UPDATES

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January to May 2021

30th May

Thanks to Joop, pictures of , an AC50 mark IV, hand-stamped serial number plate:

Recently on ebay.uk, a thick-edged small-box four-input . Note the position of the warning plaque over to the left rather than centred (which is the norm):

26th May (2)

A Foundation Bass set - serial number 1754 - ostensibly from late 1964 at . No pictures of the electronics, but some good ones of the exterior.

26th May

An exceptionally clean Super Twin set on . Serial number in the 3000s; Fane speakers.

19th May

Thanks to Rodney, a copy of the JMI circuit diagram for the AC50 Mark 1 - OS/044. This was not signed off for release at first however as there is an anomaly in the way that the 22R resistor is represented in relation to the GZ34 valve - it looks as though it is in series. The arrangement was tidied up in OS/53 and in a sheet that stood between the two, signed off by Derek Underdown.

The page on has been updated.

First circuit drawing of the AC50 Mark 1 circuit - OS/044.

A detail of a version of OS/053 to show the corrected arrangement of the 22R resistor in relation to the rectifier valve.

14th May

A further note on Hunts capacitors in AC50s with reference to the company's catalogue and pricelist of 1965. Sets of prices are given for bulk ("13 or more"). Two of the capacitors used by Vox were special order.

Also to say that the red "RTC" capacitors used in AC50 mark 1s are likely to have been Hunts too, simply rebranded. The possibilities ventured below (entry for 3rd November) can largely be put to one side.

PREAMP

25uf/50v - Hunts code MEW39T - 1s/3d each (bulk).

16uf + 16uf/450v - Hunts code JE562T - special order.

32uf/450v - Hunts code JE555T - special order.

POWER SECTION

8uf/150v - Hunts code MEW111T - 1s/3d each (bulk).

32uf/500v - Hunts code JF706T - 3s/11 and 3/4d each (bulk).

12th May

Two further amps registered with : numbers 4696, a Foundation Bass set: and 4911.

It looks as though good numbers of amps in the latter half of the 4000s were sold in the UK and Europe.

15th April

AC50 serial number 2426, currently on offer in the USA (Orlando). A Burndept-made amp - Woden transformers, two white input jacks, Cannon LNE-32 mains socket on the back panel - late 1964 / early 1965. The slider board is missing, but everything else looks to be intact.

Slider boards were a weak point in AC50 boxes, susceptible to fracture during shipping if not supported from below. Thomas Organ sold replacements.

31st March

Recent updates: serial numbers 4424, 4455, 4845 and 4987 on the 4000s page. Serial numbers 5568 and 5847 on the . Some more 5000s to come, but probably not for a little while yet.

Throughout the 5000s, less so in the 4000s, one finds patterns of export batches, principally to the USA, but also to Europe: Germany (5616 and 5619; 5173 and 5180; 5815 and 5842) and Italy (5843 and 5847). It is likely that the serial numbers were sequential in these exports - to make record keeping simple.

30th March (2)

Further pictures of , an early large box AC50 mark 2 - its Foundation Bass speaker cabinet still survives.

One of the pots has a date code ending "L" = 1964. Mustard capacitors are "A4N" and "B/64" = first and second quarters of 1964. One of the Hunts filter capacitors has "HNT" = 27 week of 1964 (first week of July).

30th March

Some updates coming shortly - mostly AC50s with serial numbers in the 5000s, probably exported to Europe (Germany in particular) early on.

28th March

Below, a better version of OS/072, the circuit diagram for the AC50 mark 3, solid state rectified. Although this copy has been well used, it is clearer in many respects than the others in general circulation.

27th March

Some notes on JMI circuit diagram OS/053 for the AC50 mark 2 - the two channel, valve rectified amp.

Good original copies of this diagram are hard to come by. There are a couple of "restored" versions in circulation, but these are not without their drawbacks.

Below, a copy that came in with a batch of material held at one time by "Modern Music" in Dudley. Serial number 1622 may have been repaired from it. Component values that are indistinct or very small in the original have been supplied in blue type. Note the dates in the info panel - first recension 17th April 1964, second 16th July 1964 - which pretty well corresponds with the release of the new Mark 2s. The second recension is given as 16th March 1964 in at least one recent digitally "corrected" version, but that makes little sense given the first date. It could be that the line "Voltages and Valve Numbers added" was simply put in the wrong box.

There will be more to come on AC50 diagrams shortly. A version of OS/044 - the circuit of the AC50 mark 1 - has come to light with JMI additions bringing the circuit into line with the Mark 2.

A detail of the info panel of OS/053.

Right hand side of OS/053. Click on the image for a large downloadable version (2400 pixels tall)

Left hand side of OS/053. Click on the image for a large downloadable version (2400 pixels tall)

19th March

AC50 serial number 1973, a Mark 2 (large box, valve rectified, two channel), from late 1964. Model and serial number are hand stamped on the plate, a relatively late instance. The speaker cabinet seems to have been issued without a logo (no sign of glue on the cloth or drilling in the baffle. Thanks to Johnny for the pictures.

14th March

Thanks to Steev, , mid 1966, now registered.

4th March

Two more "Triumph specials" - black (painted) control panels, screen-printed legends - recently come to light. See for others. The first is currently on ebay - the amp has a VOX logo and a hand-stamped serial number plate on its back panel.

The second - thanks to John for the pics - has no VOX logo or plate (none were ever fitted). White control knobs seem to be common to the amps without logos.

27th February

An output transformer from an AC50 with a serial number in the mid 3000s. The top bell, which fits tightly over the core, has thick varnished paper inside to provide insulation. As can be seen from the first photo, some of the transformer's windings lie close to the surface.

One of the "I" sections of lamination stuck to the shroud as it was being removed.

22nd February

Four AC50s with serial numbers in the 5000s : 5291, 5355, 5590 and 5693.

20th February (3)

AC50 from later 1966. The control panel is a later replacement. JMI did not mark its inputs "Hi" and "Lo".

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  • 6808

20th February (2)

Serial number 8030, late 1967 or early 1968, probably exported to Germany early on, now .

20th February

AC50 serial number 5001 has now been . Thanks to Jake for the pictures.

8th February (2)

Recently sold in Denmark, serial number 1467 - black escutcheon plate for the speaker output sockets.

8th February

Thanks to Gwen, further pictures of AC50 and the Vox Supreme speaker cabinet with which it has travelled for some time.

3rd February

Thanks to Francis, pictures of a late AC50 Mark IV and Defiant cab set - serial number 8071. The chrome tongues on top of the speaker cabinet slide into the mounting brackets on the underside of the amplifier section (much as a Defiant) to secure the amp to the cab's top. See on the Vox Supreme website for further examples. Speakers are probably ceramic Celestion T1297s.

Further pictures are . Few late AC50 guitar amp sets survive intact.

2nd February

Some updates on their way. Below, an interesting AC50 from early 1965, built solid state rectified, but on a valve rectified chassis. Note the cathode bypass capacitors with plain metal exteriors on the left-hand side of the preamp. The output transformer is missing.

10th January

AC50 serial number 1367 - a very early large box Mark 2 - is back on , with what looks to be its original Foundation Bass cab in brown grille cloth. The cab contains a later Celestion (Greenback) 18" driver. No pictures of the electronics though.

6th January

An early large box AC50 mark 2, sold on ebay last week, probably originally with a serial number in the range 1400s-1600, last third of 1964. The only visible date codes are on certain of the Mullard mustard capacitors - "A/64" = first quarter of 1964.

Black escutcheon plates for the XLRs on the back panel were used in 1964 on all but the very earliest large box AC50 mark 2s. See .

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