GIBSON: GA30 Schematic

GIBSON GA30 GUITAR AMPLIFIER 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

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: GA40 Schematic

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

GIBSON GA-40 GUITAR AMPLIFIER SCHEMATIC

VOX: Vintage AC15 Schematic

Ignoring the gigantic and overly complicated trem section - this is one of my favourite sounding AC amps, which is most likely the inspiration for the Matchless DC30 pentode channel.

VINTAGE VOX AC-15 SCHEMATIC

Version:  NO V-1-5 / OA/031

Apparently this schematic is a v3, redrawn by Thomas Organ, with some different component numbering.   I can understand why they redrew it - a lot of people, including me, struggle with some of the original Vox schematics.   It appears that they may have done the same thing for the AC10 (which practically shares the entire output stage as the AC15, with a couple of minor differences)

Below is the original, which isn't the most confusing Vox schematic that I've ever seen, but I still prefer the one above. 

SUMMARY

In the Normal channel, the AC15 is an EF86 voltage amplifier feeding a volume control, which then drives an ECC83 long-tail pair phase inverter and a cathode-biased EL84 push-pull output stage.

It's nice and direct, with limited tone shaping along the way - there is of course the bright switch, and the traditional Vox Top Cut, but apart from that, this is a fairly bare bones signal path.

I'm not going near the trem channel - just too complicated for me, and I'll never build one.  

Inputs: 

• Two jacks wired High / Low 

• Each jack feeds the EF86 grid through a 68k series resistor (R5, R6)

EF86:

• Grid leak: 1M to ground (R1)

• Plate load (Ra): 220k (R15)

• Plate supply / filtering: from +315V through 22k (R14) to the EF86 supply node, filtered by 8uF (C3). Plate voltage is marked ~90V on the schematic

• Screen supply: 1M (R20) feeding the screen, with 0.1uF (C8) bypass to ground

• Cathode: 2k2 (R13) bypassed by 25uF (C5)

• Coupling cap to volume: 0.01uF / 10nF (C12)

Volume, Brilliance, Top Cut

• Volume pot (Normal): 500k (R29)

• Brilliance switch: SW3 inserts a 250pF capacitor (C17) as a “bright” bypass around the signal feed, letting extra treble through when switched on

• Tone: Vox “Top Cut” placed after the phase inverter — a 250k pot (R49) with a 0.005uF / 5nF capacitor (C36) working across the two opposite-phase drive signals

Phase inverter (ECC83 / 12AX7)

• Type: Long-tail pair (LTP) using both triodes (V5A, V5B)

• Normal volume feeds V5A grid via 10nF (C27)

• Trem channel volume feeds V5B grid via 10nF (C28)

• Plate loads: 100k each (R44, R48)

• Supply to PI node: from +315V through 22k (R43), filtered by 8uF (C26). PI node is marked ~220V

• Shared cathode / tail: 1k2 (R42) in series with 47k (R38) to ground

• Coupling caps to power stage: 10nF each (C32, C33)

Power stage (2 x EL84 / 6BQ5, push-pull, cathode biased)

• Grid stoppers: 1k5 each (R56, R57)

• Grid leaks: 220k each (R53, R54)

• Screen resistors: 100 ohm each (R62, R63)

• Shared cathode resistor: 130 ohm, 5W (R60)

• Cathode bypass: 50uF (C39)

• Plate supply is marked ~310V

Output transformer

• Secondary taps: 15 ohm, 8 ohm, and common

• Primary: 8k p-p

Rectifier and power supply (main)

• Rectifier: EZ81 (6CA4)

• Filtering: 16uF reservoir (C10) → choke L1 (10–20H) → 16uF smoothing (C15) to the main +315V source

Speakers

• A few variations of single and twin speakers - all 12" 

• Goodmans Axiom, Fane and of course Celestian blues

EL84 OPERATING CONDITIONS

Operating conditions are similar to the EL84 datasheet. 

