2nd July

It has become clear that in 1964 a certain number of speaker cabinets were produced with two baffle boards at front: a main outer board with cut-outs for two 15" speakers (visible through the grille cloth), and an inner one, presumably of thinner ply, the same size as the outer board, with cut-outs for one 12" and one 15" speaker.

To be illustrated shortly.

The earlier method employed by JMI of modifying cabinets with cut-outs on their baffle for 2 x 15" speakers was to fix a board inside the cabinet over the upper cut-out alone. Fixings and a cut-out for a 12" speaker were provided on that.

25th June

A detail of the 0.05uf 500v Radiospares capacitor added by JMI to amplifiers earmarked for export to Thomas Organ in the USA. The cap. was generally not marked on the circuit diagrams. Other models fitted out in this way were the AC4, AC50, AC100, but not so far as one can tell the AC10, AC15, or AC30.

The purpose of the capacitor, from mains supply to ground, was to reduce arcing at switch on.

Detail of a grey panel T60 from late 1964 / early 1965 still in the USA.

24th June

One thing that has been lost sight of in some of the commentary on the T60 is that the topology of its power section (as it stood in 1963) was used some years later by JMI for the range of state amplifiers introduced in 1966-1967: the Traveller, Virtuoso, Conqueror and Dynamic Bass, and the Defiant and Foundation Bass. The Supreme and Super Foundation Bass differ in certain respects, but their lineage is clear.

The claim that this range was a Thomas Organ Company design unfortunately has no merit. The Thomas Organ Company did not enter the business of designing and manufacturing guitar amplifiers until JMI came along. And there is nothing remotely similar to the T60's power section in the scores of circuit diagrams that survive for Thomas organs. Transistor circuitry was used almost exclusively by Thomas for voicings and effects. JMI had its own transistorised effects unit too - in 1959.

Returning to the T60, what made its power section difficult is that everything had to be perfectly in balance for it to work properly - interstage transformer secondary windings, transistors, currents, voltages, and so on. For whatever reason JMI chose not to follow Mullard in using variable resistors for bias adjustment. Bias was set by fixed resistances and that was that. Unmatched or faulty transistors and you were really in trouble.

More to follow in due course.

Detail of JMI circuit diagram OS/062 for the T60, germanium PNP transistors.

Detail of JMI circuit diagram OS/133 for the power section of the Vox Conqueror and Dynamic Bass, silicon NPN transistors thoughout.

23rd June

A pair of Mullard OC35s from the late 1960s or early 1970s, presumably matched - the sort of replacements that one might buy to keep an early T60 going.

It is likely that the slightly lower-gain OC28s were adopted by JMI in 1963 in an effort to increase reliablity. There are reports nonetheless from at least one T60 owner of the regular failure of his power transistors - it ate OC28s - see this page.

20th June

The page on T60 speaker cabinets has been updated to take account of the fact that certain 2 x 15" cabs were, from December 1963, covered in basketweave. These were largely reserved for the new AC80/100 amplifier. Their wood block logos continued to have dimpled black rexine coverings until summer 1964 however.

16th June

The second principal form of T60 speaker cabinet logo, single letters on a wood block covered in rexine, used from later 1963 to the late summer of 1964, though not exclusively. One also finds older-style perspex and early dimpled plastic logos in 1964.

Single-letter wood-block logos are also found on the 2 x 15" cabinets issued with early AC80/100s.

15th June

EARLY PRICELISTS: - neither the T60 nor the AC30 Super Transonic appear in any dated JMI pricelist from 1962 that has so far come to light. The latest list known from '62 is from 30th December.

The earliest listings for the T60 come (i) in the list attached to the "Choice of the Stars" fold-out catalogue; and (ii) in an advert placed in "Melody Maker" magazine, 2nd February, 1963.

Both give the same prices across the range. Where comparison can be made, these prices differ (upwards) from those of December 1962. Which is another way of saying the undated list attached to the "Choice of the Stars" catalogue is indeed from early 1963.

The strongest likelihood is that the T60 was not really ready for sale generally until '63. More on this in due course - something has already been said though on the updates page of the AC30 website, entry for 13th June.

14th June

A black panel T60, no serial number. The latest visible elements are the potentiometers, which have the date code "KK" = November 1963 for their manufacture. Everything else is earlier: Woden interstage transformer dated November 1962, capacitors the 49th week of '62. There are signs that only two rectifier diodes were envisaged initially (earliest form of the T60 circuit) on the main tagboard, which was populated from factory in a non-standard way. On top of the chassis, rivets down the sides, a feature that has not come to light so far on other T60s.

