AC100 Mark 2s - fixed bias, brimistor in circuit

From August/September 1965 to end of days in early 1968, Jennings Musical Industries produced around 1000 AC100 mark 2 amplifiers. Although records do not survive, it is likely that 300-400 of these were sold as AC100 SDLs (Super de Luxe) for guitar, as below; and 400-500 for bass, either with a 2 x 15" speaker cabinet, or a 100W Foundation Bass unit (or two). Some amplifiers will of course have been sold separately for use with existing cabs.

Huge orders from America and Europe kept the production lines busy throughout the mid 1960s. The workforce grew accordingly. Reports of the major orders are being .

That numerous bands - including The Beatles - were seen with AC100s during the course of 1965 naturally did no harm at all, and thanks to canny marketing by Reg Clarke (Sales Director) and others at JMI, some venues - the , being one - simply became vast shop windows for the company.

Vox AC100 Mark 2, serial number 1561

A "put-together" AC100 SDL, 1966 style. Serial number 1561 with a cab belonging to another amp. But the end result is cosmetically the same as the original set will have been. Amps and cabs were wrapped and freight shipped in large cardboard boxes.


Most AC100 Mark 2s were assembled at the Vox / Burndept Works at Erith. A few, however, were made (with slight differences from the Erith-built amps) by Triumph Electronics in Purley. These have been signalled on the pages that follow, and have been .

Erith-made amps always have numbers stamped on the interior underneath the input jack sockets. Triumph amps have none.

Vox AC100 Mk 2, chassis number

The chassis number - 2029 - of a relatively late amp.

AC100 Mark 2 chassis numbers run from around 1400 to 2294 (the highest known to date), a good primary indication of how many amplifiers were assembled at Erith - around 900. Amps built by Triumph - probably another 100 - do not form part of this sequence.

These numbers do not go in step with the serial numbers on the back panel of the amp, however. Finished chassis were taken randomly from store for wiring up and fitting out on the assembly benches. Boxes with plates already affixed were then provided.

AC100 serial number 1857, for instance, was built on chassis no. 2258. Serial number 1903 has chassis 2210.

AC100 serial number 1857; chassis number 2258.

The machine-stamped serial numbers on the plates on the back panels of the wooden boxes were the "registration" if you like for the sales ledgers kept by Jack Jennings, Tom Jennings's brother. They run in sequence from around 730 to somewhere in the 2200s. Some however - probably a good many - were assigned to 100 watt Public Address amps (much as is the case with Vox AC50s).

From the point of view of electronics, the AC100 Mark 2 was pretty much identical to its immediate precursor the fixed-bias - the "100 Watt Amplifier" being the name on the circuit diagram .

The preamp of the AC100 mark 2 with two ECC82 and one ECC83, and the power amp with four Mullard EL34s, remained the same. Power output was a true 100W. The only cosmetic change was a new red warning plaque, which came in with the brimistor.

The is in effect a heat responsive resistor, introduced by the Vox engineers as a means of limiting the in-rush current while the main power valves warmed up.

Vox AC100 Mk 2, brimistor

The brimistor in an AC100 mark 2 from late 1966 or early 1967. As brimistors are prone to going bad, few survive in place.

Below the circuit diagram for the "AC 100/2 Amplifier", sheet OS/036 - dated 17th-28th June 1965 (with addition). Click as ever for a larger image:

Note the dates: 17th June "redrawn" 28th June "Brimistor and voltages added". The addition registered on 3rd September relates to the mains transformer "T1 Voltages were 210V. Heater amps added". Perhaps this was a result of more accurate measuring. All AC100 control panels give 205V as a voltage setting. Around 800 amplifiers were built to this schematic.

A late version of OS/036. This one is entitled "AC 100 Amplifier", having been adjusted from "AC 100/2 Amplifier", the last dated addition being December 1966. Click as ever for a larger image:

Supplied to Thomas Organ at some point in early 1967 and issued with the "Service Manual". There are eight notes: no. 5 records the decision to include a "Brilliant Switch", as in AC50 schematics, but as in AC50s this was never incorporated (which note 8 here states). The changes envisaged in notes 6 and 7 relate to the uprating of the power handling of a dropper resistor in the preamp (not changed in production amps), and to the layout of bias string (underchassis components in a slightly different order).