Vox AC30 - Woden transformers

1962 - 1968

The AC30/6

Woden factory, Bilston, c. 1976. Picture from the .

This page will be for the time being a series of notes - mainly practical matters. There will be nothing, apart from this sentence, on the "tone" of transformers, an utterly nonsensical subject.

An overview of Haddon and Albion transformers can be found .

Woden date codes

It is perhaps worth saying as a preliminary that the Woden date codes relate solely to the manufacture of the transformers - they do not give the date of production of the AC30. Having been sent down to Kent from Bilston, batches could sit on stock-room shelves in Dartford and Erith for some time before being wired up into chassis. And of course, we do not know how long they sat on shelves at the Woden factory - presumably it took some time to assemble larger consignments.

The codes are simple enough - two letters. The second letter is for the year - "T" = 1962, "U" = 1963, "V" = 1964, "W" = 1965 and so on. The first is for the month - "A" = January, "B" = February, etc. "I", however, is omitted, so the sequence is "A-H" = January to August; "J-M" = September to December.

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

Wodens are mainly found on the chassis assembled by Burndept for JMI, extremely rarely on those assembled by Westrex, JMI's other main contractor.

The earliest date code that has come to light in an AC30 is "HT" = August 1962. Burndept began producing chassis for JMI soon after that.

From 1962-1964, Woden mains and output transformers have handsome sea-green enamelled shrouds. From 1964-1968, the shrouds are plain metal.

Prior to 1968, Woden evidently did not make transformers for JMI in March, that is to say, no date codes with "C" as the first letter have come to light in the period 1962-1967. All other months are encompassed though.

The Woden factory generally ran off batches of different types of transformer for JMI every three months or so, and in some instances every month - but not as a rule in March, as mentioned. The only Woden with a "C" for the month that has so far come to light is in a late Super Twin - serial number 5596, choke stamped "CZ" = March 1968.

JMI part numbers

As can be seen from the pictures above, the tops of the transformers were 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 - on tranformers with "HT" and "JT" date codes (August and September 1962) - are plain numbers:

Mains: 66309.

Output: 66310.

Choke: 66311.

Transformers with "HT" and "JT" codes are found in AC30s with serial numbers in the range 5630 to the 5700s, also in 6097.

At present, it appears that JMI did not use Wodens with October 1962 date codes ("KT") in any production amplifiers. A small batch of unused output transformers - stamped "KT" but of a new type - acquired by Alan Pyne (who had worked for JMI) look to have been (rejected) test samples.

With the advent of transformers with "LT" = November 1962 date codes, the designations changed. Wodens with these date codes first appear in AC30s made ready for sale in 1963, serial numbers 6172 through to the 8000s:

Mains: 66309 & J/82.

Output: 66310 & J/83.

Choke: 66311 & J/84.

In the autumn of 1963 however, the mains tranformer - 66309 & J/82 - was superceded by a new version designated 72318 & J/82. The suspicion is that JMI asked for the secondaries to be adjusted - specifically the main HT voltage. Batches of the new unit were produced with date codes "JU" = September 1963 through to "AV" = January 1964, that last batch being fairly sizeable. It is probably no coincidence that 72318 followed (in relatively short order) the modification of the cathode bias circuit (June 1963) - the main bias resistor changing from 80R to 50R.

Mains: 72318 & J/82. Date codes "JU" to "AV" = September 1963 to January 1964.

In the late summer of 1964, at the request of JMI, Woden produced its transformers in a simplified (and therefore cheaper) form - the output transformer in particular, which had a less sophisticated internal structure. The new part numbers were:

Mains: 76852.

Output: 76853.

Choke: 76854. Also used in the Vox AC100.

Woden mains 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.

Batches

1962-1963.

In the table below, serial numbers, type of transformer (output, mains, choke), and date code are given. Just to recap, the second letter of the date code is year: "T" = 1962, "U" = 1963. The first letter is the month: "A-H" = January to August, "J-M" = September to December. Remember that the dates are the date of manufacture of the transformers, NOT the amp.

Large batches of mains, outputs and chokes were ordered in the late summer of 1962 ("HT", "JT"), late 1962 ("LT", "AU"); and late summer/autumn 1963 ("HU", "JU", "KU") - all for use in AC30s assembled by Burndept. A small batch of mains transformers came in in late 1963 ("MU").

Nothing so far with a date code in the period February to July 1963.

5634: mains, output, choke "HT" = August 1962.

5648: mains, output, choke "HT" = August 1962.

