Vox AC30 website updates

January 2022

30th January

A detail from the flyer for the Jennings "Model G" (see below, entry for 27th Jan.), late 1960 / early 1961. Nine speakers were recommended for the organ, which could put out 150 watts. The bass enclosure (lower roundel, at right) probably contained a Goodmans Audiom 90, an 18" driver developed in the early 1950s for organ and public address applications. The middle frequency wall-mounted units possibly something like the Goodmans Audiom 60; the wall-mounted tweeters perhaps embodying a pair each of Goodmans Trebax horns?

The "nine loudspeakers altogether" mentioned in the brochure will therefore have been: 1 x 18"; 2 x 12"; and 6 x Trebax tweeters.

The system will have had a fairly complex cross-over network to protect both tweeter and wall-mounted middle-frequency units from excessive power, and to protect the horns especially from damaging low frequencies.

28th January

From a brief history of JMI drawn up by Rodney Angell (paraphrased): - when Jennings struck its deal with the Thomas Organ Company in August 1964 certain restrictions were imposed, the principal one in relation to organs being that Jennings had to bring an end to its own Church and Entertainment Organ lines (the "V Series" and two new units still in development) and market only Thomas models. The Continental (and derivatives) was allowed but little else. Ten years of development and production initiated and overseen by Derek Underdown came to a halt. Thomas organs did not prove popular.

1964 was the last year in which Jennings-made Church and Entertainment organs were advertised.

That no Thomas organ with a Jennings badge has come to light in the UK so far may simply mean though that they were not much prized in the 1970s and 1980s.

As has already been mentioned, Thomas shipped component parts over to England rather than complete organs. This was partly for convenience but as Rodney notes also to avoid certain types of import duty.

In 1969 and 1970, the principal Danish importer and distributor of Vox amps did the same but in the opposite direction - relevant component parts were purchased from "Vox Sound Equipment Limited" and the various models of amp assembled in a factory outside Copenhagen.

27th January

Some material relating to the Jennings Organ Company's demonstration of its "Model G" organ - an enormous three manual unit - at the church of St James, Kidbrooke Park, near Blackheath, South London, on 4th December 1958. (1) Derek Underdown's photo of the installation; (2) an advert in "Musical Opinion" magazine, November '58; and (3) a brochure for the "Model G", based on the initial publicity, from late 1960 or early 1961 - the price of the organ rose from 1850 guineas to 1975 guineas around December '60.

1958 was the year of the big sales push for the "Model G" - further material . Of note was the shipping of a "Model G" to New Zealand, installed by JMI engineers in the Civic Auditorium, Lower Hutt.

The organ contained three 50 watt amplifiers designed by Derek Underdown - 150 watts in total! - which could be set up to drive external speakers, wall or floor mounted.

Derek Underdown's photo, thanks to Toni Standing, his daughter.

November 1958.

26th January (2)

Following on from the entry below: it does, on reflection, seem likely that Triumph took on production of the AC2/AC4 in very early 1962. This was the "Type 3" amp with pink/champagne-coloured control panel, no voltage selector, and box with diamonds aligned horizontally.

The modification noted on the circuit diagram dated 3rd April 1962 *apparently* indicates the introduction of a mains transformer with voltage selector (for 110v or 240v). Amps in the new tall case with grille cloth diamonds aligned vertically (the AC2/AC4 "Type 4") all have such selectors.

This provides a useful *preliminary* framework for dating. As for there being a "last word" - one can always hope. New information often comes to light in a piecemeal manner, hardly ever in the "right" order. Equally, a new relevance can emerge out of the blue for things that have long been known. Much therefore has to be provisional.

26th January

Thanks to Martin Kelly, a copy of the circuit diagram for the AC/2 formerly belonging to Rodney Angell, who began at JMI as an amp tester and service engineer. The sheet, which measures 13" x 8" (old fashioned "foolscap"), is identical to the one designated OS/009 published by Jim Elyea and in circulation elsewhere, but the "OS" number is lacking.

Circuit diagram for the AC/2, no "OS" number.

Quite why the circuit was drawn out new in December 1961 is not wholly clear. AC2s had been in production since late 1960. Possibly it was simply "housekeeping", John Bell, the sheet's draughtsman, having been prompted to tidy up working materials. OS/010, the Top Boost circuit; and OS/011 (first version), the Vox Reverb unit, also drawn out by him, are dated 11th December 1961 and 30th January 1962 respectively.

