Vox AC30 - Wima capacitors
Detail from the advert further below.
In the late 1950s and 1960s WWIM Tropydurs were widely used in a variety of applications - radios, broadcast equipment, jukeboxes - they were also used by JMI in early Vox AC30/4s and AC30/6s (1960 to early 1962). Very few survive in place, however, as they had exceedingly short lives, quickly becoming leaky and unreliable.
In vintage radio circles they are sometimes known as WIMA "toffee" capacitors - see this great thread (you'll need to sign up to the forum to see some pictures).
Spent WIMA Tropydurs from a TV Front AC30/4. Thanks to Valentin for the picture.
JMI bought its WIMAs from "Waycom Limited", the sole UK distributor - "200,000 to 300,000 pieces always in London...." - Empire Buildings, Duke Street Hill, SE1:
"Wireless World", July 1960
WIMA catalogue, late 1959 / early 1960. Thanks to Marc for the image.
Below an advert placed in "Wireless World" magazine by Smiths of Edgware Road in London. The prices are necessarily retail. Manufacturers - such as JMI - would buy wholesale, i.e. with a discount (sometimes substantial) for bulk. Even so, values above 0.1uf will not have been inexpensive.
"Wireless World", July 1961.
Below, a couple of Tropydurs in place in an AC30/4, the others largely replaced in the early 1960s by grey Radiospares polyester caps.
Two WIMA Tropydurs in place at the time when the photograph was taken. The grey Radiospares capacitors replace those that had been removed.
In late 1960, English papers began advertising the new WIMA Tropyfols - polyester foil dielectric sealed in resin, gold-coloured bodies, black or red print. JMI began using these in late 1961 / early 1962, again purchased from "Waycom Limited" in London.
"Wireless World", December 1960.
"Wireless World", January 1961.
The front page of a WIMA brochure for the new Tropyfols, April 1960. Thanks to Marc for the copy.
At some point between August and October 1961, i.e. in the period corresponding broadly to the change from the last black panel AC30s to the new copper panel AC30/6s, gold-coloured WIMA Tropyfol were brought in - doubtless stipulated to the two contractors which then began to assemble increasing numbers of chassis for JMI: Westrex first, then Burndept Electronics.
In AC30s produced in late 1961 and 1962, codes can sometimes be seen on 400v WIMAs. Those observed so far are "P4", "Q1, "Q2" and "Q3". "P" and "Q" would seem to be the year, the number standing for the quarter. By 1967, WIMA had gone over to the Mullard system for small-value caps, "C8" standing, for instance, for third quarter of 1968.
It is tempting to take "P" as 1961, and "Q" as 1962. Red-print WIMA Tropyfoil (Tropyfol) have a different set of codes, so too the earlier black Tropydurs.
Below, a couple of black print Tropyfols with the new (Mullard style) date codes: "B7" and "C7", standing for the second and third quarters of 1967
Although Durolits (constructed from interleaved paper and foil) were clearly available in the UK from the middle of 1962 (at the latest), there is no mention of the range in literature produced by Waycom Ltd early on. There is a reference in a trade journal of 1963 published by the Pergamon Press (Robert Maxwell's old company), but little else until the later sixties.
Below, WIMA Durolit 0.15uf, 630v coupling capacitors in serial number 5650, which was produced in the last third of 1962. Its Woden transformers have August 1962 date codes. The values are given on the capacitors in relatively small green/black print. Print size was increased in the later sixties.
Coupling capacitors C6 and C9 as indexed on the AC30/6 circuit diagram OS/065.
In mid 1962, JMI began using larger WIMA Durolits - 0.15uf, 630v - as the main coupling caps between preamp and power section - C6 and C9 on OS/065, the circuit diagram for the AC30 "Normal" voicing. In 1960 and early 1961 - that is to say in the days when WIMA Tropydurs were used - these capacitors had been 0.01uf, 500v.
AC30/6 circuit diagram, OS/065 - coupling capacitors.