Vox AC30 transformers

1960 - 1964

The AC30/4 and AC30/6

Details from OS/065 - the mains and output transformers, and the choke. Click as ever for larger images.

It may be best to start by saying that this page will contain virtually nothing on the "tone" of output transformers. There is no infallible "recipe" for a "good" AC30.

Also to say that, in spite of what is sometimes asserted, Radiospares mains and output transformers were not used (i.e. fitted at factory) in early AC30/4s - chokes yes, but not mains and output.

Introduction

1960

Transformers are the most expensive and largest item on most valve amp chassis. One has to know how big they are going to be before a chassis can be designed and put into production. Thereafter any transformer sourced from a new supplier needs to be of the same size - so the chassis and its cut-outs do not have to be redesigned.

To judge from the picture of the prototype AC30 chassis published by Jim Elyea, Dick Denney and Derek Underdown worked initially with bits and pieces. The question is who supplied the transformers that enabled Dick and Derek to finalise the new chassis - i.e. establish the position and size of the cut-outs, the thickness of the metal to avoid bending and buckling under the weight of the iron, and so on. Was there an existing unit that could be used? The answer is probably not.

A standard AC30/4 chassis: mains transformer at left, choke at right along with output transformer.

From a basic "design" standpoint certain criteria would have to be met for the transformers. For the mains: the ability to supply 5v to the rectifier and 6.3v to a given number of valves (preamp and power); the provision of a specified main voltage before rectification; centre tapping; the ability to choose from a selection of input voltages (115v-245v); and a certain amount of resilience to stand up to the heat generated in a cathode biased circuit.

Whether the transformer maker would have been given any information about "local conditions" - the box the units were to go into - is an open question. AC30s can get extremely hot, even early ones that run the valves less hard than in later amps. The chassis is extremely tight in the wooden cabinet. Heat for a mains transformer is the enemy, breaking down the insulation between windings.

For the output transformer, the criteria would be: the required power handling, in other words the output from 4 x EL84; 8ohm and 16ohm output taps; and an anode to anode impedance of around 4K ohms, though in practice this might vary between 3.3K and well over 4K.

Presumably (surmise) the general design specifications would be arrived at in consultation with the transformer manufacturer. The rest would be up to the manufacturer to define and create - the winding ratios; the grade of grain-oriented steel to use for the laminations; the type of insulation. Many of these aspects would coalesce as a matter of cost versus profit, and time versus available labour.

It is probable that Dick Denney and Derek Underdown tried several manufacturers as part of the initial setting up process - i.e. to see if there were any anomalies in terms of frequency response and so on "in the real world". Haddon was selected.

Numbers of AC30 twins produced in 1960 and the greater part of 1961 were quite small - around 700 in total. Haddon presumably could cope.

An advert placed by Haddon in "Electronic Engineering" magazine, February 1954. Encapsulated transformers of the type illustrated were often used by Leak, manufacturer of high end Hi-Fi amplifiers.

1961

From mid 1961, JMI began to increase production of AC30s exponentially. Assembly of chassis was farmed out first to Westrex, based in north London. Burndept, based in Erith, was brought in as a second contractor around mid 1962.

Westrex used transformers made by Haddon and Albion; Burndept used transformers made by Woden.

One line of argument goes that Haddons were too expensive and so Tom Jennings, ever eager to save money, settled on the idea of using "inferior" units made by Albion and Woden.

But one also has to take into account the possibility that Haddon was unable to meet or had no interest in meeting a greater demand from JMI. Transformer manufacturers had plenty to do in the 1960s, especially those with military contracts. A certain amount of effort spent in the time-consuming process of making small high-quality transformers for JMI might have been acceptable; making huge numbers might not.

Breaking down specimen transformers from new batches and recording details on sheets, as JMI did, was a means both of documenting the make-up of the units and checking that value for money was being got - i.e. that insulation was not quietly being skimped on by the manufacturer.

In the early 2010s, a thread was started on the Plexi Palace forum - "How to recognise Parmeko, Albion and Haddon transformers". Wodens are easy enough. The thread is preserved by the wayback machine.

At the top of page 3 of the thread is this comment:

It was always wonderful reading these things - and still is - as if the magician is giving one a glimpse of what goes on back-stage. If this is based on direct report from someone at JMI, then all to the good even if it does seem rather hyperbolic - "every batch....". But no "source" is given.

Having all one's eggs in one basket would clearly not be a particularly desirable thing, especially in time of high production. If all one had was Woden and Woden faltered.... But there is also the question of supply. It's all very well having "demand", but if a single supplier cannot meet it then one has to readjust.

