17th February

The page on serial numbers in the 22000s and very low 23000s, 1967-1968, is now up. The low 23000s is as far as JMI got before the company ceased trading in late Spring '68.

16th February (2)

A detail of the still sticky reverse of the JMI "Heavy Duty" label pictured last week (below, 8th Feb.). When the label was removed, it picked up a smaller label stuck on the speaker by the manufacturer, in this case either Celestion (most likely), or Goodmans.

The text of the smaller label reads: "STANDARD / 15-16 OHMS / 55 c/s".

16th February

A page on AC30s with serial numbers in the 22000s coming tomorrow, along with some updates to the page on numbers in the 20000s and 21000s.

14th February

Some brief notes on the AC30/4 currently on Reverb. The main TCC filter capacitor in the preamp has the date code "RJ" = September 1960; one of the preamp pots has a code ending in "H" = 1960. The preceding letter has a long vertical limb - so either an "H", "I", or "J" = August-October. The blue speakers - note the original pink and yellow wiring - indicate the amp was made ready for sale in early 1961.

13th February (2)

Just to note that the picture formerly at the head of the page on split front AC30/4s - The Shadows on stage in March 1961 with their new AC30s with black covering - has been replaced as it was wrongly captioned. The amps that The Shadows received in March 1961 are known from photographic evidence to have been AC30/6s. Photos are key.

The two images below, The Shadows at Abbey Road Studios at some point in 1961, came up on "Pinterest", no source attributed. The detail of one of the amps below, slightly enlarged beyond its natural size, shows the six inputs.

One can see a guitar cable plugged into the first socket of the Brilliant channel; to its right (as we look at the picture) the black input socket of the Normal channel; and to its right the white socket of the Vib-Trem channel.

13th February

The page on split front AC30/4s has been updated and will be tidied up further over the next few weeks. Shirley Douglas and Chas McDevitt are recorded as receiving new fawn split-front AC30/4s in 1960 - some months after an initial order in May of that year (Jim Elyea, p. 624). The serial numbers were 4250 and 4270. It has seemed best to adjust the span for the production of split-front AC30s to "autumn 1960 to winter 1961". It is clear from the surviving evidence that production of split-front and TV front AC30/4s overlapped for a longish period. Sales of twins were slow to begin with. AC15s were much more popular in 1960.

AC30/6s are first mentioned on 30th July 1960 in an advert placed by "Paul's Music Studio" (later known as "Musicland") which was run by Paul Jennings, Tom's son, and effectively a JMI shop. The ads were repeated on 6th and 13th August. Has anyone ever seen an original TV Front AC30/6? So far, most if not all TV Front AC30s are AC30/4s.

Below, the cover of the first edition (autumn 1960) of Shirley Douglas's "Easy Guide to Rhythm and Blues for Bass Guitar". Pages have now been included on the page for documents 1960. Further additions to "documents 1960" to come.

This copy came from Jim Marshall's shop in Hanwell.

12th February

The page on documents 1960 has been updated with further material. More to come shortly.

11th February (2)

Just to record an addition to the entry below. The Shadows are first seen with their TV Front AC30 twins in January 1961 (recording for ATV series "Cliff", broadcast in February and March, 1961). Presumably the band received the new amps in December 1960. Prior to that - for much of 1960 - they had used two-tone blue and grey TV Front AC15s.

11th February

Further notes on early AC30/4s and AC30/6s, picking up from some recent entries on this page. Provisional to updating the chronology, there is now a new documents page for 1960, gathering together most of the details referenced below.

29th April 1960. The date given for the drawing up of OA32 and OA26, the circuit diagrams for the AC30/4 and AC30/6. Modifications for the AC30/4 "Bass" model are indicated on OA32. OA31, the circuit diagram for the AC15, also bears the date 29th April 1960. "Normal" and "Bass" models are also noted.

30th April 1960: "Melody Maker" magazine indicates that "Normal" and "Bass" AC15s were available - the day after the circuit diagram had been drawn. "Normal" and "Bass" versions of the single speaker TV Front AC/30 are also advertised.

May 1960: The AC30 TWIN is first mentioned in any dated context - references in the pricelist of Spring 1960 and "Accordion Times", May 1960. This is the TV Front AC30/4 twin, price 100 guineas.

May 1960: It is expressly stated - "Accordion Times", May 1960 - that "Bass" amplifier models are for accordion.

Late July 1960: Paul's Music Studio ("Musicland"), Paul being Tom Jennings's son, advertises the AC30 with 6 inputs - price: 89 guineas.

