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.  I'm just using these as a reference for a Bell & Howell conversion.  

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 

GIBSON: BR6-F Schematic

6SJ7 preamp, 6SN7 paraphase inverter, 2 x 6V6 with local negative feedback.  

GIBSON BR6-F GUITAR AMPLIFIER SCHEMATIC

Clearly a bit of work has been done on this one - just using the pic as an example of circuit construction.

Low impedance input

6SJ7 PRE

Ra 220k

Rk 2k2 20u bypass 

Screen 2m2, 50n bypass

10n coupling cap to 500k volume pot

6SN7 PI

Ra 100k

Rk 3k3 (not bypassed)

50n coupling to 6V6

100k / 10k pair to PI grid

6V6

100k grid leak

Rk 200 ohms

470k NFB from 6V6 plate to 6SN7 plate

No grid stoppers or screen resistors 

GIBSON: GA-40 Schematic

5879 pentode premap, which is an improved version of the E8F6 - uses a Vox style cut control across the plates of the phase inverter.  Paraphase inverter, driving 2 x cathode biased 6V6.

GIBSON GA-40 GUITAR AMPLIFIER SCHEMATIC