EF86 OPERATING CONDITIONS

The preamp circuit is almost identical to the EF86 datasheet

VOX: Vintage AC6 Schematic

The Vox AC6 is a departure from the Vox AC2 & AC4, in that it has a 12AX7 preamp, negative feedback, no trem, and for some reason, the tone control is before the first triode - it's a bit of an odd-ball in the Vox lineup from the time. 

The 12AX7 preamp arrangement would give the amp a different flavour to the EF86 versions, but it's still an EL84 driving an alnico speaker in about the same size cab, so similar to its friends in that sense.

Further reading:  VoxAC30.org.uk   

VINTAGE VOX AC6, GUITAR AMPLIFIER SCHEMATIC

SUMMARY

In simple terms, the circuit uses two 12AX7 triodes feeding a volume control, which then drives the EL84. The first triode is cathode-bypassed, the second is unbypassed and receives negative feedback from the output transformer, and the tone control sits ahead of the first triode.

Some of the schematic values are hard to read, so hopefully this is close.  That said, apart from the unusual placement of the tone control, the 12AX7 stages are biased cooler than a typical Fender-style preamp.  

This cooler operating point doesn’t so much reduce gain as it shifts the clipping behaviour, with the stage tending to run out of headroom on one half-cycle first, giving asymmetrical clipping when driven.

The 220 kΩ input resistors are also a little unusual.  As drawn, they don’t just act as isolation resistors — they form a voltage divider with the 1 MΩ grid leak on the first triode.  The result is a small amount of input attenuation, so some signal level is lost before the first gain stage even begins to amplify.

The 12AX7 Preamp

Inputs:  

• Two jacks.  

• Each jack feeds the first triode's grid through a 220k series resistor

First Triode:

• Grid leak: 1M to ground

• 12AX7 plate load (Ra): 100k

• Cathode resistor (Rk): 4k7

• Bypass cap: 25uF, 50v

• Coupling cap to next triode: 0.01uF (10nF)

Second Triode:

• Grid leak: 470k to ground

• 12AX7 plate load (Ra): 100k

• Cathode resistor (Rk): 4k7

• Negative feedback from the output transformer to the cathode, 47k series resistor

• Coupling cap to volume pot: 0.05uF (50nF)

Volume & Tone control:

• Volume pot: 500k (wiper feeds the EL84 grid via a 4k3 grid stopper)

• Tone: 250k pot with a 0.005uF (5nF) cap in series to ground from the grid of the first triode

Power stage: EL84 (6BQ5) single-ended, cathode biased

• Grid stopper: 4k3

• Grid leak: via the volume pot

• Cathode resistor (Rk): 150 ohms (2W or 3W)

• Cathode bypass:  25uF, 50v

Output transformer:

• Secondary: 3 ohms

• Primary: Probably 5k or 5.2k, which was common with Vox at the time

Rectifier: EZ80 (6V4)

Power transformer:

• HT secondary: 250V-0-250V (as shown)

• Heater winding: 6.3V

• Primary shown with 250v, 230v and 85v

Power supply filtering:

• 32uF reservoir (C1) → 1k (8W) series resistor (R3) → 32uF (C2) main B+ node - likely to be 270v

• From +270V:  47k dropper (R5) → 8uf feeding 12AX7

Speaker: 

• Elac 8" alnico, 3 ohm

VOX: Vintage AC2 Schematic

Another early Vox model is the AC2.  It’s often described as being the same as an AC4, but there is one key difference — the tone control.  

Instead of sitting in the preamp like the AC4, the AC2’s tone control is wired in the power stage, between the EL84 plate and the B+ supply (effectively across the output transformer primary).  

It obviously works, but no one does this anymore - the tone control is sitting at just short of 300 volts DC, which isn’t ideal for a number of reasons.  Curiously, they chose the opposite direction for the AC6, with the tone control before the first triode of the preamp.    

Further AC2 reading:  AC2 on VoxAC30.org.uk 

VINTAGE VOX AC2, GUITAR AMPLIFIER SCHEMATIC