Vox AC30: the Treble circuit
Introduced in mid 1964
The "Treble" stamp in its normal position on the right-hand side of the aluminium preamp assembly.
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 some months earlier, however - starting mid way through the run of serial numbers in the 11000s.
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-François 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 11452T (previously 11556T):
Currently on Reverb. Hand-stamped serial number plate still. The Celestion blues have date codes "30KH" = 30th October, 1963. One of the blue Hunts capacitors has the date code "WII" = 13th week of 1963. Copper panel, no markings on the preamp upright.
The preamp of serial number 11556, however, is built according to the "Normal" circuit. Particular give-aways are the values of two capacitors on the far right-hand side of the top preamp circuit board - on the schematic "C5" and "C7". In serial number 11556 these are: 0.047uf:
Detail of serial number 11556. One can just see on the two gold WIMA capacitors the voltage rating (400), and the value 0.047.
At this point the "Treble" schematic calls for two 0.01uf capacitors (also expressed as 10000pf - not 1000pf as stated here initially).
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).
The only way to single out true "Treble" AC30s is from an examination of the circuit. Not all "Treble" chassis have "Treble" stamped in red on them.
The earliest "Treble" chassis has yet to be identified; safe to say, however, that it will have a serial number in the 11000s.
Perhaps an even better way to spot a Treble model is by the value of the coupling caps C6 and C9 after the phase inverter, 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, do let me know.