VOX AC4 BLONDE  1960/61

Inside the
Vox AC-4
(Serial No. 2752)

Cabinet & fittings
Tremolo Pedal
Electronics - chassis
Electronics - circuitry


This section provides a detailed description of VOX AC-4 Serial Number 2752.  NOTE  the logo position this was factory fitted....we all have off days!


I won't bore you with the detail, but the steps which I followed were:


The cabinet covering is of a beige-coloured oatmeal pattern.  Overall condition is very good, with only minor scuffs to show for the amp's 40 years.  The cabinet front/speaker baffle, which is attached by nuts and stud bolts, has an outer surround of white plastic piping (stapled), a gold half-round strip and brown speaker cloth with red, green and gold threads.  After a gentle wash in weak detergent solution the colour of the speaker cloth brightened quite a bit, but couldn't overcome the effects of fading over time.  Under the cloth is a layer of fine black hessian.  The logo is fixed centrally and is of a one-piece moulded plastic pattern, measuring 2.75 by 0.875 inches; there are no marks to indicate that it was ever mounted at the 'conventional' left-hand side.

  On the top  of the cabinet there is: a white moulded handle (held by brass-plated alloy brackets), two copper- coloured alloy vents, the brown control panel and (common practice in those days) the original vendor's label.  The control panel has 2 input jack sockets and black 'chicken head' pointer knobs which control volume, tremolo speed and combined tone/on-off.
Label reads:
The Band Box, Wolverhampton Limited
28 Snow Hill
Phone 21420
  At the rear of the unit, two removable panels are secured by brass woodscrews set in cup washers.  The upper panel protects the electronics and bears a standard Jennings Musical Industries Ltd. plate, which is in excellent condition.  The Serial Number 2752 is stamped on the plate - no number appears on the chassis. Chalked initials 'LB' are written inside the cabinet. 

The upper panel also holds in place an internal horizontal plywood baffle which prevents access from below.  All internal surfaces are painted matt black. 

The lower panel forms a stowage compartment for the mains lead and tremolo foot pedal. 

In the middle sits the 8 inch 3 ohm Elac loudspeaker, secured by 4 nuts, washers and stud bolts. 


  Side view showing the Elac label.  I believe that the Rola-Celestion range, later adopted by Vox, was geared to the radio and public address industries pre-1960 and then started to appear in guitar amps.
  Solder tag connections and three printed numbers; 8C/164, D133489 and 03359 (on the cone).  Is there a date code in there somewhere?


  A simple varnished wooden base, push-on/push-off single-pole switch and a crude logo of rub-down Letraset (TM) characters.


The amp chassis is of simple L-shaped construction in thin steel plate, secured to the cabinet by 2 bolts.  Because the AC-4 chassis has to be removed to change valves (gynaecologists excepted), repeated over-tightening of the securing nuts can bend the metal.  This is how it was on first inspection, so I fitted 2 hardwood strip supports under the nuts when re-assembling.

The following photos were taken after the necessary repairs had been carried out, using original 1960's British parts salvaged from a variety of junk equipment over the years.

  Valve complement: is: EZ80 rectifier, ECC83 tremolo oscillator/buffer, EF86 pre-amp and EL84 output.  Valves are secured in McMurdo sockets by simple wire retainers - no screening cans.  Sockets are marked 80, 86 etc. in black paint.
   The mains transformer (left) has the part number 'RT.290' and the logo of 'JD Electronics (Birmingham) Ltd.'  The tiny output transformer is marked '5 200 3.'
  When I viewed the 'works' for the first time, I was appalled by the quality of the wiring, much of which was (and remains) bare and uninsulated. In addition, an incompetent amateur had replaced the original tone control with an un-switched component and added a toggle switch for the mains supply, mercifully out of sight on the internal baffle panel.


Preamplifier V1 uses an EF86 low-noise high-gain pentode in a conventional voltage amplifier arrangement, with the output voltage developed across R5.  Cathode auto-bias is provided by C1 and R4, and the screen grid voltage (pin 1) is decoupled by C4.

The amplified signal at V1anode is coupled through C2 to VR2, the volume control potentiometer.  VR1 and C3 form a  'treble cut' tone control network, which shunts more and more of the higher frequencies as the resistance of VR1 is decreased.  This simple arrangement was used in most wireless sets in the valve era, but was usually placed across the loudspeaker primary.

The wiper of volume control VR2 is connected to the control grid of output stage V2 via R8, which prevents the control grid drawing excess current if driven positive, with respect to the cathode, by large signals.  The single-ended Class A power output stage drives the 3 ohm 8-inch Elac loudspeaker via output transformer TR1.

V3A is connected as a low frequency phase-shift oscillator, with C9-11, R15, R16 and VR3 forming the critical phase-shift network.  The output frequency will be that at which the foregoing components apply exactly 180 degrees shift i.e. perfect positive feedback.   VR3, the tremolo speed control, allows variation of the output frequency.  Foot-switch SW1, in the closed position, applies an earth to the V1A control grid circuit, and oscillation stops.

The output of V3B is connected to cathode-follower V3A, which buffers V3B's high-impedance output and applies the low frequency tremolo voltage to the cathode circuit of V1, thus varying the valve's bias voltage and modulating the guitar signal.

The HT output of full-wave rectifier V4 (EZ80) is smoothed by reservoir capacitor C7B and filter components R10 and C7A which reduce the AC ripple component (100Hz in UK).  The HT supply for V1 has additional anode decoupling (R7/C5), to remove HT variations due to the output stage.


The circuit diagram has been annotated to show the test points at which readings were taken.  Using a DVM (10M ohm impedance) , DC values were recorded with the signal input shorted & the tremolo switched off.  To obtain undistorted signal levels, the amp was tested with an audio signal generator connected to one input jack and set to 50mV peak-to-peak @ 800 Hz.  All readings were taken with the controls fully clockwise.  At the time of measurement,  the AC mains voltage was measured at 245v/50 Hz.


Number DC Level Sig pk-pk   Number DC Level Sig pk-pk
1 - 36mV   9 - 2V
2 228V 2V   10 324V -
3 76V -   11 - 280-0-280V AC
4 263V -   12 2.9V -
5 - 2V   13 167V 140V 6.5 Hz
6 7.9V -   14 164V 140V 6.5 Hz
7 272V -   15 1.6V -
8 252V 100V   16 25V -

The input signal level was then increased until distortion became visible on an oscilloscope connected across the loudspeaker.  The input level was then measured at 150 mV and the output was 2.05V rms (= 1.4W) into 3 ohms.


   BY BILL ACTKINS     2002