The main power transistors are Mullard OC28s.

12th June

A couple of early shop adverts for the T60: Gough and Davy, Hull, April 1963; and Eddie Moors, Bournemouth, January 1964. Moors was a JMI dealer of long standing.

April 1963.

January 1964.

11th June

A JMI flyer from around Spring 1963 illustrating both the Transonic and the T60 (standard promotional image). The sheet came from Rose's Music Shop in Brighton.

The range of amplifiers still in beige.

10th June

30th August, 1963, Jerry Williams and The Violents, a Swedish group, with a T60 set. The cab had 1 x 12" + 1 x 15" speakers. Below, the amp on stage in late October 1963 when the band shared the bill with The Beatles.

30th August, 1963.

Late October 1963.

The list of bands *photographed in 1963* using a T60 on stage is provisionally: The Beatles, Rolling Stones, Sounds Incorporated, Tornados, The Violents, The Midlands, and Peter Jay and the the Jaywalkers. Jet Harris evidently never used the amp he was given in August 1962 in public.

As further instances come to light - which they are sure to - the list will be updated.

9th June

Sounds Incorporated, October 1963, with a Continental organ and T60 speaker cabinet from early '63, bar across the "O" of "VOX".

Performance of "Keep Moving", released November 1963.

8th June

A detail from a shot of the Tornados on stage, early March, 1963, one of the earliest views of a T60 in use in concert. It looks as though the "O" of "VOX" does not have a bar across its middle. Does this mean that cabinets that do have a logo with a bar across the "O" date from before this time?

The page on T60 speaker cabinets can be found here.

March 1963, a few days before Heinz Burt, on stage to the right, left the band.

7th June

Immediately below, a detail from a photo taken on the 17th October, 1963, at the opening of McCormack's Music in Glasgow. The Rolling Stones, who were on tour at the time, are pictured with the amp. The logo of the speaker cabinet is probably perspex with black rexine backing.

Bill Wyman's T60, used on the band's 1963 tour and probably issued to him by JMI shortly in advance of it, had a perspex logo with pink backing.

17th October, 1963.

November 1963. The T60 is seen on stage in photos taken at Cardiff on 6th October.

6th June

The earlier sections of the page on T60 speaker cabinets have now been updated. More to follow shortly.

5th June

A great detail of the early T60 set owned by the Beat Cats (Berlin). Note the early form logo on the speaker cabinet, the "O" with a bar through, almost as a theta. Is this the cab with the Goodmans Alcomax speakers - illustrated on this page - that came to light in Berlin a couple of years ago?

3rd June (2)

A couple of pieces of documentation. The first, thanks to Andrew, an overview of the specifications of the Texas Instruments 1S021 diode published by Radiospares in 1965. It is probably from Radiospares that Jennings obtained its supplies, late 1962 to 1968.

Radiospares catalogue, 1965.

The second, a sheet for the T60's circuit not much seen these days, drawn up in September 1964, presumably - as in the case of other models and their sheets - to accompany exports to Thomas Organ in the USA. A good number of T60s crossed to America in late 1964 and 1965. AC30 Transonics and Lightweights were not included in the various "deals" with Thomas.

The sheet ends with Issue 7. The two sheets most commonly seen these days take the modifications through to Issue 10 or Issue 11. Copies of one or other are often found in versions of the Thomas Organ Service Manual. To date however, no Thomas copy of the September '64 diagram has come to light. There must be one somewhere though.

Still the earlier sheet number "A/027". Later ones were redesignated "OS/062".

3rd June

The preamp of the T60/AC30 Transonic has three main sections: (i) an initial gain stage of two OC44s; (ii) volume, treble and bass network, based on the circuit published by Maynard in "Radio Constructor" magazine (see below, entry for 30th May); and (iii) a further two OC44s as a "gain recovery" stage before the driver transistor.

The initial gain stage (i) generally follows that of Mullard's public address amplifier (1960-1961), though Mullard envisaged OC71s, as it did for most of its other cascaded gain stage designs. Why did Jennings prefer the OC44? Hard to say for sure, but it may be that in practice its noise floor was lower than that of the OC71.

At any rate, Jennings, in a manner of speaking, stitched the preamp of the T60/AC30 Transonic together from existing elements.

