The circuit employed in the Domino Super Reverb Twin was effectively the same as that of the AC10 SRT. Details of the first JMI sheet for the circuit - OS/026 - drawn by Mike Turner in February 1963 can be seen above and below.

OS/026, "Issue 1" and its date noted on the sheet, top right.

As Glen Lambert has kindly pointed out, JMI "borrowed" the design of the circuit - with certain necessary adjustments - from Ampeg's Reverberocket R12 of Spring 1961. See also this thread.

Quite why JMI should have copied this amp in early 1963 is not certain. One intriguing possibility is that a Reverberocket either turned up in "Musicland", the Jennings shop in Bexleyheath, or at the main shop on the Charing Cross Road, and that Dick Denney heard it, liked it, and so copied its circuit. There are differences though: the Ampeg for instance has a potentiometer for reverb depth/length. The "step" switch indicated in OS/026 is a JMI adjustment.

Detail from OS/026, first version, February, 1963.

Above, the four-position switch for the "reverb expansion" - OFF, then three pre-set settings for the depth/length, the "steps" being fixed resistors of 200 ohms, 500 ohms, and 1K ohms. Jennings had long used this form of switch for the selection of vibrato on certain models of Univox (OFF followed by three pre-sets). The reverb pre-sets set out on this sheet were replaced by a 1K potentiometer by late December 1963.

The challenge is to find an early AC10 SRT that conforms to OS/026 (Issue 1): two inputs, five control knobs, and step-switch for reverb.

An early advert for the new model:

"Melody Maker" magazine, 2nd February, 1963, promoting the new AC10 SRT. The prices of amplifiers available are given in the small right-hand panel.

Unusually, the circuit for the AC10 SRT was drawn out again eleven months later - in January 1964 - by Albert Hogben in at least two versions, both designated OS/026. A third version of OS/026 was apparently allocated (also in January '64) to the Domino Super Reverb Twin.

In terms of the main revisions, the outline is:

Issue 1 - 13th February 1963.

Issue 2 - 30th December 1963, the reverb step-switch becomes a 1K pot - some other changes too.

There are at least two versions of Issues 3 and 4 for the AC10 SRT and Domino SRT. The dates assigned are the same:

Issue 3 - 27th January 1964.

Issue 4 - 22nd October 1964.

Detail of OS/026 reprinted by Jim Elyea as sheet 8 in his portfolio. Jim's sheet 9 - also OS/026 - has exactly the same info panel. For the main difference between the two, see below.

Some months later, the circuit was drawn out yet again by Albert Hogben, this time as OS/71. Steer clear of OS/71 (copies have long been on the web) - it contains colossal errors.

14th Jan. 2024: - the section below will need to be revised and expanded. On no account use/follow sheet number 9 in Jim Elyea's schematics portfolio.

A note on the mode of HT supply outlined in three versions of OS/026, and in OS/71. All four sheets are assigned in their information panels to the Vox AC10 Super Reverb Twin (two inputs).

(1) OS/026 of 13th February, 1963 (original sheet): a single line from the rectifier valve via a 1K resistor to supply the EL84 screens, the output transformer (for the EL84 anodes), and the rest of the circuit.

(2) OS/026, issue 2, 30th December, 1963 (original sheet): - a 1K resistor to supply the EL84 screen, as above.

(3) OS/026 reprinted by Jim Elyea as no. 8 in his "Schematics Portfolio": as above. Drawn by Albert Hogben in early 1964 - Issue 3 (the sheet in view) dated 27th January, Issue 4 dated 22nd October. No mention of Issues 1 and 2.

(4) OS/026 a non-original diagram provided by Jim as no. 9 in his "Portfolio": - a dedicated line (no resistor) from the rectifier to supply the OT (for the EL84 anodes). Do not use this circuit.

(5) OS/71 (scans on the web): as (1) and (2) above. The power rating of the 1K resistor is specified as being 10W. As noted yesterday, this diagram has some pretty colossal errors (elsewhere in the circuit).

Quite how many (if any) chassis were built according to (2) is not known at present. As for Issue 2, which must have been drawn at some point between 13th February 1963 and 27th January 1964, no copy has so far come to light. As noted above, the creation of this sheet may have been prompted by the change from step-switch for reverb to potentiometer.

Issue 4, dated 22nd October, 1964, is likely to have been advanced to accompany the release of the new AC10 SRT with trapezoidal amplifier cabinet.

As for the circulation of the sheets, the first version of OS/026 (13th Feb. 1963) was evidently sent out to repairmen. The status of the versions from 1964 reprinted by Jim is not certain. The same is true of OS/71.

This JMI wallet of circuits - which included a copy of OS/026 (February 1963 version) - was sent out to a young man in Nottinghamshire in early 1965. The sheets measure 10" x 8".

A little more on OS/71. The sheet differs from its predecessors in three main respects: (i) the circuit is expressed differently; (ii) no indications of voltages are given at any point; and (iii) no date of redaction is noted in the information panel. As far as this last point is concerned, early January 1965 seems most likely. OS/70, for the Vox wireless microphone unit, is dated 31st December 1964, and OS/72, for the AC50 mark 3, is dated 7th January 1965.

It is not clear, however, what OS/71 was actually for, or for whom it was drawn. In view of the absence of circuit voltages, the sheet can hardly bave been of much use to repairmen. For all we know, it may have been one of those large sheets (generally around 14" x 12") intended for factory use only. The trouble with digital copies is that are generally circulated without reference to the "real-world" original (size, context, and so on).

Info panel of OS/71

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