Gibson GA 79 RVT

[Toxophilite]

Has anyone built one of these
It looks like a pretty cool amp
Probably have to use different preamp tubes, thus maybe different preamp but it's a pretty neat setup

https://schematicheaven.net/gibsonamps/rvt791.pdf

[TitaniumValhalla]

Has anyone built one of these
It looks like a pretty cool amp
Probably have to use different preamp tubes, thus maybe different preamp but it's a pretty neat setup

https://schematicheaven.net/gibsonamps/rvt791.pdf

Looks very interesting. If you build one I would try to stick with the 6EU7s - high quality NOS examples can be found for a pittance compared to NOS 12AX7s. If you only want to go with new production tubes they are more expensive but still in production from a few of the New Sensor brands. I bought some NOS Amperex Bugle Boy 6EU7s on ebay for a few bucks which would've been hundreds if they were 12AX7s and apart from the pinout and the heater, it's the same guts.

[Toxophilite]

hmmmm
Any reason to stick with the 6eu7s. Most of the ones I see on ebay now are around the $15 -$20US mark a piece. That's about $20 - $27 Can. I have lots of good vintage 12ax7 kicking around.(mostly free)I'm definitely going to avoid using a 7199 as they've always been pricey. I
ll keep my eyes open though and see if anything turns up


 If I used 12ax7 I wouldn't have to buy any tubes as I already have a bunch of 6BQ5
The earlier GA-88 stereo amp had 12ax7 preamp tubes


I've read that the 6EU7 tube was developed as  a competitor for the 12ax7 but that it basically did the same thing as the 7025 which was a drop  in low noise replacement for the 12ax7


Is there another advantage other than cost (which for me is moot)

[sluckey]

There's a much cleaner schematic in Hoffman's schematic library.

[shooter]

Any reason to stick with the 6eu7s

I've repaired a handful of the GA-5's that used the 6EU7, 2 had bad sockets/ tubes, I re-wired and replaced with AX, or AU, based on customers preference.  primary reason to replace, they are now more compatible with more tubes, the EU pinout isn't "standard"

[Toxophilite]

Cool and thanks I'll look that up
It makes sense to me to use a more readily available tube if it's simply a amtter of wiring the tube sockets differently
Using the schematic it just shows the grid plate and cathode

Also there's a good sub for the 7199 the 6U8 I believe (I might have one of those)

Can those in know see any difficulties in a straight preamp tube swap besides wirinmg the tube sockets for 12ax7 ?

Also I wonder why this amp is rated at 15 watts a side while the Ga-88 is 18 a side

Maybe some Russion 7189 equivilents would get a little juice out of this
I think I have an appropriate PT and a pair of those hammond organ OTs as well as an appropriate choke

i don't have a reverb transformer. I wonder how I figure out the requirements of that

[terminalgs]

6EU7 vs 12AX7:



In a guitar amp, I don't think you'll detect a big difference between a 6EU7 and 7025 or high quality 12AX7.  Magnatone/Estey used a 6EU7 on the 460 and 480 of 1961-1963.  Prior to that, they used a 12AX7 in the predecessor model 260 and 280's.  After the 460/480, they went back to the 12AX7 in the M15.  If you look at the circuits surrounding those first triodes in all those amps, they used the same plate resistors, voltages, caps., etc..,  so I don't think there is any need to re-think the GA77RVT change to a 12AX7 other than pinout.


reverb:


you can use a 6U8A subbed for a 7199.  They seem to have similar/same specs.  as far as a reverb transformer, I think you could use a Fender reverb OT 7.5K:8 or a champ 8K:4/8ohm type transformer.  Fender drove them with a parallel 12AT7, plus you have wiggle room with a reverb tank.


There was a shootout of vintage stereo amps (some guitar/amp magazine) where they compared the Magnatone (I think a M15), an Ampeg Twin-Echo, and a GA77RVT.  you might fish around for it.


The downside about the GA77RVT is the trem and reverb is only in one channel. To see if you'd really dig the concept, put two similar amps next each other and and plug your guitar in to one and daisy-chain the input to the other.  The 480/M15 at least has two separate vibrato units that are driven by the same oscillator, so there is enough variance in left and right that it really sounds stereo.  I'd never suggest a 480/M15 build because you can buy one for  what you'd spend in NOS varistors and transformers alone.  As far as the GA77RVT goes, its a big project for an amp that isn't a whole lot more that two discrete amps sharing a cab and power supply.

[sluckey]

As far as the GA77RVT goes, its a big project for an amp that isn't a whole lot more that two discrete amps sharing a cab and power supply.

I agree. I'm pretty sure that amp will sound very ho-hum next to all your AB763 reverb amps.

[PRR]

The EU was once an "improved" AX. Same basic electricals. Revised pinout may get heater pins away from signal pins. EU was used to introduce a twisted heater, but I think "all" AX got the same heater soon after. EUs were checked for hiss/hum, AX you got what you got; but 57 years on all the old ones have gone old and all the new ones are made new ways.

IMHO, if the socket is EU pinout stick in a EU; if AX pinout use an AX, and on a new-build use AX pinout and product unless you are sure of a good supply of EU. With AX pinout you can always get another AX even on a rainy Saturday in Lodi NJ.

