The solid state Jennings J40, one of the most popular amps of Tom's new range, was introduced in mid 1968, having been developed alongside the J100 - on which, see this page. Both were exhibited in public for the first time at the "British Musical Instrument Industries" Fair in August 1968. By the time the J40 was deleted from the catalogue, at least 500 had been sold.

An extract from Beat Instrumental magazine, August 1968, reporting on the BMIIshow. Mention of "transistor amplifiers from 40-100 watts" encompasses the J40 and J100. At this time the company was "Jennings Electronic Developments". In 1969 it became "Jennings Electronic Industries".

The J40 preceded the Jennings AC40 (an all valve descendant of the Vox AC30) by around eighteen months. For Tom, it was solid state amplifiers first.

Detail from the Jennings Electronic Developments catalogue of late 1968 / early 1969, produced in advance of the Frankfurt Trade Fair of late February.

To judge by the blurb above, the initial drive was to make the J40 as versatile as possible - auxiliary inputs for "tape recorder, record reproducer", and so on. See the first two examples in the section on "Surviving Amps" towards the foot of this page. It is clear that early J40s were constructed and detailed differently - see below.

In the newspaper-format catalogue printed for the Russell Hotel Trade Fair of August 1969, conventional specifications were given, however - standard form for amplifiers intended for guitarists and organists.

Detail from the Jennings catalogue of August 1969.

Above, a section on JEI in the "Focus on Amplifiers" published by "Beat Instrumental" magazine, February 1970. The J40 cost £130.

Through to late 1972 the J40's recommended retail price remained relatively stable - only a £10 increase by December of that year. In mid 1973, it was reduced by 10 per cent. A few months later the J40, J100 and J100D were deleted from the Jennings catalogue, though it is clear that some continued to be sold.

Jennings pricelist, December 1972.

Brief list of prices published in June 1973.

Circuit and assembly

sectoin in progress

The earliest J40s (serial numbers 500 to 600s?), early 1968 to mid 1969: although nothing is known definitively of their electronics beyond general arrangement, certain observations can be made:

(1): - preamp and power section are separate assemblies, one at the top of the cabinet, the other in the base, the two doubtless connected by umbilicals - flying leads - much as in early Vox AC10s and AC30s;

(2): - the power section of the early J40 in all likelihood stands in close relation to power sections of the early Bass and PA amplifiers, produced for Jennings under contract by Triumph Electronics in Purley. Tom and Dick Denney had a long history in JMI days of working with Triumph.

(3): - in company with the Jennings Organ, Bass, and PA amps, signal input and output sockets are provided on the back panel of the J40. "Slave Output" feeds the preamp out to a different power amp; "Auxiliary In" feeds the preamp of another device into the J40's power amp. On the Organ, Bassd, and PA amps these sockets are termed: "Preamp Output", "Main Amp In". It would have been an easy matter for Triumph to have produced a back panel plate specially for the J40 - simply a piece of painted aluminium with letraset legends (the favoured Triumph way of doing things). Quite whether the early back panels all took the same form is hard to say - vanishingly few early Jennings amps from 1968 survive.

The back panel of a Jennings PA100 from 1969. Compare this with the back panels of the two early J40s illustrated below.

(4) - this is not to say the J40's power amp was *identical* to that of the JEI PA50, say (available from Spring 1968), simply that it shared a common type of chassis and modularity of build.

(5) - as for the power section's circuit, no doubt designed by Dick Denney in JMI days. A version of the JEI PA amplifier was displayed by Vox at the Russell Hotel Trade Fair in August 1967 - see this page. The team in charge of Vox - "Vox Sound Equipment Limited" - from mid 1968 were old friends of Dick and Tom.

(6) - the preamp of the early J40 was in all likelihood much the same as that of later J40s. Below, a detail of an early board stamped "JED PC". "JED" is "Jennings Electronic Developments", Tom's company through to mid 1969, whereupon "Jennings Electronic Industries" was brought into being in its place.

Preamp board of an early J40. Thanks to David Musker for the photo.

(7) - also to mention thanks to Dave Musker that the closed back of the early J40 will have been necessary for safety's sake - to keep inquisitive hands away from the chassis in the base of the cabinet.

When further details relating to early J40s come to hand, as they doubtless will, they will be added here.

Standard J40s (serial numbers in the 600s on), mid 1969 to 1973 (?): a new design, preamp and power amp consolidated on a single chassis along the lines of the J100, made ready for display at the Russell Hotel Trade Fair in August 1969.

A detail from a small newsprint picture of Dick Denney (far left) at the Russell Hotel Trade Fair of '69, new-style J40 with dark strip behind its controls in the foreground. At right in the chair is John Oram, who had demonstrated the old-style J40 (among many other things) at the Frankfurt Fair of February 1969.

By the summer of 1969, the range of Organ and Bass amplifiers had been completely revamped too. Gone was the old design (as illustrated in the brochure of Spring 1968). Modularity was key. Below, the preamp board of a JEI O50, effectively the same as the J40 board pictured a little way above but without the network of capacitors for the MRB (spaces provided but unpopulated).

Preamp board, JEI O50

Although no circuit diagram for the new-style J40 has so far come to light, we can get a good way forward thanks to Steve Walsh and David Musker. Steve described the preamp as being:

"generally remarkably similar to the circuit in the Vox Phantom Special. Take the Normal Channel on a Mark 1 Conqueror but instead of those tone controls, attach the tone stack from the brilliant channel with the MRB. The presence control switches between two different caps or no connection at all. The Tone switch is unusual. It seems to break the connection between the pot wiper and three caps of MRB, resulting in a considerable reduction in volume."

Denney's MRB circuit goes back at least as far as late 1964 (the prototype Vox Guitar Organ).

The preamp assembly of a late J40. Thanks to David Musker for the photo. The underside of the board is illustrated above. The presumption is that in early J40s flying leads would connect this module to the power section in the base.

Main power transistors were 2N3054, as in the Vox Defiant. Conquerors and Supremes had 2N3055s. The extent to which the circuit adopted for the power section of later J40s mirrors that of the Defiant will need to be taken up at some other point. Simply to say for the time being that the secondary of the Defiant's mains transformer is 36v-0-36v. The Jennings Organ and PA amps have secondaries of 27v-0-27v; the J100 and J200's are 30v-0-30v.

As for the manufacturer/assembler of the electronics - Jennings did not have its own lines - the suggestion that "assembly of chassis is likely to have been undertaken by the APT Electronic Group of Byfleet and Ascot", ventured a while ago on this page, now seems too strong. APT is certainly possible, but Triumph has a good claim too.

Speakers

Speakers were generally Celestion alnico T1096s, the 15ohm version of the T1088 common in Vox AC30s, wired in parallel for a total impedance of 8ohms. The point was made on the sticker pasted inside the speaker compartment of J40s.

JEI speaker impedance warning sticker.

Early - serial number unknown

An early Jennings J40, secondary panel at rear. The back panel has a central cut-out similar to the ones that Denney devised for the Beatles' early AC50 speaker cabinets.

Serial number 546

Power section at rear, as in the amp above. Note too that the graphics on the control panel are different from those of the amps below. Serial number plate of the early type, no mention of patents at foot.