VOX: Vintage AC4 Schematic

Despite the name - and even the familiar look - the original Vox AC4 has very little in common with the modern “AC4” amps wearing the same badge.  The cabinet and the single EL84 output stage are about all they share.

The AC4 started life as the AC2 in 1958.  Fair to say that the AC4 is probably best known due to its modern counterparts, despite being a different beast.

The circuit, layout and feel are very much of their era — the preamp and power stage are almost text box examples from valve datasheets - and if you want the deeper backstory, the Vox Showroom & AC30.org both have an excellent history of this little amp.  

VINTAGE VOX AC-4 GUITAR AMPLIFIER SCHEMATIC

SUMMARY

It's a classic single-ended amp design, point-to-point construction, valve rectifier.  High gain from the EF86 firing pretty much straight through to the EL84, into an open back cab with an alnico speaker.   

There's very little loss in terms of low-end or with almost no frequency shaping along the way (at least before it hits the output transformer and small alnico speaker).  The tone control just rolls off some treble; no other shaping takes place. 

There is one unusual thing about the preamp - a huge 5.6meg screen resistor is taming the EF86: it starves the screen to drop gm and gain, improves linearity, and when you push it the screen current dynamics produce that soft ‘screen compression’ feel pentodes are famous for.  By comparison, 1meg is the datasheet value when paired with Ra 200k.

The trem wiggles the bias of the EF86 (cathode coupled), which is a fairly typical design from the era, not unlike a Vibro Champ.

The EF86 Preamp

Inputs:  Two jacks.  Each jack feeds the EF86 grid through a 100k series resistor

Grid leak: 1M to ground

EF86 plate load (Ra): 220k

EF86 screen supply: 

• 5.6M (Rg2) from the preamp B+ node

• Screen bypass: 0.1uF (100n) to ground

EF86 cathode:

• Cathode resistor (Rk): 1k5

• Bypass cap: 25uF

• Cathode voltage: ~2.7V

• Trem oscillator connects to the cathode

Coupling cap to volume/tone network: 0.047uF (47nF)

Volume & Tone control:

• Volume pot: 1M (wiper feeds the EL84 grid via a 6.8k grid stopper)

• Tone: 1M pot with a 0.001uF (1nF) cap in series to ground from the signal node (treble-cut)

• Power switch is shown as part of the tone control assembly

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

• Grid stopper: 6.8k

• Grid leak: via the volume pot

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

• Cathode bypass:  25uF

• Cathode voltage: ~8.5V

Output transformer:

• Secondary: 3 ohms

• Primary: 5k / 5.2k as stamped on output transformers 

Rectifier: EZ80 (6V4)

Power transformer:

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

• Heater winding: 6.3V, 2A (as shown)

• Primary shown with 230V / 115V options via selector plug

Power supply filtering:

• 32uF reservoir (C1) → 1k (5W) series resistor (R3) → 32uF (C2) main B+ node labelled +270V

• From +270V:  22k dropper (R5) → 8uF cap, ~260V node feeding the EF86 & 12AX7

Vibrato oscillator: ECC83 (12AX7)

• Cathode resistor: 3.3k with 25uF bypass (cathode marked ~1.7V)

• Speed pot: 1M

• Coupling caps shown: 0.02uF and 0.01uF, plus 0.01uF to ground in the network

• Footswitch shown to switch vibrato on/off

Speaker: 

• Elac 8" alnico, 3 ohm, sometimes Goodmans

EL84 OPERATING CONDITIONS

Operating conditions are similar to the EL84 datasheet.  I included both the 5.2k and 7k primary impedance conditions, as some modern transformers offer both options (or at least 5k and 8k).   

Many AC4s had transformers stamped with 5000, or 5200, indicating their primary impedance.  Given how common EL84 based amps would have been at the time, a stock transformer from any number of manufacturers would have been readily available.

EF86 OPERATING CONDITIONS

The circuit is very similar to the EF86 datasheet, but that massive screen resistor is hardly textbook.