Below, shots of one of the first Mullard semiconductor data booklets - from 1957 - containing sheets for 10 types of transistor and 9 diodes. The OC44 was new in December 1956, the OC71 had reached its settled form by November.

Whether the transistorised preamp/effects module developed by JMI in 1959 for its "Entertainment" organs had OC44s is unknown.

Mullard semiconductor data book, 1957.

Advance pages for the OC44, December 1956..

2nd June

It turns out that the "1S021" rectifier diodes in the power section of the T60/AC30 Transonic, mentioned below, were manufactured by Texas Instruments. Texas had opened a facility in Bedford around 1960 - see this page - parictipating in various British trade shows in the early 1960s. Over the course of the T60's production (through to early 1968), Jennings used around 8000 1S021s.

So far, no earlier analogy has come to light for the use of the Mullard OC35 as an audio output transistor. Mullard recommended the OC26 and OC36. The OC35 was "general purpose", not that that can really have mattered for what was primarily a bass amplifier.

It may be that the power section of the T60 as we have it was for the most part a Jennings design (a sort of working forward from the Mullard circuit book). Whether CTH Electronics and Audix, both early to market with solid state public address amplifiers, followed Mullard to the letter is not known.

January 1961.

In later 1963, Jennings moved from the OC35 to the OC28, perhaps in the search for greater stability and consistency. Now and again, one finds OC26s. In terms of surviving T60s, the majority have - or now have - OC28s. The pincipal question marks hang over serial numbers 239 and 268 (black panel, later 1963), which have modern OC28s, possibly introduced by repairmen in line with later and more common versions of the circuit diagram. All this will be tabulated in due course.

An early Transonic amplifier section, however, does retain its original OC35s. Thanks to Steve for the picture.

1st June

Picking up from yesterday's entry, a short note on the power section of the T60/AC30 Transonic amplifiers. This stands half-way between two circuits for a Public Address amplifier published by Mullard in 1960 - one with a 14V supply and an output transformer for the speaker; the other with a 28V supply and a speaker directly coupled to the output transistors (as in the case of the T60).

A peculiarity of the JMI power section is the use of "1S021" rectifier diodes about which there is little documentation. Transistor equivalents books from the 1960s and 1970s give the BYX22 as a direct replacement. Mullard envisaged generation of the HT in its circuits from accumulators (batteries) rather than a mains transformer.

The power transistors in the T60 (OC35s rather than OC26s) are arranged as the Mullard 14V circuit (above) published in the "Reference Manual of Transistor Circuits" of 1960 and 1961; in its HT voltage and directly coupled speaker, the T60 follows the Mullard 28V design.

Detail of the T60 circuit of early 1963. The initial design had only two 1S201s. The manufacturer of the diodes is unknown at present.

Two of the four "1S021" rectifier diodes in a T60 from the autumn of 1963.

30th May

A little more on JMI's work on transistorisation in 1962. Below, a detail from a circuit for a transistorised guitar preamp published in "Radio Constructor" magazine, September, 1961, by D.E. Maynard, alongside a detail from JMI's circuit for the T60 bass amplifier (as used also in the AC30 Transonic and AC30 Lightweight amplifiers).

Allowing for necessary differences - the guitar preamp was battery-powered for instance - the arrangement of the tone controls is pretty much the same. Towards the end of his article, Maynard offered a number of modified circuits (for low-impedance input signals, etc.), one using a 10K volume pot following an initial gain stage, and 10uf coupling caps, things also present (later on) in JMI's T60.

It seems likely that Les Hills and the design team at JMI used circuits or parts of circuits such as these as their starting point. Quite what prompted the employment in the T60 - and the "44" and "55" organs - of Mullard OC35s as output transistors is not clear at the moment though.

Just to note that in 1966/1967 Triumph Electronics, one of JMI's principal contractors, brought to market a transistor preamp incorporated in a guitar strap - a neat idea. Perhaps based on Maynard's article?

27th May

Below, a couple of instances of early T60 bass amplifier sets, the first made for Jet Harris and illustrated in the music press in August 1962, the second seen on stage with Sounds Incorporated, who had one of the first Vox Continental organs, in late 1962.

JMI's approach initially seems to have been to provide select bands with T60s and Continentals as a sort of new transistor-based package. The Tornados also had one of each by early 1963, seen on stage in March.