Overview of AC30 transformers

In the case of transformers for Vox AC50s, JMI evidently dismantled specimen units from batches and detailed the internal structure on sheets. These sheets still survive (ownership known). Whether the same went on for AC30 transformers is unknown - probably so though.

ALBION - from late 1961

Little is known at present about the company, though on page 5 of the Plexi Palace thread mentioned above, there is this eyewitness account:

JMI may also have applied its own labels.

A good deal of Albion's business seems to have been in multi-tap transformers for instrumentation. The JMI part numbers were the same as for Haddon:

Mains: OH114.

Output: OH083.

Choke: PE241.

Click as ever for larger images. Albion mains transformer, part no. OH114; output transformer OH083.

WODEN - from autumn 1962

Woden factory, c. 1976. Picture from the Wolverhampton Express and Star Archive.

The company was based in Bilston, near Wolverhampton (West Midlands). Transformers are marked with JMI part numbers (for reordering) and date codes of manufacture. The date codes are NOT the date of manufacture of the AC30. Early part numbers are:

Mains: 66309. In 1963 the designation became 66309 & J/82.

Output: 66310. In 1963 the designation became 66310 & J/83.

Choke: 66311. In 1963 the designation became 66310 & J/84.

Above, mains transformer and choke with the date code "HT" = August 1962. Output transformer: "JT" = September 1962.

In 1964, the transformers made for Vox by Woden were produced in a simplified (and therefore cheaper) form - see the section further down this page. The new part numbers were:

Mains: 76852.

Output: 76853.

Choke: 76854. Also used in the Vox AC100.

PARMEKO - 1964 and 1965

Based in Leicester (Vernon Street). Produced transformers for audio applications - radios and hi-fi - and instrumentation. The JMI part numbers were:

Mains: 66313.

Output: 66314.

Choke: 66429.

Note that in 1965 Vox AC100s have transformers with part numbers: 66775 and 66776, the choke being a Woden (noted above). In 1966 AC50s have: 66522; 66523 and 66524. These are simply JMI part numbers, not the designation of manufacturer.

OUTPUT TRANSFORMERS

Haddon output transformers

Below, three Haddon output transformers from 1960, 1961, and 1962/1963, respectively.

From 1960-1961 a pair of red wires was provided for the HT. The pair later became single.

Just to mention that transformers made by Haddon had been used by Jennings in the AC/30 of 1958-1959, and in AC1/15s produced in the same period.

1960

As Glen Lambert has pointed out, the laminations are built up in layers, in this case layers of three and four - one set of three plain end forward, the next of four at 180 degrees, with cut-outs forward.

The underside of the transformer pictured above.

1961

Still double red wires for the HT.

1962/1963

Still double red wires for the HT. Thanks to Marc for the picture, and the one above.

Albion output transformers

Insofar as one can tell from outward appearances, the output transformers that Albion made for JMI retained the same internal structure from 1962-1965. This does not mean to say however that construction was always the same from year to year, or that exactly the same materials were used.

In terms of wires, a single red for HT; single green, blue and black for the speaker terminal block.

Serial number 5329. For further pictures, see this page.

Below, an Albion output transformer from c. 1964, same arrangement as above: single red wire for HT and single wires for the output taps.

Above, an Albion output transformer from c. 1964.

Albion output transformer of AC30 Super Twin serial number 3349, early 1965.

Thanks to Glen Lambert, pictures of an Albion output transformer (of 1964) taken after its laminations had been removed. Glen kindly outlined the salient aspects:

- the plain rectangular tube section paxolin former - not really a bobbin at all. The wire was simply wrapped around and built up in neat layers with very fine waxy paper in between each layer.

- the grey loops of wire are the links between the interleaves. These are a way of distributing the layers more evenly throughout the "bobbin", for both mechanical and electrical benefit.

- the '64 label used by Albion.

- the single red wire used for the centre of the Primary, as opposed to using two red wires (as per Woden).

- everything is papery in its construction, and plenty of wax.

The underside of the transformer: at left the wires that feed to the output block - black (common), green (8ohm), and blue (16ohm). The Albion label across the outer cloth wrapper. On the far side of the underside, the yellow wires (to the plates of the EL84s); and red (voltage supply).

MAINS TRANSFORMERS

Note that in Haddon and Albion units the (primary) wires to the voltage selector pass from the top of the transformer, entering the chassis relatively high up via a grommeted pass-through. In contrast, the primaries of Wodens issue from the underside of the transformer, passing up to the selector via a grommeted hole through both plinth and preamp upright. Chassis assembled by Burndept for Vox are therefore slightly different from those produced by Westrex. Burndept used Wodens pretty well exclusively.