5650: mains, output, choke "HT" = August 1962.

6097: mains and choke "HT" = August 1962; output "JT" = October 1962.

6664: mains, output, choke "HT" = August 1962.

unknown: mains "LT" = November 1962.

6948: mains, output, choke "AU" = January 1963.

7061: output and choke "AU" = January 1963; "HU" = August 1963.

7109: mains, output, choke "AU" = January 1963.

unknown: mains, output, choke "AU" = January 1963.

8739: mains, output, choke "JU" = September 1963.

8922: output and choke "JU" = September 1963.

9434: mains, output, choke "KU" = October 1963.

9479: mains, output, choke "JU" = September 1963.

9677: mains "JU" = September 1963; output "LU" = November 1963..

9775: choke "HU" = August 1963; mains and output "JU" = September 1963.

9781: mains, output, choke "AV" = January 1964.

unknown: mains, output, choke "KU" = October 1963.

9916: mains "KU" = October 1963.

9917: mains "HU" = August 1963; output "JU" = September 1963; choke "KU" = October 1963.

9986: output and choke "KU" = October 1963; mains "LU" = November 1963.

10317: choke "JU" = September 1963; output "KU" = October 1963; mains "MU" = December 1963.

10578: choke "JU" = September 1963; mains and output "MU" = December 1963.

Below, a number of AC30 Trebles from 1964 (serial numbers in the 11000s) with transfomers from 1963. Normal and Bass AC30s assembled by Burndept at this time have new Wodens (date codes "AV" and "BV"). It seems likely that the benches responsible for producing the Trebles were, at some point, supplied with older units to use up.

11718T: output "JU" = September 1963; choke "KU" = October 1963; mains transformer an Albion.

11769T: choke "HU" = August 1963; output "JU" = September 1963; mains"MU" = December 1963.

11873T: mains "MU" = December 1963; output and choke "AV" = January 1964.

11874T: choke "JU" = September 1963; mains and output "MU" = December 1963.

unknown Treble: output "KU" = September 1963; mains "MU" = Deember 1963.

unknown Treble: mains "MU" = December 1963; output and choke "AV" = January 1964.

unknown Treble: a strange assemblage - Twin cabinet, Super Twin serial number plate (Normal voicing), Treble chassis - choke "MU" = December 1963; mains and output "AV" = January 1964.

1964 - forthcoming.

Two large batches of transformers: early 1964 ("AV" and "BV"), and late summer ("HV", "JV" and "KV"). Albions and Parmekos were used for large runs in mid 1964 by both Burndept and Westrex.

1965-1966.

A note on the Woden transformers in AC30s, late 1965 / early 1966, serial numbers in the 20000s and 21000s - a sort of snapshot.

In the table below, serial numbers, type of transformer (output, mains, choke), and date code are given. Just to recap, the second letter of the date code is year: "V" = 1964, "W" = 1965, "X" = 1966. The first letter is the month: "A-H" = January to August, "J-M" = September to December.

The gold-coloured chokes used in this period in both AC30s and AC100s are un-dated. Only the JMI part number is stamped = 66429.

20269: output "KV" = October 1964.

20523: output "KV" = October 1964; mains "JW" = September 1965.

20750: mains "HW" = August 1965; output and choke "MW" = December 1965.

21019: mains and output "DX" = April 1966.

21086: mains and output "HW" = August 1965; choke "DX" = April 1966.

21098: output "KV" = October 1964; mains "HW" = September 1965; choke = "AX" = January 1966.

21192: mains, output, and choke "DX" = April 1966.

Evidently output transformers from October 1964 (code "KV") were still in store in early 1966. These had been ordered when serial numbers were in the 16000s. The batch was large - no intervening date code before August 1965 ("HW") has come to light.

Insofar as Super Twins are concerned, serial numbers 4744, 4784, 4831, and 4834 all have: mains and choke "KV" = October 1964; choke "AW" = January 1965. By the 5200s, date codes have become "KX"

Super Reverb Twin serial number 5301 has: mains "EW" = May 1965, and choke "HW" = August 1965.

1968.

An interesting replacement Woden mains transformer in AC30 Super Twin 2731. In pencil on top: "Replaced / 9th July '68 / AG".

Unfortunately the fore-edge has rusted, so it is not possible to be sure of the date of the transformer's manufacture. But it will probably not be too far removed from the original unit in (very late JMI) Super Twin serial number 5596 - Woden date code "BZ" = February 1968.

AC30 Super Twin, serial number 2731.

AC30 Super Twin, serial number 5596.

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