OS/009 - for the AC/2 - has a note of a modification relating to the mains transformer dated 3rd April 1962. If Rodney's recollection is correct, it may be that the sheet, or at least some version of it, was passed to Triumph Electronics at around this point to serve as a basis for production of a kit amplifier to be sold in shops and catalogues.

As far as normal (non kit) production of the AC2/AC4 is concerned, yesterday's entry, corrected thanks to Marc Wichern's eagle eye, and the will need to be revised. It looks as though Triumph had started making chassis by serial number 2211. The tell-tale features of Triumph are there - the consolidated earth buss-rail, and in particular, the insulated stand-offs on the chassis.

AC4 serial number 2211. The earth rail runs across the middle of the chassis. The stand-offs are indicated with arrows.

That the drawing up of the diagram coincided with the start of Triumph's contract and the recasting of the AC2 as the AC4 (new pink/champagne-coloured control panel) is certainly possible, but further component date codes are really required to test the position out.

25th January

Some notes on later Vox AC2s / early AC4s - to be incorporated :

At some point around serial number 2400-2500 JMI updated the format of the amp: - a taller box with a slider board for the chassis; grille cloth with diamonds upright instead of horizontal.

Production of the chassis for these later amps was contracted out to Triumph Electronics in Purley. According to the late Rodney Angell, who worked for JMI from 1960 to 1968, Triumph had been brought in initially with a view to producing Vox amplifier kits for sale in department stores and so on - a sort of parallel to JMI's "Pacifica" kit guitar, introduced in late 1960. Kit amplifiers - presumably the AC2/AC4 or some version of it - evidently never went beyond the planning stage, but Triumph handled production of standard AC4 chassis thereafter, also of the the AC10.

AC4 serial number 2961, recently sold - components with date codes indicating assembly in Spring or early Summer 1963 - corrected.

AC2 serial number 2961.

24th January (2)

A further instance of older transformers (probably left-overs from earlier days) being pressed into service by "Vox Sound Limited" in the second and third quarters of 1971 - AC30 TB serial number 25233. Others - both on - are serial numbers 25119 and 25658 (which has a mixture of Haddon and Albion).

VSL moved production from the West Street Works in Erith to the Birch-Stolec factory in St Leonards-on-Sea in Spring 1971 - presumably in clearing out, numbers of old parts came to light. A chronological overview of "Vox Sound Limited" can be found on the .

24th January

Five late "Vox Sound Limited" AC30s, last quarter of 1972, all with printed circuit boards, have now been added to the - serial numbers 25933, 25949, 1094, 1151 and 1237.

23rd January (2)

Just to illustrate the difference between the two types of printed circuit board, introduced by the Birch-Stolec factory in 1972:

AC30 Top Boost board. Note the thickness of the traces. Relatively few plain AC30 Top Boost amps were produced with PCBs.

AC30 Top Boost Reverb board. Comparatively thin traces, and the printed part code "PCB 70-1".

Although heat often plays havoc with these boards, bending the paxolin and making traces intermittent, these amps, when in good order, do sound good.

23rd January

Vox Sound Limited, AC30 Top Boost Reverb, serial number 1237, late 1972, one of the last to be built in the on the Ponswood Road Estate, St Leonards-on-Sea (Hastings). A printed circuit board amp. Speakers are Celestion greenbacks.

As far as the printed circuit boards are concerned, there were two types - one for a small number of plain Top Boost AC30s (thick traces on the board, no board number), and one for the Top Boost Reverbs (thin traces, board numbered "PCB 70-1").

Some additions coming soon on the page linked above.

17th January (2)

Just to note that further pictures of AC30 serial number 22765 have been . Its Woden mains transformer and choke have the date code "KY" = October 1967. The latest Woden date codes so far recorded are "BZ" and "CZ" (February and March 1968) in Super Twin amplifier section .

17th January

Thanks to Darcy, pics of an AC30 Twin with serial number in the 9200s, Normal voicing, Top Boost added on the back panel. The Woden transformers have October 1963 date codes; cathode resistor 47ohms. Probably pebble vinyl originally, now black basketweave. The speakers are later replacements - silver Celestion T1088s.

13th January

Thanks to Luca, pictures of AC30 Top Boost serial number 22765 - close to the end of the JMI run. Now added to .