Presumably it was easier (and perhaps cheaper) to have chassis made with an extra hole than to ask Woden to produce a transformer with a different type of primary.

Serial number 4763. Haddon mains transformer, wires passing through the upper chassis to the voltage selector.

Serial number 5329. Albion mains transformer, as above, wires pass through the upper chassis to the voltage selector.

Serial number 6097. Woden mains transformer, wires passing from the underchassis up to the selector. Note the presence of an unused hole higher up (so an Albion could also be fitted).

AC30 from early 1963. Picture showing the wires feeding from the lower chassis.

Woden mains transformers

Below, a Woden output transformer made in January 1963 (date code "AU"). Note the twin red wires for the HT, and the doubled-up green (8ohm), blue (15ohm), and black (common) for the speaker terminal block.

Output transformer of an AC30 assembled in early 1963 - see this page.

Speaker terminal block.

Woden produced output transformers of the type represented above - fairly complex internally - from autumn 1962. They are likely to have been expensive and, more to the point, time-consuming to produce. The JMI part no. is: 66310 & J/83.

In the late summer of 1964, a new type of output transformer with a simplified internal structure was arrived at: a single red wire for the HT supply; and single windings for the impedance taps. The JMI part no. for this new type is: 76853.

Woden output transformer produced in October 1964 = "KV". The plain metal shroud (instead of the enamelled green of the previous type) was doubtless a further means of cutting down on cost and time needed for production.

Below, a knock-down of a Woden. Its secondary has two pairs of wires for the preamp and power amp valve heaters (6.3v), and a twisted pair of red for the rectifier (5v).

Primary and secondary wires on the underside of the transformer.

Outer wrapper removed, revealing the 5v and 6.3v windings.

Foil screen over the primary winding.

Foil screen removed, revealing primary.

Primary windings.

Windings removed down to the former.

CHOKES

Radiospares

The earliest chokes were standard Radiospares catalogue items. They accompanied Haddon mains and output transformers in AC30/4s and pre- and some post-LOC AC30/6s. In late 1961 Haddon produced its own choke, as did Albion (and Woden later on - late summer of 1962).

The AC30/4 and AC30/6 circuit diagrams both give the value as 10-20HY and 100MA. "HY" stands for "Henrys", the measure of inductance.

Detail from OS/065, the AC30/6 Normal voicing.

The outward facing side of the choke on an AC30/6 pre-LOC chassis.

The chassis side of the choke on another chassis, original label present: 20 Henrys, 500 ohms (DC resistance), 100 Ma.

The reason why the circuit diagram gives 10-20HY inductance becomes clear when one refers to the Radiospares catalogue of 1961: the two values were available in housings of the same size and format. If one was out of stock, the other could be used. As far as the schematic was concerned, JMI evidently thought it well to reference both (just in case), leaving the sheets uncorrected as copies were made. By the end of 1962 chokes were 19H.

Detail from the 1961 Radiospares catalogue.

1961 Radiospares catalogue cover.

Although no definitive example of a 10H choke has come to light so far in an early AC30, many are "unknowns" - only the outward facing side of the housing normally being recorded in pictures. Labels on the "inner" side are often discoloured or absent.

Below, photos of a Radiospares "Heavy Duty" 20H choke from a pre-LOC AC30/6 during repair - thanks to Glen. The laminations are in two sections: one in the shape of an "E", the other (which lies across the top") an "I". The bobbin - with its windings - slots down over the central limb of the "E".

(1) The bobbin and windings removed from the laminations. (2) The Radiospares wrapper removed - the varnished outer insulation underneath. Note the stamped number "29".

(3) The windings. (4) The repaired windings wrapped again, and the bobbin mounted on the "E"-shaped section of laminations.

(5) and (6) The choke reassembled.

Below, pictures of a Radiospares 10H choke from the early 1960s alongside the type of choke used with increasingly regularity in AC30s assembled in 1965 and early 1966. Some good examples here - serial numbers 20269, 20523 and so on. The part number of this later type is 66429.

66429s were also used in the summer of 1965 for a run of new fixed bias AC100s built to the "100W Amplifier" circuit diagram - see the examples on this page. The one illustrated below is actually from one of these AC100s. No maker's mark is ever given; but there are often three-letter date codes stamped in white.

Although the frames (housings) are different, the formers are identical: five slots on one side, plain on the other but with a sort of tongue on top. It may be that the later chokes were also supplied by Radiospares.

Radiospares, however, was generally not a manufacturer - simply a commissioner of items made by others and branded with the Radiospares name. The principal manufacturer of the transformers supplied by the company is reckoned to be R.F.Gilson of Wimbledon (South London). But whether Gilson produced these chokes is unknown.

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