Production of treble preamp chassis

Jennings Musical Industries

A solid state modular treble preamp section - well used. The octal plug plugs into the power section. The unit measures 21 inches wide; 3 3/4 inches deep, and 7 1/4" inches tall.

Defiant serial number 1031. Rear view of the the preamp module with its back off, the reverb tray suspended at the foot underneath the circuit boards.

Below, some notes on the assembly of treble preamp chassis for the solid state range at the Burndept Electronics / Vox Works in Erith in 1967 - the common unit that went into Virtuosos (with reverb), Conquerors, Defiants and Supremes. The Traveller was constructed differently.

Chassis numbers

Perhaps the first thing to say is that all preamp chassis were stamped with numbers, the sequence beginning at 01000 (or 01001). The first "0" regularly stamped poorly, resembling a "C" - see the third picture, below

Very early Conqueror and Dynamic Bass preamp chassis designate the model of amplifier for which they were destined. "30W T. PRE/AMP SER. No." for the 30 watt Conqueror. "30W B. PRE/AMP SER. No." for the 30 watt Dynamic Bass.

This seems to have been dropped fairly quickly. Preamps were subsequently designated "PRE/AMP T. SER. No." The need to store sets for the different models separately was therefore removed - the units used for the Virtuoso, Conqueror, Defiant and Supreme were all identical cosmetically and electronically in any case.

No examples have yet emerged, however, of chassis stamped "50W T. PRE/AMP SER. No." (for the Defiant) and "100W T. PRE/AMP SER. No." (for the Supreme).

Conqueror serial number 1046. Treble preamp chassis no. 1045.

Dynamic Bass serial number 1135. Bass preamp chassis no. 1001 - either the first or second bass casing made.

Supreme serial number 1094 - preamp chassis no. 1619. Note the standard format: "PRE/AMP T. SER. No.".

It has been said that the stamping of chassis numbers by Burndept, the company that assisted Vox in the making of the solid state range, was a legacy of wartime work. In actual fact it was simply Burndept's standard practice. Many companies that made equipment during the war did not stamp numnbers at all. Practice varied with manufacturer.


A detail from a photo of one of the workrooms in the Burndept / Vox Works in Erith taken in June 1969 (NOT June 1967 as has been supposed). On the benches, Defiant power sections.

1) - Preamp chassis casings were stamped out, numbered, and passivated (chemically treated to prevent rust). Certainly in late 1967, Burndept and Vox used the company "Holmes Plating" in Blackheath to do the passivating.

2) - When the chassis casings were ready, control panels were fitted and the units were populated (in batches) with components on the workbenches in the Vox Works at Erith. Following inspection, they were then put to one side in no particular order. It seems that finished preamp chassis could sit unused for some time.

A Foundation Bass preamp chassis, stamped "VIB.". The same stamp is also found in treble amplifiers. Later on one finds large numbers stamped in preamp sections and circular "JMI" inspection stamps.

3) - When the time came for a treble amplifier to be made up, a preamp chassis was selected from the store, along with a suitable power amp section (30W for a Conqueror, 50W for a Defiant, and so on), and a wooden box. Numbered stickers were added to the box and its back-board, and to the preamp and power amp. The various elements could then be reunited if something went astray in the final assembly process, or indeed later on. As the numbers on these stickers do not go very high, batches of no more than a dozen or so were done at a time, and the sequence restarted at "1" for the next. Preamp and power amp were then fitted into the cabinet.

Left to right, stickers on the back-board, box and rear of the preamp section in Supreme serial number 1058. The surface of the one on the back of the preamp has been washed over. None survives on the power section. Such stickers can also be seen on the boxes of Supreme serial no. 1030 (7), Supreme 1087 (3), and an early Super Foundation Bass (1).

4) - A serial number plate was added to the back-board, and the number noted in Jack Jennings's register. The serial numbers of the various models were recorded in order. Probably at this point the amplifier section was matched up with a suitable speaker cabinet. Speaker cabinets were not numbered in the JMI period.

5) - Following final inspection, tags were attached to the amplifier and its speaker cabinet, and sets of covers for the two sections provided.

Defiant serial number 1102 with its original covers and inspection tags.

6) - The amp (with warranty cards attached) was then boxed, its model name and serial number written on the outside, and the box stacked in the store ready for despatch. When known, the destination might be written on the box too.

7) - Before the amps were sent out, destination and purchaser were noted against the serial number in the registers.

Below, a digest of known component date codes noted against treble preamp chassis numbers - an explanation of how the various codes work . As one might expect, the higher the chassis number, the later the component date codes, generally speaking. But there is nothing at present to indicate that chassis were populated strictly in numerical order.

A couple of chassis with component dates unknown are included simply to give an indication of final use - in other words the resulting amplifier.

One of the interesting things to emerge from the table is that production of Supremes seems to have lagged slightly behind the Conqueror and Defiant. Serial number 1030 for instance has component date codes of March 1967.

Whether troubles encountered by The Stones - who had standard production units as early as mid March '67 - set the sale of other early Supremes back is hard to say. It could easily be though that JMI was waiting to see. Early Supreme power sections were biased very hot (as numerous incinerated tagboards attest). Transistor / bias failure may have been a real concern in March '67.

By late April 1967, around 100 Defiants had been produced - see serial number 1102 . Presumably around the same number of Conquerors (or perhaps more) had also been made by that point.