Gibson GA-50T Tube Amp What to Replace Debate...

[Crumbs]

Okay still on the slow path. I've had distractions which have prevented me from working on it for awhile. I've been building out a DIYLC Layout for the tag board I bought to try to make maintainability easier. Trying to minimize jumper paths. When I've got that done I might post it for suggestions.

Current problem
the schematic doesn't tell me how it is wired, especially not after 75 years of undocumented mods. I've been documenting connections as I disassemble in case I want to revert.

I've found an odd connection. If you look at R18, P3 in the tremolo circuit lower left it shows 9.1k R18 between one leg of 1k P3 potentiometer to V7A pin 3 cathode. Instead R18 is wired pin 3 of P3 to pin 1 of lower P1 270k pot with the P3 wiper not wired to ground but instead to V7A pin 3. Obviously there were other tremolo circuit changes but curious if anyone knows why someone might do this.

P3 is curious in general. There are only 6 knobs on the front but 7 pots. P3 is attached to the back of P1 the tremolo frequency adjustment. When I put this back together how do I know what the P3 resistor set point is? I assume it is based on oscillation harmonics but having never built (or investigated) a tremolo circuit would like guidance on figuring out how to set it before reassembly.



Schematic source: https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga50t.pdf found on https://el34world.com/charts/Schematics/files/Gibson/Gibson_Schematics.htm. Was used to create overlay in post #55 which I've repeated here with more annotations

 

[Mr Boochie]

I have not worked on this amp. But I can tell you during these production years — it was not uncommon for Gibson or Ampeg to pull amps off the assembly line and modify them in-house. They never annotated the stapled schematic, owner's, or service manuals. Techs would sit on the conveyor belts, plug their guitars in, and play the amps. Then, tell the engineers, "The amps need this or that." There's a famous photo of Dennis Kager doing this. It's how he got started as an amp builder.


Working on these vintage units requires some head-scratching. I always restore them to the state I found them (old part values and placement), not to the generic schematic.

Merle Travis played through the GA-50T in this well-known song using his famous Gretsch. It's a historic amp. I hope you get it back in working shape.

 

[Mr Boochie]

Okay still on the slow path. I've had distractions which have prevented me from working on it for awhile. I've been building out a DIYLC Layout for the tag board I bought to try to make maintainability easier. Trying to minimize jumper paths. When I've got that done I might post it for suggestions.

Current problem
the schematic doesn't tell me how it is wired, especially not after 75 years of undocumented mods. I've been documenting connections as I disassemble in case I want to revert.

I've found an odd connection. If you look at R18, P3 in the tremolo circuit lower left it shows 9.1k R18 between one leg of 1k P3 potentiometer to V7A pin 3 cathode. Instead R18 is wired pin 3 of P3 to pin 1 of lower P1 270k pot with the P3 wiper not wired to ground but instead to V7A pin 3. Obviously there were other tremolo circuit changes but curious if anyone knows why someone might do this.

P3 is curious in general. There are only 6 knobs on the front but 7 pots. P3 is attached to the back of P1 the tremolo frequency adjustment. When I put this back together how do I know what the P3 resistor set point is? I assume it is based on oscillation harmonics but having never built (or investigated) a tremolo circuit would like guidance on figuring out how to set it before reassembly.


Schematic source: https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga50t.pdf found on https://el34world.com/charts/Schematics/files/Gibson/Gibson_Schematics.htm. Was used to create overlay in post #55 which I've repeated here with more annotations

1.) Gibson may have done those undocumented mods.

2.) Do the parts look to be the original age as the rest? If so, this may have been a Gibson mod. I would not change them. Companies tweaked reverb and tremolo circuits in those early days. There was no cookie-cutter production.

3.) Put the tremolo circuit back as you found it. Play the amp and go from there.

Good luck!

BTW: finding a good original Jensen P8R 8-inch alnico can be as tricky as the P12P 12-inch. The P12P is an excellent speaker. I hope those speakers are in good shape.

 

[gadget73]

not sure I understand what you're describing with the P3 pot stuff. Can you scribble it out?


as-drawn changing P3 changes the bias point on V7A, but what functional effect that has I don't know. Overall the circuit is a low frequency oscillator that mixes it's output with the input signal from the guitar. The feedback that makes it oscillate is C12 back to the grid of V7A, the double P1 pot looks like it would change the frequency, P2 changes the level. I'm guessing if P3 is set to far wrong it won't oscillate reliably but beyond that guess I don't know what the official procedure is for setting it. Might be as simple as making sure it starts no matter where the speed pot is set and tweaking it as needed.

 

[Destructor]

I hope you found a good home for the mouse.