Gibson GA-30RVT Invader with 6v6s schematic

[glebert]

I've got a chassis for an 8 tube Invader that I thought was the same as the Epiphone EA-12RVT, which had 7591s. It seems like it is wired for 6V6 or 6L6 pinout, I'm thinking it was probably a mod, but I've read that there were early models that had 6V6 tubes. I know Gibson was pretty notorious for changing amp designs and not releasing schematics for them but it would be interesting if someone had a schematic with 6V6s.

If this is one that someone converted to 6V6 that seems like it will be pretty low gain because of the lower transconductance vs the 7591.

[Mars Amp Repair]

Hello glebert,
I'm not an amp modder, so I'm sure someone on this forum can give you better information. I found this below posted on another forum. I'm not sure of it's accuracy, but thought I'd throw it in...perhaps I should have thrown it out!!!
glen
"7591s is not compatible with 6V6, 6L6 or EL34 as plug & play. The EL34s will burn out the transformer filament winding with double the current draw alone. The 6V6s will be subject to severe over voltage plus likely melt the 6V6 plates and the EL34 & 6L6 will be biased too low burning out the power transformer and likely the audio transformers. All in all real ugly business.

Modifying the Fishers for 6L6GB is doable, but tube height will likely interfere with a cabinet. The filament draw for four 6L6GB tubes is .4 amps higher. I am not sure if enough reserve is available for the additional current draw, but likely ok in the Fisher receivers that support the tuner filaments. Changing dial lights to LED would reduce overall current draw. Readjusting the bias to about -24 volts for 60ma idle current per pair of 6L6s will be required with the screen voltage lowered to 250 volts using a voltage regulator. I have not performed the engineering for this proposed mod, but the fore mentioned concerns come forth immediately".

[g1]

It's probably worth comparing the output section to the 30RV which has 6V6's. Schematic is in the Gibson Master service manual, where you may also find others that are close.

[J M Fahey]

I've got a chassis for an 8 tube Invader that I thought was the same as the Epiphone EA-12RVT, which had 7591s. It seems like it is wired for 6V6 or 6L6 pinout, I'm thinking it was probably a mod, but I've read that there were early models that had 6V6 tubes. I know Gibson was pretty notorious for changing amp designs and not releasing schematics for them but it would be interesting if someone had a schematic with 6V6s.

Here you have an early GV30, without the fancy letters, with 6V6, from1948


It claims 14W output, vs. later "letters" 25W so those quite probably use 7591 at significantly higher voltage to get claimed 25W

That said, what is significantly higher votage?
It might still be within modern 6V6 ratings.

I guess (may be wrong) that there is a Russian tube which has been sold both as a beefed up/premium 6V6 and as a low end 6L6.

That one "should" stand your amp nicely.

I fact I suspect the "reissue" 7591 *is* such a tube, relabelled as such, and charged twice the price.
The cheapskate in me would just buy a Russian 6V6.

If this is one that someone converted to 6V6 that seems like it will be pretty low gain because of the lower transconductance vs the 7591.

Maybe somewhat lower gain, doubt it woukd be "significant"
Probably 1 or 2 extra nothes on the volume knob.

Maybe not even that, since modern pickups are often stronger than older ones.

[glebert]

Hello glebert,
I'm not an amp modder, so I'm sure someone on this forum can give you better information. I found this below posted on another forum. I'm not sure of it's accuracy, but thought I'd throw it in...perhaps I should have thrown it out!!!
glen
"7591s is not compatible with 6V6, 6L6 or EL34 as plug & play. The EL34s will burn out the transformer filament winding with double the current draw alone. The 6V6s will be subject to severe over voltage plus likely melt the 6V6 plates and the EL34 & 6L6 will be biased too low burning out the power transformer and likely the audio transformers. All in all real ugly business.

Modifying the Fishers for 6L6GB is doable, but tube height will likely interfere with a cabinet. The filament draw for four 6L6GB tubes is .4 amps higher. I am not sure if enough reserve is available for the additional current draw, but likely ok in the Fisher receivers that support the tuner filaments. Changing dial lights to LED would reduce overall current draw. Readjusting the bias to about -24 volts for 60ma idle current per pair of 6L6s will be required with the screen voltage lowered to 250 volts using a voltage regulator. I have not performed the engineering for this proposed mod, but the fore mentioned concerns come forth immediately".

Thanks for the info, I think I had found similar info in various places. The pinout for 6x6 and 7591 are different, so definitely not a direct sub. Mine has the plate voltage coming in on pin 3 and -48v bias voltage on pin 5, so no way it is set up for 7591 (also not cathode biased like the GA30 above). I know 7591s were really hard to get for a while, so perhaps this was modded in that era.

With the fairly high plate voltage (over 450 unloaded) I decided to try it with some spare 6L6 tube I had, and heater current for those should be just 100mA higher than 7591 so I'm not too worried about it (but I do need to check that heater voltage). Powered up on LBL and variac and I get some white noise out of the power tubes. Put a 6EU7 in the PI and the noise level goes up, so it seems to be at least partially functional. Oddly enough I was ready to scrap this amp and now I really want to see if it can turn into something cool or if it is just a turd.

This is the schematic I've been using, but now have to check for the mods/changes. This is the schematic that people say is accurate(ish?) for the 8 tube version of this amp.

ea_12rvt-futura.pdf

[glebert]

I did find a schematic for 1963 GA25RVT that has 8 tubes and 6V6 power tubes. My gut is that my amp lies somewhere in between these, either from the factory or because of modifications. The tubes are the same type, but the 12AU7 is in a different place. For anyone working on old Gibson amps I highly recommend making sure you know which socket is wired for which tube. For 12au7 vs 6eu7 the heater wiring is a pretty easy tip off.

Got the amp making sound end-to-end (with more dead spots than working on the volume pot). Reverb and tremolo are really cool sounding.