Vox AC10 "Amplifier No.2" (circuit)

Early to mid 1960

Glen Lambert

Circuit

There was no separate schematic for the AC10 Amplifier No.2. Indications are that no more than 100 were made in total. At present, only two examples with two-inputs and four with four-inputs are known to survive. The main overview page on the model can be found here.

A schematic for AC15 Amplifier No.2 was drawn 4 December 1959 and designated as OS/005. The AC15 version appeared in a 1x12" TV front Cab with a Goodmans Audiom 60 speaker.

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.

The return to the G1/10 - see this page - in terms of electronic design suggests that Derek Underdown had a hand in the development of Amplifier No.2. The EL84 output tubes now have added protection with 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.

Mains transformer secondary voltages on the AC15 are stated on the schematic as being 300-0-300V (see below) but in practice these are likely to have been higher. Operating voltages for the AC15 version will have been in the region of 350V before the choke. Both AC10 and AC15 versions of the circuit needed some form of limitation to power output in order to prevent transient high current spikes and to protect the single 10" and 15" speakers (which were rated at 10 and 15 watts respectively). The Goodmans Audiom 60 in particular had a reputation of being all too easy to damage.

Tone Control

Dick Denney's AC1/15 of 1958 used a tone 'cut' control wired across the input grids of the power stage (which was later to become standard in all JMI amplifiers). Earlier amplifiers designed by Derek Underdown used a simple low pass filter tone control wired at the output of the first stage of the preamp. The Amplifier No.2 Amps followed the general lines of Derek's Underdown's tone circuit arrangement, which provided a separate control for each channel. Two users could therefore adjust their tone settings individually - it was still commonplace for amplifiers to be shared by musicians. The schematic of the AC15 version places the tone control for the EF86 channel at the input grid of the preamp, This is not an ideal location as in certain circumstances the input signal could be unduly loaded down. It is unclear at present whether any amps were actually built this way.

Input Jacks

On the initial AC10 Amp No.2 built in early 1960 the input jack configuration simply followed the G1/10 fascia layout, and hence had just one input for each channel. Subsequent builds from mid 1960 were made with dual inputs for each channel. These were of equal level with two 220K resistors forming a "divide by two" if only one input was used. If both inputs were used the 220k resistors served as isolation between the instruments, and a shared 1M load resistor as a reference to ground came into play.

Comparison of first production run and balance of the AC10 Amplifier No.2 black and silver photo-anodised control panel.

AC10 Amplifier No.2 serial number 3236, general layout of chassis showing the smaller channel mounted OT attached to the upper.

This stands in contrast to the future AC15 and AC30 amplifiers which had input jacks configured as high and low pairs. Being produced in tandem, the AC10 No.2 amplifier and the AC15 No.2 naturally have significant points in common, notably: dual inputs wired in the same way, and 100 ohm protection/power limiting resistors at the anodes of the EL84s to attenuate the high HT. Both these features were retained through to the end of AC10 production.

The inputs (some text not germane at this point greyed out).

Vibravox

The main channel on the No.2 amps was now pre-amplified by the high fidelity EF86 pentode, which supplied an increased clean level at wider bandwidth to the output section, although somewhat attenuated though by the 3-pole high pass filter, which was required due to the tremolo oscillation frequency. See the portion of the circuit diagram illustrated below.

The Vibravox circuit was a simple preamp based tremolo brought across from the G1/10 add-on. Some parts were built into the main circuit but others were seperated off to an optional rear 'Vibravox' add-on unit. The Tremolo effect worked by varying the anode voltage of EF86 channel. This methodology was retained by the AC10 through to the end of its production life. The tremolo circuit made use of the pentode side of the ECF82 tube, which was a dual envelope having a pentode, which was used for the low frequency oscillator having enough drive to modulate the tremolo frequency directly onto the EF86, whilst the medium power triode side was simply used for the pre-amp of the Normal channel.

A schema of the ECF82, pentode on the left hand side, triode on the right.

Schematic OS/005 includes an enclosed dashed region indicating the contents of the optional rear add-on Vibravox. JMI retained the generic Vibravox name, even though the circuit was tremolo only. The controls on the back panel were a 2Meg Tremolo Speed pot, a Tremolo Deep/Soft plastic push switch (of the type used on bedside lamps of the era), and a 4-way paxolin wafer socket for connection of a footswitch. A two-way pedal connection will not work correctly on the No.2 amplifiers.

The dotted line encloses the tremolo circuitry. The 3-pole high pass filter can be seen above the ECF82 - a network of 0.005uf capacitors and 1Meg resistors.

Characteristics of the No. 2 Amplifiers

The Vox Amplifier No.2 was built in 10W and 15W versions:

Features In common:

- Contempo style chassis.

