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