Below, circuit diagram OS/013, drawn in November 1969, the earliest surviving sheet (so far) for the AC30 with integrated Top Boost: an image of the whole for orientation, and a larger detail of the Top Boost circuit itself.

A second copy of the sheet survives, not singed as this one is, but generally less distinct.

The original company name in the panel will have been "Vox Sound Equipment Limited". When "Vox Sound Limited" came into being in January 1970, many of the sheets were adjusted accordingly.

The circuit corresponds with JMI OS/010, drawn out by John Bell in December 1961 - see this page for an overview of the surviving versions.

COPPER PANEL CHASSIS WITH TOP BOOST CONTROLS IN PANEL The majority were made by Westrex (the exceptions being a small number of chassis made early on by Burndept). Components for the most part have date codes from summer 1963 ("UF" to "UH" - June to August). Manufacture took place either in very late 1963 or early 1964. The "Expanded Frequency" AC30 (AC30X), which normally incorporated such chassis, was advertised for the first time in February 1964.

In terms of serial numbers, the lowest that have come to light so far are 10346 and 10348.

Detail of AC30 Super Twin serial number 10346. The Top Boost controls (Treble and Bass) are the two to the left of the Tone Cut (furthest right).

Detail of the underchassis of AC30 Twin serial number 10348. Westrex marked these chassis with an "X" in a circle. This one has year of manufacture: - "64".

No JMI circuit diagram is currently known for the AC30TB. The earliest we have is OS/013 (two copies only), drawn up either in January or November 1969 by "Vox Sound Equipment Limited". One of the copies, illustrated at the head of this page, is marked "Serial number 9619. Modified". 9619 is a JMI number, not part of a sequence ever used by VSEL.

Wallets of circuit diagrams supplied to repairmen by JMI generally only had sheets for the AC30 Normal, Treble, and Bass, along with OS/010 for the *add-on* Top Boost circuit.

As mentioned above, Westrex produced the majority of the chassis with copper-coloured Top Boost panels. Burndept concentrated on Normal, Bass, and Treble runs.

The number of chassis made?: 100 has for years been the estimate most often quoted and the figure regularly cited when an amp comes up for sale ("only 100 made!"); over 500 has also been supposed, but that seems to be based on at least one wrong hypothesis. 500 in any case would be almost one in four AC30s during the period concerned.

Where Super Twin amplifier sections with such chassis are concerned we have: - serial numbers 10346, 2583TB, 2690 (not designated Top Boost on its plate however), and an amplifier issued with a pair of Line Source 40 public address speakers. It may be that AC30 public address sets of this sort were not uncommon. The chassis currently in a Super Twin box with a serial plate bearing the number 12897 is a later replacement for the original, however.

The chassis now in the Super Twin box with serial number 2592TB was not issued from factory in it either. The box has Top Boost controls on its back panel.

An overview of chassis known to survive (February 2025) can be found on this page.

The question of how many chassis were made remains though. Around 300 is most likely. As far as can be determined all copper panel chassis with integrated Top Boost controls have 50R bias resistors.

THE FIRST GREY PANEL CHASSIS WITH TOP BOOST CONTROLS IN PANEL - preliminary notes.

As part of the move to the new-style grey control panels, JMI handed production of Top Boost chassis over to Burndept. Burndept, as standard practice, stamped its chassis with numbers, indicating the voicing with a flash of paint and legend stamped in red. The colour for units with in-panel Top Boost was green, the legend "T/B Boost" (Treble/Bass Boost). See this page.

As mentioned above, there was naturally some "cross-over" - at least one Burndept-made chassis was given a copper panel. By the same token, at least two Westrex-made units survive with a grey panel - AC30 Super Twin serial number 2673TB, and another serial number unknown.

AC30 Super Twin serial number 2763T, grey control panel, chassis assembled by Westrex, stamped "T.B." at left.

Serial number unknown. Chassis assembled by Westrex. Grey control panel with integrated Top Boost controls. As above, "T.B." stamped on the preamp upright. Note "165" in red crayon. The same writer wrote this and other numbers (119, 130) on various other JMI amplifiers.

Known early examples of Burndept-made Top Boost chassis, arranged below in order of chassis number, are:

03916 - a reference only to this chassis being a Top Boost, perhaps incorrect. No further details at present.

2901TB - Super Twin, chassis 4476, Mullard capacitors with date codes "D3N", "A4N", "14N".

unknown - Twin, chassis 4530, Mullard caps with "14N" (1st quarter of 1964).

12411TB - Twin, chassis 4551, Mullards "A/64" and "14N".

unknown - Twin, chassis 04565, copper panel (original), Mullards "14N".

unknown - Twin, chassis 04590, copper panel (repro?), Mullards "14N".

11657TB - Twin, chassis 4616, Mullards "14N".

12019T - Twin, chassis 4690, Mullards "D3N", "A4N", "14N".

13098T - Twin, chassis 05135, Mullards "14N" and "A/64".

13763TB - Twin, chassis 05136, Mullards "14N" and "A/64".

2739T - Super Twin, chassis 5214, Hunts cap. "WYT" = 10th week 1964, and main preamp filter = "WHT" 12th week 1964; three Mullards with "B4N" .

12405TB - Twin, external shots only.

This list could of course be extended if Twins with serial numbers in the 13000s or higher and Super Twins with numbers in the 3000s and higher were to be included. A version of this list, again arranged by chassis number, can be found on this page.

Evidence from grey panel AC30 Expanded Frequency is a little lacking at the moment. As noted elsewhere on this site, the new serial number sequence, which corresponds pretty much with introduction of grey panels, began at 500 and stretched into the low 600s before the model was deleted from the catalogue. The highest number known at present is 604. See this page.

Back to the page on add-on Vox AC30 Top Boost units.