Vox AC30: the Treble circuit

Introduced in Spring 1964

Detail of serial number 11613T, chassis number 3083. The "Treble" stamp in its normal position on the right-hand side of the aluminium preamp assembly, and the splash of red paint for visual identification - see further .

The existing schematic for the "Treble" model was drawn out on 10th September 1964 (OS/056), the day before the "Normal" and "Bass" circuits were newly drawn out.

A detail of OS/056 - "For Works Use Only", the circuit checked by Dick Denney.

"Treble" model AC30s had been put into production much earlier, however - probably in the first months of 1964, starting early in the run of .

As Glen Beauchamp has pointed out, JMI seems not to have advertised this version of the amplifier. We have entries for the Normal, Bass, Top Boost, and the various types of Super Twin in catalogues, pricelists, and so on, but nothing of the Treble. Was it left to the salesmen to lead customers to the model - and perhaps to explain the significance of "T" stamped on the serial number plate?

The key changes in relation to the "Normal" circuit are few but effective - principally the reduction in value of a group of coupling capacitors with a view to allowing more treble through.

The 0.15uf caps after the phase inverter (C6 and C9 on the schematic) in the AC30/6N and AC30/6B were reduced to 0.047uf (47000pf).

And the 0.047uf caps (C5 and C7) on the grids of the valve were reduced to 0.01uf. As Pierre-Fran├žois has kindly pointed out, these mirror the 0.01uf caps in the same position in the AC15 (schematic OA/031, drawn on 29th April 1960), and in the AC30/4 of the same date.

The lowest serial number (by quite some margin) to have surfaced so far for a "Treble" AC30 is 11066T.

AC30 Treble, serial number 11066T.

Below, a short overview of Treble chassis numbers (not serial numbers) and component date codes. Early 1964 is clearly in view. If one goes by the latest dated element - the Woden transfomer dated February 1964 - then early Spring 1964. Quite whether further precision will be obtainable seems unlikely. As mentioned above, JMI issued no special promotion for the new model.

2901 - Pots "KK" = November 63. Wodens KU" = October 63.

2986 - No component date codes known at present.

3002 - Woden output "MU" = December 1963.

3018 - Hunts cap with "TDI" = 48th week of 1963. Wodem mains "MU" = December 1963; output "KU" = October '63.

3053 - Hunts cap. 48th week 63. Wodens "KU" and "MU" = October and December 63.

3083 - Hunts cap. 45th week '63. Wodens "AV" and "BV" = January and February 64.

3086 - No component date codes visible.

3088 - As above, no component date codes visible.

3094 - Wodens "HU", "JU" and "MU" = August, September, December 63.

3098 - Hunts cap with "TDI" = 48th week of 1963.

3104 - Wodens "JU" and "MU" = September and December 63.

3265 - Pots "KK" = November 63. Hunts cap. 47th week 63.

3332? - Woden mains "MU" = December 1963; output and choke "AV" = January 1964.

3401 - Woden mains and choke "AV" = January 1964.

3443 - Welwyn resistor "UG" = July 1963; Wodens "AV" = January 1964.

3449 - Woden mains and output "AV" = January 1964. Paint and treble stamp mostly scratched away.

3450 - Hunts cap 41st week 1963.

3465 - Woden mains "MU" = December 1963; choke and output "AV" = January 1964.

3545 - Woden mains "AV" = January 1964; main preamp filter cap. "TDI" = 48th week 1963.

3565 - One Woden certain "AV" = January 64.

3571 - Wodens: mains "MU" = December 1963; output and choke "AV" = January 64.

3582 (?) - Wodens "AV" = January 64.

3592 - Wodens "AV" = January 64.

At present, all chassis currently known in the range 2901-3592 are TREBLE. It will be interesting to see if the pattern can be broken. Either side of this range, evidence is a little fragmentary at the moment though. Chassis 2685 is a BASS, as is chassis 3889.

The presence of a red stamp on the preamp chassis is always a good indication that the amp is indeed a Treble model. But the only way to be be really sure is to check the electronics..

1) The two 0.01uf capacitors (also expressed as 10000pf) in the Treble circuit diagram - C5 and C7.

C5 and C7 marked on the AC30 "Treble" circuit diagram.

Below, serial number 16649, marked as "Treble" on serial number plate and chassis. The mustard capacitors are of the correct value - 10000pf (0.01uf).

Detail of serial number 16649. The 10000pf caps.

2) Further indicators of a Treble model are the presence of 330K resistors and 220pf capacitors at the volume controls, as marked in green on the picture below. The resistors lower the output from the volume controls (in comparison with other models) at all settings - a means of preserving the treble-enhancing qualities of the 220pf capacitor as volume is increased. Thanks to Brian for his help with this section.

The 330k resistor and 220pf at the volume control of the Brilliant channel. One can just make out the 330k and 220pf for the Normal channel to their left a little below.

Detail of serial number 11613.

3) Another means of identifying a Treble model is by the value of the coupling caps C6 and C9 after the phase inverter. They are positioned on the right hand side of the lower preamp tagboard: 0.047uf (47000pf):

Detail of the AC30/6 "Treble" model schematic - C6 and C9 marked.

Serial number 16649 again - Mullard mustard 47000pf coupling caps followng V2. B/64 indicates that the capacitors were produced in the second quarter of 1964.

If anyone knows of further details that would be useful on this page, .