The circuit diagrams on this page all come from hard copies - sometimes copies of copies - made numbers of decades ago. It seemed rather redundant to gather together copies of things that are currently in circulation in publications and on the web. The circuit for the AC30/4 will be provided on a separate page.

For the AC30/6 "Normal", at least six iterations of the circuit diagram were produced between Spring 1960 and the end of 1960. These are likely to have been struck off from a large format master sheet. Master sheets could be up to 40" and 30". The introduction of a visible "list of changes" in early 1961 may have been come as a response to some problem or error that had arisen. Now and again one still finds changes that were not recorded openly, however.

[O]A/026 it should be said was the sheet included in the Thomas Organ Service Manual of 1966, and seems also to have been the one more frequently sent out to service engineers in the United Kingdom. OS/065 was circulated in the UK too though.

Note that no JMI sheet has come to light so far for the AC30/6 with top boost controls integrated in the control panel.

Circuit diagrams of course are only part of the picture. Engineering drawings of various elements - transformers and so on - will also have been made. The company that printed the engineering drawings for "Vox Sound Limited" in its original incorporation (1970-1973), was "A. & F. Royal Limited" in Crawley, Sussex. It is not known at present who printed sheets for JMI.

Click on the images below for larger downloadable versions.

JMI: 1960-1968.

AC30/6 "Normal"

At least six iterations of the diagram for this circuit were produced: (1) of 1960 (no sheet number); (2) of the second half of 1961/early 1962 (no sheet number); (3) mid 1963 (no sheet number); (4) mid 1963 (designated A/026); and (5 and 6) September 1964 (designated OS/065 and A/026).

The circuit was initially drawn on the 29th April 1960 by Dick Denney (RD) and traced (ie. drawn as a "mechanical" drawing) by Derek Underdown (DWU).

(1) A copy of the original form of the diagram - with no list of changes - is known to survive, albeit in a slightly fragmentary form. It may be possible to give further details in due course. This is at present the only known original sheet for the "Pre-LOC" (pre-list of changes) circuit. Drawing number 18 in Jim Elyea's schematics portfolio was made for him, a sort of winding back from later sheets.

(2) Below, a copy of the circuit diagram for the AC30/6 produced in the second half of 1961 / early 1962. That capacitors of 0.05 and 0.005mfd were specified indicates that black WIMA Tropydurs are in view. In later diagrams these values were silently changed to 0.047 and 0.0047mfd, the closest available in the newer brands/ranges adopted - gold-coloured WIMA Tropyfols, Mullard mustards, and so on.

Measures 16 1/2" wide x 13" tall. Click for a larger downloadable version.

The list of changes as at May 1961.

WIMA Tropydurs, 0.05mfd, simply to indicate appearance.

(3) Number 19 in Jim's schematics portfolio - a print of an original sheet produced in mid 1963. Where the sheet from later 1961/early 1962 specifies capacitors of 0.05mfd and 0.005mfd, this sheet has 0.047mfd and 0.0047mfd, all silently updated. A note relating to add-on Top Boost (OS/010) is given a little way under the list of changes.

(4) Number 20 in Jim's schematics portfolio - a print of an original sheet produced in mid 1963. Much as above, but designated A/026 in its information panel lower right. Technically speaking the sheet should be OA/026, but this version and the one below (number 6) are both plain A/026. Note that OS/010, the diagram for add-on Top Boost, references this sheet number, not OS/065 - see below.

(5) OS/065. Quite when this designation was introduced is not fully clear at present. It may be, however, that the reference number was introduced in the wake of the order from Thomas Organ in late August 1964. Putting the circuit diagrams in order will have been a necessary preliminary prior to despatch to the Thomas Service Center in Evanston.

The latest date generally given on sheets numbered as OS/065 - in effect those that have circulated on the web for years (often with completely misleading filenames/captions) - is 11th September 1964.

The sheet is a copy of a copy, probably from "Modern Music" in Dudley. Values are good and legible, except in one instance, which has been supplied in red. Someone has added a potential negative feedback loop!

First change: 7th February 1960. Main "List of Changes": 8th May 1961. ISSUE 2: 14th June 1963. ISSUE 3: 11th October 1963. ISSUE 4: 11th September 1964, no changes marked, the entry made by Dick Denney. 11th Sept. 1964 is the date on or after which the modifications were incorporated in the main circuit (ie. in the main body of the diagram). Two of the additions were made by Mike Turner (MT).

