Vox AC10 "Amplifier No.2"

Early to mid 1960

Glen Lambert

"AC10 Amplifier No. 2" takes its name from its close relationship (in terms of electronics) with "AC/15 Amplifier No. 2", which is expressly designated as "No. 2" on JMI diagram OS/005. Although no formal schematic for this version of the AC10 seems to have been produced, "Amplifier No. 2" was clearly in view: the amplifier (or rather circuit) that succeeded it was clearly designated "AC/10 Amplifier no. 3" by JMI (diagram OS/008). Effectively, our cues come from the workshop floor. A buyer of course would go into a shop simply to buy a Jennings "AC10" with or without tremolo, not an "Amplifier No. 2" or for that matter an "Amplifier no. 3". In pricelists the AC10 was just the AC10 (or AC/10).

AC10 Amplifier no. 2 represents a complete departure from Dick Denney's AC1/10 and AC1/15 for instance. The return to the G1/10 (in terms of electronic design) suggests that Derek Underdown had a hand in its development. The EL84 output tubes now had added 100 ohm resistors wired in series between the anodes and the OT primaries. This was a feature last used on the AC2/30 amplifier of late 1957 / early 1958, presumably also applied here to protect the speakers and output stage due to the relatively high operating and transient voltages of these amps.

The AC10 and AC15 Amplifiers No.2 were two channel, four control amplifiers built on a Contempo chassis mounted on a wooden plinth at the top of the cabinet. It's thought that The AC10 Amp No.2 chassis was in production a little before the AC15 due to its earlier features such as the unique hinged plinth which allowed the chassis to neatly swing up into an old style island control cutout, and down for access to the valves. This quirky arrangemant was only ever used on the AC10 Amp No.2, whereas the AC15 Amp No.2 saw the introduction of the more conventional 'slide-in' chassis with the control cutout extended to the rear of the cab, a concept that was used on virtually all JMI amps from that time forward.

AC10 Amplifier No.2 drop down hinged plinth - a unique assembly only used on this version in 1960. The two brass thumbscrews at the top held the whole thing in place.

AC10 Amplifier no. 2 also saw the exclusive use of a new type of TV front cabinet measuring 15 3/4" x 16" x 9", external corners and internal grill opening both much more 'squared off' than other Jennings TV front cabs. The new "square" AC10 cabs were covered in the familiar diamond embossed ivory coloured levant. The old-style 'island' opening for the control panel, however, was retained on all builds. The AC15 on the other hand appeared in a two-tone grey and oatmeal bookcloth covering, some units having an alternate loose weave 'checkered' grille cloth instead of the standard brown diamond.

In terms of fittings, the AC10 had gold-topped cream-coloured Roberts radio knobs: a moulded plastic 'TEN' flag lower left on the front; and the regular gold Vox logo top right.

Both the AC10 and AC15 had two-piece "Contempo" style chassis. The AC10 chassis was fully aluminium, and probably the earliest chassis made by JMI in this format. The AC15 was made with an aluminium upper section and passivated steel lower (to cater for the larger transformers). The circuit, as mentioned, was a return to a design akin to the tremolo version of the G1/10, but with the negative feedback loop omitted.

Two early examples of two-input AC10 Amp No.2 in newly issued 'squared' TV Cab. Serial number 3143 (at left) shown on p. 117 and p. 342 of Jim Elyeas's Vox Book described as having a traffolyte fascia panel (although not shown), and two views of serial number 3158 (right) privately owned. The ventilation opening under the grille cloth on the upper front baffle was carried over from the AC1/10 as the power tubes were also mounted at the top. The two-input cabs also retained the brass "Jennings" plaque at the top. This was dropped in subsequent four-input builds.

Channels

The first AC10 Amp No.2 had just one input for each channel, effectively following the form of the G1/10 and AC1/10 fascia panel. The balance of AC10 units from mid 1960 onward were four-input build, which simply meant an added parallel jack for each channel. The AC15 version doesn't appear to have been offered in two-input build. Both AC10 and AC15 Amp No.2 had separate volume and tone controls for each channel, presumably with a view to allowing musicians sharing the amp to set tone individually.

Both channels were voiced in the same way - a voicing that would be called "Normal" on other JMI amps - but for Channel 1, one had the option of tremolo, enabled by the attachment of a "Vibravox" module on the rear panel. This was connected via a B9A socket on the internal tagboard. A small aluminium housed sub-assembly with two extra controls (speed pot and depth switch) was then added in a cutout on the rear panel above the speaker, transforming Channel 1 to tremolo. The added controls were a Radio Spares "Alldry" 4-way socket for pedal connection, speed control knob, and two depth settings via push switch. Due to the nature of the design, the pedal required a four conductor flex and DPST switch. The tremolo circuit was virtually identical to the optional tremolo designed as an option for the G1/10 TV Front amp. The AC10 Amp No.2 was offered in 'Normal' version for 35gns and with Tremolo for 45gns. The AC15 Amp No.2 was offered only with Tremolo and priced at 65 gns.

Pricelist from Spring of 1960 showing the AC10 offered with and without Tremolo, AC15 only with Tremolo.

Views of a four-input mid 1960 AC10 Amplifier No.2, serial number unknown, showing the fold-down aluminium contempo chassis, rear panel tremolo unit unplugged for access. The original dark grey two-piece round footswitch is shown. Note the "Jennings" brass plate is lacking on this cabinet.

The amp pictured above has its original 15ohm six-spoked brown Plessey 10 inch speaker of the type used in the Univox organ, which was the defacto standard AC10 speaker from 1959 through to 1961 when they changed over to a species of Goodmans Axiette with red painted magnets. Richard Allan 1012 speakers (also used later on in the smaller Domino amps) may have been fitted in some instances when Plesseys were unavailable.

Circuit

As has been mentioned, there was no separate schematic for the AC10 Amplifer No.2 although it was so close to the AC15 build that one is likely to have been deemed un-neccessary. There is certainly no gap in sequence of early JMI sheets that have come down to us (OS/001 to OS/011). The schematic for AC15 Amplifier No.2 was drawn on 4th December 1959 and designated as OS/005. Indications are that no more than 100 of the AC10 Amplifer No.2 were made in total. To date only two examples with two inputs and four with four-inputs are known to survive.

Detail of the information panel. Examples of AC15s built to this circuit can be found on this page. Schematic OS/005 represents a complete departure from Denneys previous AC.1 (Amplifier No.1) amp.

More on the specifics of the circuit can be found on this page.