Gibson GA-77RVT 1963/64 specific schematic

[CB Amp]

Hello,

I've got a Gibson GA-77RVT Crestline (#710095, 1963 or early 64), with a very specific circuitry, distinct from the schematic found on the web or in the Tube Amp Book.
It has an extra OA2 tube (compared to "normal" ones), the bass pots are 10Meg (!). I re-draw the schematic, it's completely distinct from the standard GA-77RVT. But it seems that some people did some mods on this amp (really crispy sound with a lot of treble, reverb on both channels).

The amp has the following tubes : 6EU7, 6EU7, 12AU7, 6EU7, 6EU7, 6CG7, 6L6, 6L6, OA2.
Does anybody have the original schematic of this model ? It would help me a lot.
Thank you.

Here is a pic of the amp (found on the web, but mine looks exactly the same) :

[hollisdevillo]

yeah you might have an in between model or something. you might want to check on schematics around that time and see which ones match closer to yours. it sounds more like you have a 77-ret vanguard. it has similar tubes. has that oa2. it's not uncommon for the techs to make changes in the amp and not the schematic.

[capnjuan]

Hi CB; this is a link the the GA77RET mentioned earlier http://www.schematicheaven.com/gibsonamps/ga77ret.pdf The RET model has a Tel Ray oil can in it, not conventional spring reverb. Anyway, the RET model has an OA2 showing and it looks like a voltage regulator in the tremolo circuit. More here: http://www.r-type.org/exhib/aaa0266.htm If the voltage to the plate of V3(?)B exceeded the tube's rating, it would shunt excess voltage to ground. Whether your amp is an RET may not matter; your amp might have been made after the general schematic was drafted - never say never with Gibson! Good luck with your amp. John

[CB Amp]

Thanks for your replies.
I had a look at the RET model and other amps that could use the same circuitry, but I did not succeed...

My amp has the same vibrato circuit as the RET, and about the same power supply and power amp. But everything else is different, preamp, reverb of course, phase inverter. And that's where my problems are... There is some reverb on both channels and too much treble. So I drawn the schematic, there are strange things, but nothing that could justify this... I don't understand.

I don't have any scanner, but I will try to take a picture of my schem and the circuitry. Perhaps you'll find something...

[capnjuan]

Ok; good luck! J

[CB Amp]

Here is the schematic :



The amp chassis :



Inside (awfull layout...) :

[capnjuan]

Nice work CB and very quick! It looks like the OA2 is doing what it did before, it's just wired in differently. I wish I could be of more help with your trem and reverb ... but I have nothing to offer. Good luck! J

[Enzo]

A schematic for your amp may or may not exist. I have for one Gibson model three diffferent published schematics, plus three yet again different hand drawn schematics. Six versions all under the same model name. This is a standard occurence with old Giobson amps. They changed the circuitry all the time, and not always did it get documented. And the Epiphone models were freely mixed and matched along the way.

Many times I have sanned through the Gibson Master manual looking at tube complement and control features to determine which model some amp might be, after it doesn't match the schematic for the name on the panel.

You can download that whole book from Schematic Heaven, if you'd like it.

[capnjuan]

Phantom Gibson models

I bought a GA20T Ranger on eBay 4 months ago. I just assumed it matched the public domain schematic; 12AY7, 5879, 6SQ7, and 12AX7. When it got here, I pulled the tube covers off and it had 3X6EU7s and one 5879 ; as far as I know, there is no schematic for this version. Not the end of the world, a fact of life w/ Gibsons. J

[rl1968]

Try this schematic at el34world. This was the best site for schematics that I found when restoring my Gibson Mercury I Amp.


http://www.el34world.com/charts/Sche...n/GA-77RVT.pdf

[kirk5443]

print

this print for this model is 100 % correct! thanks!!

Here is the schematic :



The amp chassis :



Inside (awfull layout...) :

[kman007]

I am currently working on a speaker replacement in this Vanguard Crestline, supposed to be a 1964 but a different style. Had the oil can reverb but was robbed.

What ohms rating speakers should I use? What is the resistance rating of the output? Anyone know and can help? [/ATTACH]