(6) A/026. Much as OS/065 above, the list of changes running through to 11th September 1964.

OS/056

AC30/6 Treble

Dated 10th September 1964. Drawn by Dick Denney (DRD) and checked by him. The modifications noted on OS/065 are incorporated. Only a few divergences from the AC30/6 Normal circuit: C2, C3, C5, C6, C7, C9 and C10; the cathode circuit of V1; and at each of the three volume controls, a 330k resistor and 220pf (not numbered) to enhance the treble response further. This sheet was part of the Thomas Organ service manual for amplifiers first issued in the USA in 1966.

OS/057

AC30/6 Bass

Dated 11th September 1964. Drawn by Dick Denney (DRD) and checked by him. The modifications noted on OS/065 are incorporated. Only a few divergences from the AC30/6 Normal circuit, all capacitors: C2, C3, C5, C6, C7, C9 and C10. This sheet was part of the Thomas Organ service manual for amplifiers, as above.

OS/010.

AC30/6, add-on Top Boost unit, 1961

There were at least four iterations of OS/010. A short account of them can be found on this page. Below, the earliest known.

OS/010 from the JMI wallet pictured below. Dated 11th December 1961. Drawn by John Bell. Note that the sheet references [0]A/026, not OS/065. The HT voltage is given as 230v. An addition of 19th June, 1963, is signalled, but there is no note of what it was. There is no legend inside the panel, top right.

A JMI wallet of circuit diagrams, one of five currently known. These were issued by JMI to its engineers and sometimes sent out to repair shops. Twelve sheets survive in this one.

A62-005

Reverb unit, 1962

Dated 30th January 1962. Drawn by John Bell.

OS/075

AC30/6, add-on Reverb unit, 1965

Dated 15th January 1965. Drawn by Albert Hogben. Circulated as part of the Thomas Organ service manual for amplifiers (as above).

OS/075/R

AC30/6, add-on Reverb unit, 1965

Dated 14 October 1965. Drawn by Albert Hogben. Changes to 26th January 1966. A simplified version of OS/075.

Vox Sound Equipment Limited: 1968-1969.

OS/013

AC30 Top Boost.

Dated 20th November 1969. Based on OS/065 (copied onto a larger sheet), the Top Boost circuitry added above it by draughtsman "PG".

The two pictures above from another copy, clearer in some respects, feinter in others. Note that in the top panel there is a large gap between "Sound" and "Limited". "Vox Sound Equipment Limited", the name of the previous company (1968-1969), originally stood there, "Equipment" having been erased. In the lowest but one panel, the hand-written "Vox Sound Ltd" may have been supplied over the erasure of "Vox Sound Equipment Limited".

The sheet was drawn by the person that drew the VSL diagram below. Its number - OS/013 - should on the face of things be a JMI number. OS/011 (redrawn in 1964) is for the JMI Reverb unit; OS/012 (dated 15/09/58) is for a "Jennings Organ Company" V-series organ power section. No JMI sheet numbered OS/013 is known. If OS/013 is not simply a mistake for OS/213, which would certainly be a VSL sheet, then it may be that VSEL or VSL simply used an unallocated number in the old sequence. There are quite a few gaps.

Sheet for Italian-made AC30s

The information panel.

The sheet as a whole.

Above, the circuit diagram for the AC30 drawn up in Italy by E.M.E - thanks to Michael. The date of the drawing is given as "17/11/68", On 28th Feb. 1972, the sheet was stamped and signed off. The circuit encompassed is the AC30 Top Boost.

E.M.E - Elettronica Musicale Europea - was the joint venture between Jennings, Thomas Organ, and Eko, set up in mid 1966 in a huge factory in Recanati (a little way south of Ancona), funded in part by the Italian government.

Vox Sound Limited: 1970-1973.

Sheet without a number

The compiler of the diagrams below had evidently been taught to write on the continent - the stem of the number "7" for instance has a cross stroke. Other continental traits are visible throughout.

AC30/6 Top Boost

Dated 12th February 1971. No information panel. A later addition has been made at the input of V1 (ECC83).

OS/249

Solid state AC30/6 with Reverb

Drawn 30th May 1972. Solid state amplifier. Changes through to 2nd February 1973.

Dallas Musical Industries: 1974.

SC/V/1313

AC30/6 Top Boost

Dated 24th June 1974. The sheet is an original intended for service engineers.

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