UPDATES

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June to December 2021

21st December

Recently on ebay, AC50 serial numbers 1234 and 1322, small box, four inputs (diamond), respectively summer and autumn of 1964, nothing to do with the Beatles, but good in their own right.

13th December

Thanks to Volker, further pictures of AC50 serial number 4878, probably very late 1965, or early 1966, have been . Back board and baffle have "66" in chalk, enabling them to be matched up with the box for which they were made.

Two of the pots have the date code "JM" = October 1965. The underchassis is initialled by "BN" and "DE". The latter is Dave Earp.

7th December

AC50 serial number 1477 (or perhaps 1473?) has just turned up in Germany - - along with a later AC50 Super Twin speaker cabinet - Fane speakers.

In company with serial numbers 1360 and 1384, a wooden block covered with black basketweave vinyl supports the speaker socket. There is no engraved traffolyte escutcheon plate. Serial numbers 1354, 1367, and 1383 have plain aluminium plates instead of an escutcheon. See .

26th November

A rough-print picture of Geoff Johnson at the Frankfurt Trade Fair, February 1968. Geoff's company, Triumph Electronics, produced around 800 AC50 chassis under contract for JMI, late 1963 to early 1965, a pretty impressive output given the cramped nature of the premises. Other models were also handled by Triumph at this time. In 1966 its main drive became the 4- and 7-series amps.

By July 1967 Triumph had developed its own amplification range, promoting them in the music trade press. Presumably at this point the contract with JMI was near its end, if not over. In mid 1968 however, Triumph began building small runs of amps for "Jennings Electronic Industries", Tom Jennings's new business.

Geoff Johnson demonstrating Triumph's strap with built-in preamp at the Frankfurt Fair, February 1968. Rosetti were Triumph's distributors.

20th November

Below, a picture from August 1964 posted a few days ago on the updates page of the - a hitherto unknown JMI design for a stand, probably never put into production. What was it for? The presence of brakes suggests a unit that could put out a considerable amount of bass. Perhaps for an AC50 amplifier section with a new "large box" speaker cabinet containing two 15" drivers? The Beatles had certainly used such cabinets for their concerts in Stockholm in late July '64. But those cabs had more conventional trolleys, and the basket top of the one below looks a little too precarious for an AC50 amplifier section, so the question remains.

On the hand-wheels, "VOX" is in old-style cursive script. Selmer later devised side stands for its Thunderbird amplifiers in the form of a Z without a top bar, but it is unlikely that these sprang in any way from the JMI design.

Picture from autumn 1964.

14th November

115 Dartford Road, September 1964. Large box AC50 Foundation Bass sets being packed for export to the States. Amplifiers were tested in the three small rooms at the back of the factory space.

Published in the local Dartford press, 4th September, 1964 - in terms of these exports, quick action - the "Million Dollar Deal" with Thomas Organ had been made only 10 days earlier at the Russell Hotel Trade Fair in late August.

9th November

AC50 Mark 3 (solid state rectified) chassis number 1944 now in a handsome reproduction small box. Thanks to Michael for the pictures.

4th October

Pics of serial number 1891, a valve rectified Mark 2, now added to this page. There are a few AC50s up for sale at the moment on various platforms around the world: serial numbers 1881, 1958, 3228, 3543, 4107, 4551, 5439, 5626, 6027, and 6822. Several others have no serial number plate or are represented with pictures not showing the plate well enough to be read.

12th September

Serial number 3771 now . Number 5213 is pictured on . Thanks to Ihor for signalling the existence of both amps.

6th September (2)

Currently on , AC50 Foundation Bass, serial number 3543. A replaced mains transformer, but otherwise in reasonably good order. Pot codes "GM" = August 1965. Interesting to see a Goodmans Audiom 91 blue label speaker in the cabinet, nominally 100 watts handling. Goodmans reckoned that these drivers had a limit of around 75-80 watts when used for bass. Closed-back cabinets, however, will have brought this notional limit up to around 90 watts - plenty for an AC50 in any eventuality.

6th September

An unusual plate - seems genuine: hand-stamped, the last digit of the serial number re-stamped, and a stray "3" in the rating panel.

5th September

Music trade press, May 1969, reversed captions to the pictures though! In spite of that, the note to some extent explains the strange amps - control panels of aluminium painted black, legends screen printed, white knobs, and so on. A further three (at least) survive besides, one currently on ebay (now with black knobs). It seems unlikely that Triumph Electronics itself was involved, even though the principal components employed in these amps definitely came from there. Triumph was in south London (Purley), not south-east, and leaving the company name-plate on chassis would be a colossally stupid thing to do if one were the perpetrator.

Music trade press, May 1969.

27th August

A great pic. of the new Foundation Bass speaker cabinet from the preview report of the Jennings stand at the Russell Hotel Trade Fair, August 1963. Production cabinets were made slightly differently, the radius at the corners being much less pronounced.

Music trade press, August 1963.