- Schematically the same with some component value differences.

- Four knob control panel layout: 2 x Volume 2 x Tone.

- Photo-anodised black fascia panel, AC10 with silver legends, AC15 with gold.

- EF86, ECF82, ECC83, EZ81, 2 x EL84 tube lineup.

- Long-tailed pair phase inverter.

- 100 ohm protection series resistors in the EL84 anodes.

- Optional rear panel tremolo via B9A socket.

- B9A type, three position voltage selector.

- Choke mounted at far right of chassis - 8 Henry 120mA.

- 130ohm 8W common cathode resistor.

Two views of AC15 Amplifier No.2 serial number 3962 with graphical corrections to the fascia. Showing similar layout to the AC10. The ECF82 was in a shock mount base likely due to the medium gain triode being pushed to highest possible gain.

Features particular to the AC15 Amp No.2: - HT approx 350Vdc

- Passivated steel lower chassis.

- Lay-down 15W OT nominally rated 300 - 0 - 300V @120mA (although voltages were higher in practice as the AC15 was a 15-18w amp).

- 12" Goodmans Audiom 60 speaker - 15 ohm.

- Alternate power lamp assembly with tall red lens.

- More powerful mains transformer (2" lamination stack).

- Slide-in of chassis allowed by control cutout to rear of the cabinet.

- Cabinets generally finished in two tone grey.

- Larger voltage dual filter cap.

- Channel 1 & 2 inputs arranged left to right per later AC15 (designations CH.I & CH.II do not match the OS/005 schematic).

- Add-on tremolo fascia black with gold legends.

- All three preamp tubes placed in line on the chassis with a 24-way tagboard incorporating a B9A socket for add-on tremolo.

AC15 Amplifier No.2 serial number 3962. Two-tone Cabinet internal view showing tagboard with integrated B9A tremolo socket and Goodmans Audiom 60 speaker.

Features particular to the AC10 Amp No.2: - - HT approx 320Vdc

- Both chassis sections aluminium with side struts.

- Horizontal frame-mounted 10W output transformer.

- Black photo anodised panel with silver legends.

- 10" six-spoked brown Plessey or Richard Allan 1012 speaker - 15ohm.

- Smaller mains transformer (1 1/2" stack).

- Hinged chassis positioned in an island cutout in the top of the cabinet, which was a squared version TV Front used only for the AC10 Amp No.2.

- Intially built in early 1960 with a single input for each channel and the balance built in mid 1960 with two inputs for each channel.

- Channel 1 & 2 inputs arranged front to back per later AC10. These positions were reversed on AC15 Amplifier No.3 and onward.

- Add-on tremolo fascia plain silver with black legends.

- Phase inverter tube mounted lower on the chassis with a paxolin mounting strip incorporating B9A socket. Additional components wired point-to-point from the phase inverter's base.

Tremolo (and other matters)

If the amp was to have no tremolo unit, a rubber blank was placed over the B9A socket.

AC10 Amplifier No.2 with B9A socket blanked off. One can see the phase inverter 12AX7 (ECC83) valve socket to the right of the board.

It seems the tremolo was fairly standard to the 15W model, some even having the tremolo hardwired with flywires instead of the plug and B9A socket arrangement.

The same 'Vibravox' add-on was used for both 10W and 15W versions, generally connected by a B9A plug on the tagboard. The main amp circuit was cleverly designed such that the effect channel would automatically revert to a standard audio channel if no Tremolo unit was plugged in. There were only passive components in the rear add-on unit, which was housed in a small aluminium casing, the circuit potted inside, and the assembly simply screwed together. The one difference between AC10 and AC15 rear Tremolo unit was the colour of the fascia panel: silver with black legends for AC10, black with gold legends for the AC15.

AC10 'Vibravox' Tremolo panel on the left; AC15 unit on the right.

When OS/008 appeared (drawn on 9th September, 1960) the two models were split, with the AC10 Amplifier No.3 now having integrated Tremolo Amplitude and Speed controls on the main panel, but retaining many features of OS/005 including the simple dual inputs and HT protection resistors on the output tube anodes. It also changed to JMI's new standard single Tone Cut control wired to the phase inverter stage rather than the dual tone controls of the No.2 circuit. An unexpected quirk of the design resulted in the Amplitude control also working somewhat like a drive/gain control for the EF86 when the tremolo effect was not enabled.

A detail of OS/008 showing the position of the tone cut control wired across the EL84 input grids.

The 15W version branched off to become the fully fledged AC15 with a return to integrated full phase shifting Vibrato/Tremolo onto a far more extensive chassis, and soon after became the smaller brother of the newly-released AC30/4. Examples of the third circuit AC15 can be seen on this page.