The AC30 Lightweight - also known as the "Transistor Lightweight 30" - was introduced by Jennings in early 1964 as a species of successor to the fabulous and deeply unconventional AC30 Transonic. The design was straightforward: the chassis (as in the Transonic) a Vox T60 transistor amplifier, voiced for guitar, and with the addition of a tremolo circuit; the speakers three 3/4 ohm Elacs - 10N/82 (blue) or 10N/86 (silver) - wired in series for a total impedance of around 10-11 ohms; and a new cabinet with full grille cloth front. A page on Elac speakers is available here. The cabinet was perhaps more complex than most - a semi-open back with a cut-out for the tremolo controls in the upper panel, and a sloping upper section to close off the amplifier compartment, this last a result of the need to match the angle of the T60 chassis' control panel.

In the "Precision in Sound" newspaper format catalogue of September 1964, a "Transistor Lightweight 30" is represented in a box orientated the other way round - controls in a sloping front-facing compartment, grille cloth below, presumably a flat back, dimensions identical. Just such an amp can be seen in pictures of the Jennings stand taken at the Russell Hotel Trade Fair in August 1964.

Later on, surplus boxes were later taken up - probably after the collapse of JMI in early 1968 - by the makers of the so-called "Mystery Amp". More on that can be found on this page.

Dimensions, circuit diagrams, and serial number plates

It may be useful first to give some dimensions of the standard AC30 Lightweight as (i) JMI only noted the size of the later short-lived version with front-facing controls (in the "Precision in Sound" newspaper format catalogue); and (ii) Jim Elyea, page 424, gives dimensions for something other than the standard amp.

Overall size: 24" wide x 21 1/2" tall x 11" deep. The top, bottom, and sides are 3/8" birch ply. Internal runners/braces are 3/4" square.

Slider board: 23 3/16" long x 9 5/8" wide x 3/8" thick. The cut-out is 13 7/16" long x 5 1/2" wide. The cutaway, to avoid the speaker, is 9 3/4" wide along the outer edge of the board, tapering to 5 3/4". It is 1 7/8" deep.

Baffle: 23" wide x 19 1/8" tall x 3/8" thick. The cutouts for the speakers are 8 1/2 in diameter, the centre bar in each 3/4" wide.

Fixing screws for the back boards are 2BA. All other screws - set screws on the baffle, internal fixings - are 4BA.

Lightweight baffle. Around the edge a sort of lower "border", 3/4" wide, 3/16" deep, to help the baffle sit tight up against the inner runners of the cabinet - a further cost in what will already have been an expensive item.

The relevant circuit diagrams are OS/062 (for the main amplifier), and OS/041 for the transistor-driven vibrato.

Detail of OS/062 for the T60, re-drawn in this form in early 1964 to incorporate the changes necessary for the AC30 Lightweight.

Detail of OS/041.

As for serial number plates, initially amps were numbered in line with the T60 sequence, beginning at 500, the model designation being "A.C./30L". See below, serial number 504. The series ran through to the 600s. Thereafter, Lightweights were given numbers in the main AC30 sequence - 12247L and 13939L being two such examples.

Below, one of the only instances of a Jennings promotion for the amp - an overview for an electrical trade press journal (which carried pieces on the activities of JMI from time to time).

Electrical trade press journal, March 1964.

Detail of a JMI pricelist, Feb/March 1964.

The amp was probably displayed in public for the first time at the Frankfurt Trade Fair (Musikmesse), April 1964. In late 1964 a new version with a front-facing control panel was formulated, with a sort of "Fender" feel to it. No coincidence perhaps that the "Million Dollar Deal" made with Thomas Organ - to distribute Vox equipment in the USA - had just been announced. Production numbers - if any point beyond pre-production had indeed been reached - are likely to have been extremely small.

Detail from the "Precision in Sound" newspaper-format catalogue of autumn 1964. There is not quite enough detail in the original picture to read the legends on the control panel unforunately.

Below, a short section on later sales - primarily for the moment from "Exchange and Mart" magazine. To give some context, the list price of the model in late 1964 was £94 10s.

November 1965. 80 guineas is £84.

March 1966.

Surviving Amplifiers

Updated

Serial number 504

This Lightweight / T60 has Albion transformers: mains "OR163", interstage transformer "OR161". The seller noted that one of the pots had the date code "JK" = October 1963 for its manufacture.

Serial number 505 - currently in the UK

As in the case of serial number 505, the transformers are Albions. The speakers are early silver Elacs with "tin can" magnets, no bell cover.

Serial number 508 - currently in the USA

Pictures to follow.

Serial number 509 - currently in the UK

Thanks to Eric for the pictures. Silver Elac 10N/86s with later "bell" magnet covers (perhaps from a Line Source column?). The wiring, with insulated staples to fasten sections to the baffle, looks to be original.

Serial number in the 500s

Sold some time ago.

NEW

Serial number 538

Recorded by Jim Elyea.

Serial numbers now in the main AC30 sequence

An "L" suffix added to the number to denote "Lightweight"

Serial number 12374L

A rare instance of a black panel (used on T60s early on). The speakers are blue Elacs, 10N/82. Thanks to Kevin for the pictures. Below, a couple of shots of the tremolo assembly. The transistors in the tremolo circuit are OC44s (three). One of the filter capacitors in the main amplifier section is dated "IDI" = 38th week of 1963 for its manufacture.

Serial number 13939L

Further pictures to follow.

Serial number in the 14000s (?)

Brown grille cloth, but amplifier section with a grey panel. Where T60s are concerned, grey panels come in around serial number 800. On the grille cloth lower right, the original "LIGHTWEIGHT" runner survives. The silver Elacs have early "tin can" magnets, no bell cover.

NEW

Serial number 15205 L - currently in the UK

One of the original silver Elacs in place. The tremolo module was removed at some point in the 1980s or 1990s. The pots in the amplifier chassis are dated "BL" = February 1964 for their manufacture.

NEW

Serial number 15246 L - currently in North America

An AC30 Transistor Lightweight, the "Lightweight" flag still present lower right on the front grille. Tremolo controls on the back panel. The speakers are early silver Elacs - most likely 10N/86s (4ohms apiece) - with "tin can" magnets. Thanks to Chris for the pictures.