23rd August

Thanks to Adrian, a couple of pics of a diamond-input AC50 mark 1, original black cloth, probably early autumn 1964, in great condition. The speaker cabinet is a 2x15", c. 1967.

19th August

Some updates coming soon. Just to signal that the Trade Fairs section on the has been expanded over recent weeks. What is still lacking though, among other things, is a photo of the Frankfurt Fair of February '64, at which the new AC50 was first displayed and demonstrated to the public and dealers. The search continues.

13th July

An early-ish AC50 Mark 2 chassis, last third of 1964. Sold recently in the USA. Various component date codes are evident. Pots, "DK" and "GL" = April 1963 and July 1964. Mullard capacitors "D3N" and "B/64" = last quarter of 1963 and second of 1964. Hunts capacitors "WST" and "HWT" = 12th week and 21st week of 1964. Note the dome voltage selector.

1st July

A further report - British music trade journal, December 1963 - relating to the Beatles' AC50s. This came a few weeks after Reg's communication to "Record Retailer" magazine (of late November). See the entry for 13th June further down this page. Prior to his appointment as General Sales Manager and Publicity Officer in early October, Reg had worked in the Jennings shop on Charing Cross Road - for five years it is said.

25th June

"Music Trades Review", July 1963, the earliest Jennings ad to have come to light so far for the AC50 Foundation Bass. The ad was repeated in "Beat Instrumental" in August. The actual dimensions of a small-box two-input AC50 are 19 x 10 1/2 x 6 1/2 inches - 7 inches tall including feet. The chassis initially envisaged (i.e. to fit the box described in the blurb) will therefore have been proportioned slightly differently from production amps.

Given the date of this ad, the design process for the AC50 and setting up of production evidently took around four to five months, John and George of the Beatles receiving the first units at the end of November (see the entry below, 13th June).

20th June

A paragraph (published on 25th January 1964) on the coming Jennings display at the Frankfurt Musikmesse - "new high-power 50 watt amplifiers".

15th June

An interesting report, February 1964, on the Beatles' new amps - "extra powerful amplification equipment". In the second para, the note: "specially made in narrow cases for them". The desire for a "look" suitable for The Beatles therefore seems to have been the principal driving force in the design of the thin-edged boxes of the first AC50s and AC80/100s - not just a general aesthetic sense of balance and proportion.

Just to add that the early thin-edged AC50 boxes enclose their chassis extremely tightly. Valves made in Germany in the 1960s-1980s, branded RFT, Telefunken, and so on, will not fit. They are too tall. The left hand EL34 will stick against the screws for the handle protruding through the top of the box. Valves that do fit are: Mullards (XF1-XF4); EL34s made in Japan using old Mullard tooling - branded RCA, Raytheon, etc; and large bottle American-made 6CA7s - GE, Sylvania, and RCA. 1960s-1970s Teslas may be fine too, but no first-hand info is available at the mo.

13th June

A report via Reg Clark of JMI about the Beatles' new AC50s in the weekly magazine "Record Retailer and Music Industry News", 28th November, 1963. Mention of 60 watts is likely to have resulted from a misunderstanding, Reg having said something along the lines of the new amps being "around twice as loud as their existing ones" - though not strictly true, great promotional puff. Note that the statement is not a direct quote from Reg: though new in his job at JMI, he will have known that the AC50s were indeed AC50s, not AC60s.

The "end of this week" means Friday 29th November, i.e. the day after the "Record Retailer" had appeared in print. Details of the new amps had presumably been communicated by Reg earlier in the week.

So there we have it. Reg Clark was clear that John and George would have their new amps by 29th November 1963. Perhaps they had already arrived by the time the magazine hit the news-stands.

10th June

A note on the AC50 bias circuit designed by Derek Underdown. As has already been indicated, Derek simply re-purposed the system he had adopted in the 1950s for the Jennings Organ Company's 30- and 50-Watt amplifiers. A key part of the circuit is the neon indicator lamp, used to help stabilise the voltage. In the 1950s, the lamp was known as a "CC3L". In the 1960s, the designation was "NT2", though the specifications remained the same. Some units were also marked CV2266, a military number, "CV" standing for "Common Valve". An alternative was the CV2213.

The neon lamp in an two-input AC50 mark 1 (serial number 1034).

Detail of the circuit diagram for the Jennings Organ Company 30-Watt amplifier.

Detail of the circuit diagram for the Jennings Organ Company 50-Watt amplifier.

The operating parameters of the neon lamp (pdf of the specifications of 1955):

Max. striking voltage - 85v

Max. anode current - 0.65mA

Min. anode current - 0.05mA

Regulation change in voltage for a change in cathode current from 0.5-0.3mA - 3v

An earlier form of voltage stabiliser - the Mullard 7475 - was used by Derek for the Jennings organ power pack sections.

5th June

Recently surfaced in the UK, a further thin-edged two-input AC50 from early 1964. Good honest wear; original transformers in place. Some renewed components in the power section.

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