AC30 serial number 22765.

7th January

Text entries have now been added for four AC30s with serial numbers in the - 19206B, 19535TB, 19666TB and 19674TB. Serial number 19535 has a chassis with the serial number stamped on the left-hand side of the preamp rather than the right. It should be possible to add pictures (for the most part "establishing" shots) shortly.

6th January

Typical of AC15s and AC30/6s made at Dartford Road in the latter part of 1960 through to Spring 1961 are the double spring retainers used to keep the EL84s in place. These were made by "Electrothermal Engineering Limited", based in East London. Its catalogue is impressive in extent but not easy to use, arranged as it is primarily by valve "CV" number.

Double spring valve retainer on an early split-front AC15.

Although the second edition (above) does not encompass the EL84, it is clear that JMI used Electrothermal's VR-3 spring retainer (which was interchangeable with the VR-7), intended for valves of the same height - the UL41, U78, 6X4, EZ90, and so. For the military this retainer was designated as the Z290291. Examples of this last regularly appear for sale on ebay.

Double spring retainers were used by JMI on all AC30/4s (the whole run), however, through to the winter of 1961, when the model was discontinued.

From Spring 1961, all AC30/6s made by Westrex (and all AC30/6s later produced by Burndept) were fitted with simple wire clips of the familiar type. It seems likely that use of wire clips in AC15s began in Spring 1961 too.

Wire valve clip in an early AC30 Super Twin.

5th January

A couple of pictures of serial number 11066T, the earliest Treble model to have come to light so far, . The amp was sold in early December by Gardiner Houlgate, auctioneers.

Also to note that a further amp has emerged - an AC30 Twin - with "Westrex Treble" stamped on its chassis. The serial number, as given on the plate, is 12740 T. Another example is , though that is designated normal "N" on its plate - so perhaps not really a treble model at all.

In 1964 Westrex made precious few Treble and Top Boost chassis, concentrating in the main on Normal and Bass. Those were generally used for Super Twin amplifier sections. Burndept produced the bulk of the Trebles and Top Boosts (along with large numbers of Normal and Bass), which were used both for Twins and Super Twins.

4th January (2)

Reported (by a reliable source) as having been brought in to a repair shop in the Midlands in the late 1970s, two TV Front AC30s both with *twin output transformers* - ie. two AC15 power sections brought together to form an AC30/4.

From an electronic standpoint this will not have been difficult to do. The question is why? Had suitable 30 watt transformers for four EL84s run out or not yet been produced? The expense of using two transformers will have been relatively large naturally. There is also the question of total impedance in relation to the speakers - presumably only a 4 or 8 ohm total was possible with a pair of AC15 OTs.

4th January

Added to the snippets on Geoff Harris (below, entry for 30th December): -

In March 1967, Geoff's official title was (or had become) "Organ Sales Manager" - (stated in a reply to a letter asking for a set of organ circuit diagrams to be sent out to New Zealand). The "Thomas Organ Division" had come to an abrupt end in late January / early February 1967 when Joe Benaron announced, with Tom's agreement, that Thomas would take over from JMI the marketing and distribution of Thomas organs in the UK. Probably quiet sighs of relief all round.

3rd January

A rather austere advert in a trade magazine for the "British Valve Association". Notable members in early days were Cossor, GEC, Marconi, and Mullard. Up to around 1956, the Association behaved as a cartel, regulating (fixing) the prices of numerous types of valve. As membership grew - Brimar and Mazda joined in the early 1960s - over-arching concerns became the protection of patents and regulation of quality. From the mid 1950s and probably before, the valves used by JMI generally had the "BVA" logo.

Advert from early 1963. The members listed in the runner at the foot of the page are: Brimar, Cossor, Emiscope, Emitron, Ever Ready, Ferranti, Marconi, Mazda, Mullard, Philips. Philips was the parent company of Mullard.

2nd January

Advertised a little while ago now, a superb AC30 PA set probably from the first third of 1964. The copper panel Super Twin amplifier section has integrated Top Boost. Note that the leads for the speakers - of the same type as those provided with AC50s and AC100s - are wired in, presumably from factory - XLR ends for the Line Source 40 public address columns, but no sockets on the amp's back panel.

Sets such as this were regularly listed in small ads in "Melody Maker" magazine in the second half of the 1960s.

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