much the same thing under the skin. I always called it the squint test. Hold the circuit diagram at arms length and squint. Now the details are gone and all you see if the basic outline of the circuit. AMazing how similar they all are.

[Robert M. Martinelli]

You will find many amps resemble other amps, they are all pretty much the same thing under the skin. I always called it the squint test. Hold the circuit diagram at arms length and squint. Now the details are gone and all you see if the basic outline of the circuit. AMazing how similar they all are.

Amen to that!

Enzo, I have to thank you once again for teaching me a testing procedure I didn't know already....I've been tinkering since the age of eight, now I' m in my mid-forties and never heard about "squint testing" and how useful it could be....thanks!

[EETStudent]

Haha EFK! I just got around to reading your idea on totally rewiring the circuit. Yes I have thought about that extensively, but I think I'm just going to keep it as it is. It has really started to grow on me for one particular reason...I decided to see what adding a master volume would do. Viola! Adding a post-phase inverter master volume really extended the tonal range of this puppy! Don't get wrong, I can't get anything near screaming distortion, but run each channel around 9 and attenuate with the MV and you get a really nice crunchy clean sound. I am all about one-trick ponies, but a master volume really opened this thing up. AND a lot of the hiss disappears when I keep the master volume low, leading me to believe the problem originally was in the phase-inverter. I love this amp now.

[bfanjoy]

buzz

Hi all,
Just signed up for the forum after searching for ga40rvt buzzes and cave accross this thread. I just bought this amp a wwek ago and it definately buzzes at certain frequencies - very annoying. Sweetwater shipped me another but the same issue plus more cabinet buzz. I'm thinking of returning but just wanted to see if there was any updates on this to fix because other that thisI like the amp. - Thanks, Buddy