Autumn 1960: The first split-front AC15 appears. The lowest serial numbesr known at present is 4376N.

19th November 1960: The "VOX 30 Watt, 6 input, TWIN vibrato" is 89 guineas, as above. "Normal" and "Bass" versions of the AC30/4 are available.

December 1960: The Shadows receive three new TV Front AC30s along with two tall stands for Bruce and Hank's amps, and one shorter one for Jet Harris's.

Between December 1960 and January 1961: The Celestion T530 blue is introduced. Prior to that the T530 had a hammertone/oyster finish, as the CT3757.

February 1961: the AC30/4 and AC30/6 circuits are modified slightly (one change).

May 1961: The AC30/6 circuit is revised - the "List of Changes".

Mid 1961: AC30/4s are designated "AC30 4W" on their serial number plates, AC30/6s are "AC30 6W". In the first half of 1961 the two versions are simply "AC30 W".

Mid 1961: Bass versions of the AC30/6 are for the first time designated with a "B" on their serial number plates.

Various accounts over the years have explained, or sought to explain, how the AC30 Twin came into being. The difficulty is fitting them (as they stand) with the chronology which emerges from the documents. Certainly most split-front AC30/4s that have turned up so far have Celestion blues - in other words, speakers that must have been produced after late 1960/early 1961. But the question is - how many are original to the amplifiers in question? What of very early split front AC30/6s?

10th February

Thanks to Philippe, pictures of serial number 21086 - probably mid 1966 or perhaps a little later - have now been added here. A number of amps with serial numbers in the 21000s (and new-style three-line serial number plates) have chassis containing much older components.

Serial number 21086

9th February

A couple of shots of serial number 5595N (with original footswitch) - probably ready for sale in the third quarter of 1962. The tone pot has the date code "GJ" = July 1962, and the blue Hunts capacitor "HHH" = 22nd week of '62 (late May). Thanks to Alun for the pictures.

Serial number 5595N

8th February (3)

A couple of pictures of serial number 5156N, an early AC30 Super Twin that came to light recently. The output transformer is a Radiospares, perhaps an early replacement. Mains transformer and choke are Haddons. The chassis has the phantom cut-out for the rectifier valve. The main Hunts filter capacitor bears the date code "HSW" = 25th week of 1961. 5156N is likely to have been ready for sale in the early spring of 1962.

8th February (2)

A shot of a Jennings "Heavy Duty" label. These were normally applied to the Celestion "spider frame" 15 inch speakers - the T900 and T1074 (blue), and T1109 and T1106 (silver) - fitted in "Expanded Frequency" AC30s. One also finds them on certain Fane 12" and 15" drivers. So far none has turned up on an 18" speaker.

8th February

Thanks to Fabrizio, further pictures of AC30 Super Twin 3617 (top boost module added later) have been added here.

7th February (2)

Just to note that the material relating to Radiospares chokes posted in recent entries on this page has now been assembled at the end of the page on AC30 transformers.

7th February

A page on Wima capacitors has now been started, bringing together the various entries on the updates pages earlier this year and last.

Spent WIMA Tropydurs from a TV Front AC30/4. Thanks to Valentin for the picture.

6th February (2)

The page on black panel AC30/6s has been tidied up - a couple of entries were out of sequence. Entries have now been made for serial numbers 4693 and 4835, both in Finland, and both with later (replacement) copper panel chassis. Number 4835 also heads the page on copper panel amps. It is just possible that the new style of panel came in at some point in the 4800s. But this in a way is asking to proved wrong. The earliest amp with a copper panel that has come to light so far is serial number 4955.

6th February

A detail of serial number 4693, originally a black panel AC30/6, probably from the summer of 1961.

Early on, AC30/4s and AC30/6s were designated simply "AC30 W", the "W" perhaps standing for "Wide Frequency" in contrast to earlier amps that were generally marketed as "Hi-Fi".

From around serial number 4600, perhaps a little before, AC30/4s came to be designated "AC30 4W" and AC30/6s "AC30 6W" to differentiate the two models simply for purposes of record - ie. in the JMI Sales and Dispatch register. But this did not last long. By serial number 4754, AC30/6s were just "AC30". Probably by this time production of the AC30/4 had stopped, and JMI for the most part - note the qualification - continued selling finished amps until stocks ran out in late 1961.

In September '61 JMI announced that the AC30/4 was being discontinued, but that for orders placed before October 1st the price would be 95 guineas.

