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