Simple mod for Gibson GA-19 RVT to bring amp to life

[SoulFetish]

Of course, the GA-19 RVT version I was working on uses 6EU7s in the preamp with a 6C4 7-pin triode as the reverb driver.

(see schematic below which includes changes.)

A customer brought us the amp to be fitted with an Earthed 3-cond. AC cord, and general service. He complained that the amp lacked low end and asked if there was anything we could do to improve the overall gain and bass response, without drastically changing or modifying the amp (I think he was shy about footing the labor charge that would accompany anything to drastic. The general service on a 65 year old amp is time consuming enough).
After listening to the amp and looking at the schematic, I had a couple of ideas which were really simple to implement and wanted to try. Really nothing groundbreaking, but I love the result. Even into the original inefficient stock speaker.

The first was to remove the lossy tone network following the first stage coupling cap. Even with that removed, the -3dB roll off of the .001uF with a 1M load (2 x 2M pots) is 159Hz.
Changing the .001uF to .0047uF brings the HPF down to about 34Hz. Plenty of bass for guitar.
The last change was to solder a 56k resistor in parallel with the 100k grid stopper for input #1. It's a modest change but subjectively I think there's a bit more bite.


(edit: new schematic updated with RC network (R9,R10, C5, & C7) removed, and highlighting modifications with the additional change to C22, increasing the value from 500pF to .002µF as discussed below)









This is the original schematic and circuit.
https://el34world.com/charts/Schemat...on_ga19rvt.pdf

[mozz]

What about that 500pf?

[SoulFetish]

What about that 500pf?

the 500pF feeding the phase inverter benefits from the bootstrapped input of the Cathodyne PI. The voltage at the junction of the 470k grid leak and 100k cathode resistor follow the grid, the effect of this positive feedback essentially increases the the grid leak impedance to approximately Rg / (1-A). ~according to Blencowe, this can exceed 10M
Even if we take a conservative guess of an effective 7M value for Rg, the 500pF rolls off at about 45Hz. Having said that, He makes a good case for using larger capacitor values here to keep the input impedance from rising high enough to become overly sensitive to hum.
So, replacing the 500pF cap here with even a .001-.002µF cap would probably do the trick I suppose.
I'm glad you brought it up.

[SoulFetish]

Oh yeah, one thing I would seriously consider is in the event that the 6C4 needs to be replaced, would be to not replace the 6C4 at all. They are miserable, noisey little shits by and large, and it's a PITA to buy enough of them to find one that isn't. I would probably substitute it for a Mosfet reverb driver, or keep the 7-Pin format and use a 7-pin pentode tube.

[Leo_Gnardo]

I've worked on a number of these and similar Gibson and Epiphone amps. 40+ years ago we used to think they were tin-horn junk, they certainly sounded it. Loaded with problems and potential problems, don't expect Fender tone outta these. All these years later, turns out they can really be gems! Besides the usual filter caps needing complete replacement, I find some of the cathode bypass electrolytics to have lost their value, similar for interstage caps typically discs. Lots of drifted and noisy CC resistors too, items not to be ignored when having a go at any old amp. It's wonderful to see the owner's surprise when their dusty relic shows it can roar with the best.

[Chuck H]

The first was to remove the lossy tone network following the first stage coupling cap.

Are you talking about that weird Gibson modular component with the T filter circuit inside? I don't see it in the posted schematic. Was it in the amp but not shown in the schematic?

[SoulFetish]

Are you talking about that weird Gibson modular component with the T filter circuit inside? I don't see it in the posted schematic. Was it in the amp but not shown in the schematic?

yeah. Look at the bottom schematic.

edit: that's weird. It won't save the uploaded jpg of the original schematic. I'll post a link under it.

[Chuck H]

I remember an old GA model I had for a short time. I took out that T filter and it sang like a cello. Not an agressive overdrive but nice and smooth with a bristly top end. Not exactly "my" sound but it begged to be experimented with musically I hope you get that from this amp.

[g1]

yeah. Look at the bottom schematic.

edit: that's weird. It won't save the uploaded jpg of the original schematic. I'll post a link under it.

Still not showing. You may be seeing it because of your cache, but it gives 'invalid file specified' message. I assume it is the drawing that was originally up with the red X's at the filter components?

[SoulFetish]

Still not showing. You may be seeing it because of your cache, but it gives 'invalid file specified' message. I assume it is the drawing that was originally up with the red X's at the filter components?

Here is what it should say:

"(edit: new schematic updated with RC network (R9,R10, C5, & C7) removed, and highlighting modifications with the additional change to C22, increasing the value from 500pF to .002µF as discussed below)




This is the original schematic and circuit.
https://el34world.com/charts/Schemat...on_ga19rvt.pdf
"

when I click the post to edit, it shows up as the edited post I previously saved. but it's not updating the thread.