Why no diamond grille cloth on the early T60s? Perhaps at the time JMI did not have rolls that were tall enough for the speaker cabinet. That the one shown by JMI at the Russell Trade Fair in late August 1962 had a standard diamond front (and a standard vertical "pie" logo) has to be borne in mind however.

Jet evidently never used his T60 publicly, either with the Shadows or afterwards (info. kindly supplied by Terry Webster), preferring, when he had the choice, AC30s.

On the left, the trade press photo; on the right a detail from an ad in the mainstream music press, Jet Harris unfortunately wrongly spelt in both. The VOX logo looks a bit half-hearted, certainly in relation to the one pictured below.

Sounds Incorporated on stage, late 1962. The rear panel of the amp has an extra control, probably for tremolo, as below. By mid 1963 the band had a new "standard" T60.

The panel of a Vox Transonic amplifier, effectively a T60 chassis in a space-age-themed cabinet. Controls: volume, treble, bass, and tremolo.

23rd May

Prelim. notes on the Mullard OC35 germanium transistor, used by JMI for the T60 (and Transonic) amplifiers; the power section of the Vox Continental; and the amplifiers incorporated in the "44" and "55" organs.

The circuits in view were, in the first instance, the work of Les Hills, who had played a leading part in designing the Univox for Tom in 1951. Les reportedly rarely ventured past the front gate of his house in Belvedere, so people from JMI normally had to go to him.

Where full-time members of the Jennings R&D Department are concerned, Alan Harding had the greatest experience of working with transistors - he developed the Vox Radio Microphone system for instance - but others will naturally have contributed to the designs, not least Derek, Dick, Ken McDonnell, and Tom, who had the final say on functionality and appearance.

Working examples of all three organs, along with the T60 and Transonic, were exhibited at the Russell Hotel Trade Fair in late August 1962.

So far as one can tell, the first adverts for the Mullard OC35 appeared in early 1960. Below, the cover of the Australian edition of Mullard's "Outlook" news and technical reviews bulletin for March and April.

Initially in the UK the retail price of the device was 48 shillings - an enormous amount (November 1960). Bulk / trade prices will have been slightly less. By the summer of 1961, retail had come down to 25 shillings. Late in the year it fell further to 15 shillings, the price of a Mullard EL34.

As for data sheets, it is clear that in June 1962 Mullard circulated new, or at least fuller details in its handbooks and technical service data collections. It is to be hoped that earlier documentation can be illustrated in due course.

Mullard Outlook, Australian edition - image from this page.

Mullard data sheet, June 1962.

2025

17th April

Late 1964, a detail of a shot of Dick Denney demonstrating the Guitar Organ in the main R&D room in 119 Dartford Road, a Vox T60 chassis at left. With the move to grey panels came a new type of chassis, a row of ventilation holes on the underside at front.

T60 serial number 815.

10th April

An even earlier instance of a T60 in a shop in the "provinces": Gamlins, Cardiff, February 1963 - amplifier section with two 2x15" cabinets, the format initially advanced by Jennings.

February 1963.

6th April

One of the earliest adverts for the T60 from a shop outside London, Yardley's, Birmingham, May 1963.

7th March

Thanks to Gordon, pictures of serial number 506 (copper panel) have been added here.

16th February

The page on the Vox Transonic has been updated and will be updated further soon. The page will also be ported to the Vox AC30 website.

1st March

The page on T60s with black panels has been rearranged slightly and updated.

28th February (3)

Some more on the Vox Transonic on the way. For the time being, a rough print picture of Peter Jay and the Jaywalkers published in "Melody Maker" magazine, August 1963.

The Transonics, just in view, have four control knobs. Note the T60 bass in beige.

28th February (2)

Just to add that the T60 speaker cabinet that now bears the serial number plate of T60 amplifier number 248 (black panel) cannot have been the original as it has black basketweave vinyl. Basketweave seems to come in with the first T60s with copper panels.

28th February

Three more amps added - 676 (copper panel); and 921, and 2041 (grey panels). 2041 brings the picture to late 1967 / early 1968. T60s were still being advertised in the United Kingdom and Germany in 1967. The new range of solid state amps, introduced in early Spring, gradually pushed older models (aside from the AC30 and AC50) out though.

27th February

Updates are now in progress. Thanks to Jake, serial number 664, copper panel, has now been registered; and thanks to Ash, serial number 721, also copper panel. On the black panels page, the serial number plate of 248 has been posted.

Serial number 721.

6th February

After a longish period of inactivity, the site will be underway again soon - a good number of updates coming.