[Toxophilite]

if I built it would be for fun not because I'm trying for a sound
I have most of the parts already and it poses some interesting challenges


I have space echo RE -150 that allows me to separate the dry and echo and I usually use it with two amps with the dry from amp one jacked into amp two and the echo sent to amp two as well. As The line out comes from just before the speaker it passes along the reverb and tremelo to amp two ..though come to think of it it might be fun to have those things a bit more separate. I guess I'm not much of a stranger to the sound potential and I do have a practical use for it other than just piling up with the other amps....which will happen too of course.


I've always wanted a single stereo amp. Just like the idea and I though it would be fun, keep me off the street so to speak!

[sluckey]

That's as good a reason as there is.

[Toxophilite]

Indeed!
Hey terminalgs I forgot to thank you for the info on the tubes and preamp.Thanks
I also checked out that article.
The idea of separate bias tremelos on each power amp section is appealing..if it wouldn't be so space consuming. If only to have different trems happening simultaneously or similar ones that weren't synced


The idea would be to keep the whole thing reasonably compact and I would have a way to mix the first amps pre into the second amp's power amp.
 I also kind of like the Gibson GA83S setup that has a main speaker and a smaller speaker mounted on either side of the amp.
However that strange truncated triangular cab on the GA-79RVT is neat too
I'm curious about the GA79 power amp though, is there no negative feedback?

[sluckey]

No NFB on any of those 3 amps.

[terminalgs]

The idea of separate bias tremelos on each power amp section is appealing..if it wouldn't be so space consuming. If only to have different trems happening simultaneously or similar ones that weren't synced

a single oscillator could drive both depth pots right?  both the GA83S and the Magnatone 280 use a cathodyne to produce two oscillation signals 180 out of phase from one another.  On the Magnatone, they put a stereo/mono switch to let the two different vibrato circuits either get fed the same phase signal, or different phase signal.  that switch, stereo/mono sounds great in mono but awesome-great in stereo (so they really didn't need a switch!),  stereo being the two vibratos are fed out of phase signals.  as one vibe rises the other falls (at least that's how I understood Bonham's second patent on the vibe).


That GA83S trem is more interesting than the GA77RVT's and probably sounds way cooler (1) since its stereo and (2) push-pull wiggle ALA a twin 6G8A ALA Revibe.


I think the closer to your guitar that the stereo split occurs, the better, more lush the stereo will sound.  Maybe better to split before the two preamps?  That GA77RVT has two ways to do it:  (1) the stereo switch in the middle, which would send the channel-1+reverb to the lower power section, and (2) the stereo input jack (hooked to a ES-345 stereo guitar Gibson hoped!-- you can also insert a mono plug part way and engage the tip and ring of the jack with the tip of a mono plug...)  It's odd that on the GA77RVT they didn't use mix resistors between the dry and wet reverb signals, nor the two channels when the  'stereo' switch is flipped.

[Toxophilite]

cool stuff
When I go 'Stereo' it's mostly for the echo
and I send some dry to channel two so that I can retain a 2 amp fuller dry signal when the echo is turned off
I'm not separating my pickups
I split the echo and dry at my space echo prior to the amp


I guess with no nfb this is probably a more midranged focused amp (less of a flat linear response)
that breaks up earlier?

[PRR]

> I guess with no nfb

Ampeg VT-40 w/Distortion is no NFB. On test the amp+OT was flat all across the audio band. I did not measure the stock overload (it was on bench due to smoke) but the down-volted overload was gentle, not rude.

[Toxophilite]

Thanks
What I'm understanding is that I probably can't generalize based on one element of the circuit


i was thinking of fender tweeds which often has less or no NFB from what I remember
From the couple I built , 5E3 deluxe and 5F11 vibrolux (I converted the former to the latter after a shot while and then sold the vibrolux 'head'
They generally seemed to start breaking up pretty early and more midrange(boxy) focused but that's likely any sort of comparison is an apples and oranges scenario with this gibson circuit

My general and likely limited and/or mistaken understanding of a negative feedback circuit was that it's implemented to achieve a couple things.
- flatter response
- more controlled/ less noisy amplifier with perhaps more non distorted potential volume

[HotBluePlates]

... negative feedback ... implemented to achieve a couple things.
- flatter response
- more controlled/ less noisy amplifier with perhaps more non distorted potential volume

For most guitar amps, the principle use of negative feedback seems to be from speaker to phase inverter/driver.  I've come around to believing this is about controlling speaker-flap at the bass resonant frequency more than anything else (and maybe evening frequency response that would otherwise skew a bit due to rising speaker impedance).

I think there are other considerations within hi-fi design, but I'm less convinced these days that guitar amp manufacturers were especially motivated by any of those other factors.


Separately, I tried a real GA-79 RVT almost 30 years ago.  After drooling over pictures of it in books for a couple years, I was fairly unimpressed by the actual sound of the amp.  It was "alright" (not remarkably bad or good).

In hindsight, it seems a bit of a gimmick to mate with Gibson's stereo guitars, the ES-345 & ES-355.  It seems even a lot of collectors don't actually make use of the stereo wiring, and often have the guitars re-wired for mono.

[PRR]

A tertiary "need" for NFB is to reduce power supply cost, especially on SE amps.

I'm pretty sure the AA-Champ could not have tolerable buzz without the NFB loop correcting the power stage buzz.

P-P amps are less fussy. They can benefit, but are often tolerable with very minimal first-cap filtering and no NFB.