6th February

Thanks to Simon, pictures of serial number 6887 B, saved from being thrown away (cleared out of a former dance hall) in the late sixties / early seventies. The date codes of the Celestion T530s are "03DH" = 3rd April 1963, so the amp was probably ready for sale in May/June of that year. Further pics to come.

5th February

A detail from circuit diagram OA/031 for the AC15, "Works Use Only", drawn out on the same day as OA/032, the circuit for the AC30/4. The required values for "Bass" amplifiers are indicated in the same way too - see towards the end of yesterday's entry.

4th February (2)

Some more on circuit diagrams. OS/056 and OS/057 - respectively the AC30 Treble and AC30 Bass - were both drawn up neat in September 1964, one on the 10th, the other on the 11th, which is date of the last entry on OS/065 - ISSUE 4 (see a little way down this page).

It may be that Jennings was getting the diagrams in order for Thomas Organ in the USA. AC30 Trebles were certainly on the market well before the autumn of '64.

A detail from OS/056.

A detail from OS/057.

Neither OS/056 nor OS/57, as we have them, bear marginal notes of additions or changes. What of the AC30/6 Bass? It appears that there was no such thing very early on (in 1960 and early 1961) - at least as far as serial number plates are concerned. The earliest AC30/6s with a "B" designated on their plates seem to date from the summer and autumn of 1961 - among the last of the black panel amps.

Taking for the moment these late black panel amps as our starting point, the question is whether there was a separate circuit diagram in the period 1961-1962, say, or whether the differences were marked on a forerunner of OS/065, much as they had been marked on OA/032, the circuit diagram for the AC30/4.

Details from OA/032 - an example of values marked with "Bass" alternatives on the diagram.

In a sense, we come back to the point made a few days ago - that the surviving diagrams, or at least those most often copied and disseminated, are the tip of a small iceberg.

4th February

A detail of a shot taken backstage at the Manchester ABC - sometimes called the Manchester Apollo - 20th November, 1963, during the performance by The Beatles that was filmed by Pathe News. Note the Geloso XLR connector on the AC30 Super Twin speaker cabinet (belonging to another band).

Athough Geloso connectors are known on several surviving AC30 Super Twins, it is good nonetheless to have contemporary record.

Manchester ABC, 20th November, 1963.

An original Geloso on the speaker cabinet of serial number 5329.

Picture from the IMDB page on the Pathe newsreel of the concert. There are some good excerpts on youtube.

3rd February (3)

AC30/6 circuit diagrams (schematics) OA/026 and OS/065. To all intents and purposes, these are one and the same - "Works Use Only"; first drawn out on 29th April 1960; same list of additions and dates down the right-hand side; the final date 11th September 1964.

A/026 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.

Detail from a copy sent out to "Ultrasonics Limited", a service shop in Lincolnshire.

A copy of a copy, slightly imperfect at bottom, from a repair shop in the Midlands.

Below, the "Modifications" listed on the upper right hand of the sheets. There are five sets:

7th February 1961: A capacitor added over a resistor on the plates of the ECC82.

2nd May 1961: The "List of Changes" - substantial modifications. At this point the two lower preamp boards (one of 6 terminals, the other of 35) of early AC30/6s become a single large board (of 42 terminals).

14th June 1963 - ISSUE 2. R74, presumably in the ECC83 oscillator stage, deleted. The cathode bias resistor of the EL84s (R24) changed from 80R to 50R. Secondary voltage of the mains transformer before rectification now 280v (instead of 285v).

11th October 1963 - ISSUE 3. The sheet now titled "Normal". This was added to the original information panel - see above.

11th September - ISSUE 4. The sheet as we have it.

Leaving aside their quirks, A/026 and OS/065 have all the appearance of being master sheets, corrected and brought up to date by erasure and redrawing. The notes ISS. suggest that at certain points other sheets - ISSUES - were produced for use outside the factory.

Clearly in 1962, OA/026 and OS/065 would not have changes relating to 1963. Surely somewhere there must be earlier versions/issues? Westrex and Burndept are likely to have had multiple copies along with their own wiring diagrams.

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

Detail of the upper right-hand side of OS/065.

3rd February (2)

Thanks to Peter, some corrections have now been slipped into the page on AC30 cabinets. Sides and bottom are normally constructed of 1/2" birch ply, but thickened at front with a 1/4" strip to match the 3/4" ply of the top. Baffles are normally 3/4" thick.