UPDATES

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January to April 2022

24th April

Thanks to Mark, pictures of a superb AC50 PA set - . Brown cloth all round, and old-style "VOX" logo on the hand-wheels of the LS40 tilt-back stands. AC30 Super Twin amplifier sections and AC50s were sometimes supplied by JMI in place of the dedicated PA amps that the company produced (from early 1964). See for example, the advert at the top of .

AC50 Mark 2, serial number 1420.

5th April

A detail from a newsprint picture of The Barron Knights' Christmas Show at Olympia (late December 1964 / early January 1965) published in mid January 1965. On stage, four large box AC50 amplifier sections in a row driving two AC50 Super Twin cabinets, and two Foundation Bass cabinets.

The Barron Knights with four large box Vox AC50s

London, Olympia, late 1964 / early 1965.

3rd April

Autumn 1964, Jim Gretty of Hessy's Music in Liverpool serving a customer. In the background an early AC50 in thin-edged box on top of a T60 or 2x15" speaker cabinet.

The grille cloth of the amplifier section is 2 and 2/3 diamonds tall, so the amp in view is not an early AC80/100.

13th March (3)

Serial number 2020, valve rectified, recently sold - a replaced handle, voltage selector removed, but otherwise in good order.

13th March (2)

Some pics of AC50 serial number 1958 - on the plate, model hand stamped, number machine stamped.

Serial number 1958.

13th March

Currently on Reverb, , valve rectified, late 1964 (mustard caps with date codes in first and second quarters of 1964, at least one pot with "GL" = July 1964), serial number and model hand stamped; and , serial number hand stamped, number machine stamped, the chassis a Mark 3, diode rectified.

11th March

Some fairly substantial updates recently to the - particularly the new pages on the three early versions of the AC15. A series of AC50 updates coming shortly though.

10th February

Thanks to Will, pics of AC50 Super Twin serial number 3357, mid 1965, and one of the cleanest around. In the preamp, visible date codes are pots: "DM" = April 1965; and the TCC cathode bypass caps: "WB" = February '65. At right, the large grey Radiospares 0.05uf snubber capacitor, fitted to amps made for export to the USA.

Serial number 3357.

2nd February

A detail of the information panel and bias circuit of OS/053 (first version). The sheet was printed for circulation within JMI only - not for the outside world. Note that there are no dates or numbers in the panel. The sheet is otherwise much as the standard version of OS/053 but with the bias circuit of OS/044, and without the notes on voltages, component types, valve pin numbers, and the 470R resistor on the 15ohm terminal of the output transformer.

AC50 mark 2 - OS/053 - first version. The bias circuit is still that of OS/044. Thanks to Martin for the pic. from which this detail is taken.

Evidently the electronics of the AC50 mark 2 did not take long to design. OS/044 was the starting point. A second channel - the Brilliant Channel - was added, mirroring the existing one but with capacitors to block bass and accentuate the treble (500pf and 100uf). 500pf had been used in the Briiliant Channel of the AC30. OS/053 first version was then drawn out. Shortly after, to allow more flexibility, the bias circuit was changed; and at some point after that, sheets for the now finished circuit were produced.

It is well to remember that all we are seeing are "snapshots". The circuit diagrams issued by JMI are in large part not design blue-prints - they were for the benefit of repairmen. JMI's contractors - Westrex, Triumph and Burndept - had their own working circuit and "build" diagrams.

29th January (2)

Following on from the entry below, the bias circuits of OS/044; AC50 mark 2 (first version); AC50 mark 2 (standard). It does look as though Rodney's sheet is intermediate, having the bias circuit of the AC50 mark 1.

AC50 mark 1 - OS/044

AC50 mark 2 - first version

AC50 mark 2 - standard version

29th January

Below, a detail from a hitherto unknown copy of a JMI circuit diagram for the AC50 mark 2, also owned by Rodney Angell. In relation to the "standard" sheet (OS/053), various things are lacking - valve pin numbers, voltages, certain component names, and so on. In terms of component values, the bias circuit certainly looks to be different (though the extent to which that is a function of the drawn layout is unknown at present).

It should be possible to make a full version of the drawing available soon.

Rodney Angell's sheet for the AC50 mark 2.

Standard OS/053.

27th January

Thanks to Martin Kelly, a copy of Rodney Angell's circuit diagram for the AC50 Mark 1 - OS/044 - a large version will be set up . Before moving to the organ department, Rodney was an amp tester and service engineer at JMI. Note that the numbering of components has been started in pencil in the upper half of the sheet. Size is old-fashioned foolscap (13" x 8").

25th January

Some updates to the page on - serial numbers 1234, 1322, and a late amp with black grille cloth.

20th January

The power section of PA50 serial number 7597. The Hunts caps have date codes "YDU" and "UYD" = 8th week of 1966 (in UYD the year-letter is prefixed, in YDU suffixed). The transformers were manufactured by Drake: mains (1202-112B); output (784-125); choke (352-169). The main filter caps (made by CCL) are later replacements - date codes "J7" = October 1967, "B8" = February 1968.

The Hunts suppressor capacitor (across the rectifier bridge network) blew violently at some point, shorting out the power section - an easy fix.

The EL34s are probably the original Mullard Xf2s. The impedance selector is (from factory) a terminal block rather than a paxolin board with a selector screw. No bias pots were fitted (note the blanking plates in the pic immediately below).

19th January

Some pics of the preamp assembly of PA50 serial number 7595, early 1967. A number of date codes can be made out: potentiometer "LN" = December 1966; Hunts capacitors "UYA" = 9th week of 1966; Mullard mustard capacitors, one with "D6N" = last quarter of 1966. The ECC83s are Mullard "Foreign Made", code 165/46I4. The chassis is stamped "INSPECTED. No. 1. S.E.L." Shots of the power section to follow.

18th January

Further pictures of AC50 serial number 1705 have been . The latest date codes are those of the pots - "GL" = July 1964. The underchassis is superbly clean.

AC50 serial number 1705.

17th January

Thanks to Chris, pictures of AC50 serial number 1705, valve rectified, last third of 1964, coming soon.

10th January

Thanks to Martin, pictures coming shortly of PA50 serial number 7595 (all valve). All PA50s had standard AC50 serial number plates with numbers in the same sequence. PA50 number 7597 can be seen - transistor front end, valve power section.

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