GIBSON: GA-30 Schematic

GIBSON GA-30 GUITAR AMPLIFIER SCHEMATICS

This is one of the GA-30 amps - there were a few by the sounds of it.

While they share a similar topology, there are clearly some differences from the one above.

Apart from changing the circuit, their appearance also varies over time - I'm not going to try and document them all, and I'm by no means an expert on the topic.  

Text below is for the first schematic 

6SJ7 MIC PRE

Grid leak 1m

Grid stopper (none)

Ra 470k

Rk 2k2 20u bypass 

Screen 2m2, 50n bypass

10n coupling cap to 500k volume pot.  100k mixing resistor to PI

6SJ7 INST PRE

Grid leak 470k

Grid stopper 47k (forms a voltage divider with the grid leak)

Ra 220k

Rk 2k2 (not bypassed) 

Screen 2m2, 50n bypass

20n coupling cap to 500k volume pot.  100k mixing resistor to PI

470k NFB from OT secondaries, 700pf in series with bypass switch 

6SN7 PHASE INVERTER

Ra 100k on each plate

Rk 1k shared (not bypassed)

50n coupling to 6V6

220k / 4k7 pair to PI grid

Tone 2m pot, mixing 250pf and 50n to grid of PI input 

6V6

220k grid leak

Rk 250 ohms

No grid stoppers or screen resistors