The "100W Amplifier" - fixed bias

Produced for Vox by Burndept Electronics, Erith

June/July 1965, the immediate precursor to the AC100 Mk2

Serial numbers c. 430 - 724

In March 1965, the Vox Works / Burndept Electronics factory in Erith, already busy producing AC30s and AC50s for Vox, began making the cathode biased AC80/100, taking up the baton, after a brief hiatus in early 1965 when few amps were made, from Westrex in West London.

Some 130 AC80/100s rolled off the line at Erith, the last probably emerging in late May / early June '65.

Above, views of the Works. For further info, see this page.

Demand from the USA was the thing that drove production at the Vox / Burndept Works forward; and a good proportion of the amps produced were destined for export. More on this page.

What seems to have brought the fixed bias "100W Amplifier" into being was, on the one hand, a continuing process of design and development, and on the other, the feeling that the cathode bias AC80/100 was running too close to its limits in terms of heat generated and stresses placed on vital components.

Not only did the "100W Amplifier" run cooler by far, but it delivered a true 100W.

If adverse comment on the AC80/100 did reach England from the US, it will for the most part have arrived too late. By the time large numbers of AC80/100s did actually reach the States in the summer of 1965, Vox had already moved over to the new fixed bias circuit. Only a very few amps made by Westrex in 1964 seem to have been exported.

Indeed, if there were early whispers surrounding the AC80/100's perceived unreliability (ie. in late 1964 and early 1965), they will have come in the main from English bands, either at home or on tour. Whether the JMI repair registers contributed evidence is anybody's guess.

Above, The Who in Copenhagen, 25th September 1965, with six AC100s. At this date they are likely to be the new "100W Amplifier", less likely the AC100 Mark 2. One can tell that they are not cathode biased by the presence of two screws on the top edge of the backboard. Cathode biased amps had only one. It seems doubtful that these amps found their way back to JMI in one piece.

Components old and new

Many elements could simply be carried over from the AC80/100 - switches, jack sockets, voltage selectors, Erie resistors, and so on. No new supply lines needed setting up. Other components, however, had to be sourced anew, key among these being the transformers and elements of the bias circuit.

For its axial and main filter capacitors, Vox returned to TCC and Daly, which had been used briefly in late 1964, but set aside in favour of Dubilier, which had been used in the early AC80/100s.

As a side note, it is well to say that the chassis base, plain metal in the case of the AC180/100, was proofed again rust - "passivated" - giving it a light gold appearance.

"100W Amplifier" Schematic

Dated 30th May 1965.

Just under three weeks after this, on the 17th June, a new schematic was produced: the "AC100/2 Amplifier". And eleven days after that, on the 28th June, the addition of the brimistor is recorded.

The addition of the brimistor required new holes to be provided in the chassis (for the two standoffs) behind the EL34 nearest to the choke.

As ever, click for larger images. At right, the holes for the brimistor standoffs (long since removed).

Around three hundred "100W Amplifiers" were produced at the Burndept works in Erith - , all without the brimistor.

Although the dates given on the schematics are unlikely to correspond exactly with the start of production in the factory, the time lag between production and the drawing up of the schematics in these cases is not likely to have been great.

If we allow that production of the "100W Amplifier" lasted five or perhaps six weeks, approximately 10 per day must have rolled off the line.

Early to mid 1965 was an intense period of activity for Vox/Burndept, particularly in view of the orders received from the Thomas Organ Co. in the States, the first being in September 1964 ($1m) and the second, mid '65 ($2-4m). Both were reported in "Beat Instrumental magazine.


The transformers, instantly identifiable by their black shrouds, mark a departure from the Wodens of the AC80/100 and are of a new type and specification. They were not made by Albion (as has often been suggested) or "sourced" by Geoff Johnson at Triumph.

It seems most likely that they were manufactured by a company that had close links either with Royston Industries (which owned Burndept and JMI), or with Burndept itself.

The mains transformer was marked "66775", the choke "66429", and the output transformer, at first, not all. In AC100 Mk2 amps, it is "66776".

Transformers made by the same company were used in the range of solid state amps also assembled by Burndept for Vox.

Rectifier diodes

These were Mullard BY100 (also used in the AC80/100, the AC50, and AC100 Mk2). In August 1965, they cost 11s 6d each (retail) in old money, around £10 today in relative terms. But Vox presumably had a better (wholesale) price.

Main filter capacitors (100uf 500v)

Made by Daly Condensers of Ealing, West London (West Lodge Works, The Green, Ealing, W5), superceding the Dubiliers in AC80/100s.

TCC Capacitors

The higher value axial capacitors in the AC80/100s made at Burndept were for the most part gold-coloured Dubiliers. For the "100W Amplifier", the supply line changed to green TCC "Micromites":

Pic. from the wonderful Mullard Magic website. Note the statement of "Peak Working" and "Surge" voltages.

The caps in question, as marked on the schematic (for which, see higher up this page), are:

C1 - 8uf, preamp main filter (but 16uf always fitted in practice).

C3, C4 - *25uf, preamp cathode bypass.

C9 - 32uf, preamp main filter.

C10 - 50uf, preamp main filter (but 32uf always fitted in practice).

C14 - 8uf 150v, bias smoothing.

* cathode bypass caps were TCC "Micropack"

So far no amp has come to light with 8uf and 50uf main preamp filter caps. The values were evidently simply carried over from the "AC80/100" schematic (and no AC80/100 has come to light with these values either).

Below, the Telegraph Condenser Company factory, Wales Farm Road, North Acton:

Pic. from the Britainfromabove website. The factory is printed dark in the photo, its environs slightly greyed out.

Bias circuit

For the smoothing capacitor C15 in the bias circuit, the schematic notes 0.25uf (an unusual value), 750v (an unusual rating). Hunts certainly made caps of these specifications, but evidently not in great numbers. The part actually used at the Burndept factory, however, was a blue/grey RadioSpares paper cap, 0.25uf, 1000v, well above spec., as in serial no. 531:

These came in packs of three, as in the pics below:

In later fixed bias amps - the AC100 Mk2 - one often finds RadioSpares mixed dialectic caps (polythene/foil) of 0.22uf, 600v. The change made little difference to the circuit operationally.

Zener diodes

Above, close-up shots of a zener diode temporarily removed from a "100W Amplifier" (serial number in the 500s, though lacking its plate on the back panel). Made by "International Rectifier", 15v (part no. MZ15), and rated at 3/4W. Below a shot of an MZ14 zener (not used by Vox) with its original box:

Thanks to the experts in the forum for identifying the MV15 zener as being made by IR.

A design handbook for the zener diode was issued annually in the early 1960s by the International Rectifier Co. and is likely to have been available to the Burndept engineers. A subsiduary International Rectifier Co. Ltd factory was set up in Oxted, Surrey, in 1960.

Extracts from the handbook. In the far left column, the JEDEC equivalents.

Below, the standard arrangement of zeners in the "100W Amplifier" as in ( AC100 serial no. 531):

The 20v zener extends across the width of the board; the 15v bridges two adjacent tags.