There is some valid concern over who actually "designed" the AC50. The answer is JMI, but with a slight twist.
In the "Vox Story" it is said that: "JMI's design team was already fully committed in 1962 with the Continental organ and the various solid-state amp ventures, so Triumph Electronics, one of the AC30 chassis contractors, offered a 50-watt design based on cathode-biased EL34 valves. This featured two channels, each with its own tone circuit derived from the Top Boost circuit...."
Well there are unfortunately three misconceptions in that: (i) Triumph made AC4 and AC10 chassis, but not the AC30; (2) the AC50 was never cathode biased; and (3) the AC50 was of course at first single channel (in 1963 and early 1964).
Producing a cathode biased AC50 in any case would not have provided much advantage in terms of power over the AC30. A fixed bias circuit was needed.
It seems vanishingly unlikely that Denney, co-author of the "Vox Story", completely mis-remembered Triumph playing some part. Everything falls into place though if one sets the design of the fixed bias circuit at Triumph's door - i.e. the arrangement of lamp, diodes, and resistor/capacitor network.
The working prototype was made at JMI - the tone circuit certainly being the JMI Top Boost circuit - and sent to Triumph for copying, as Graham Huggett notes.
There is no doubt that Denney designed the AC80/100 himself. Cathode bias was his thing.
As a coda: you will find old talk on the internet of cathode biased AC50s actually being seen in the wild. They are unicorns, or yetis, perhaps both.
Above, a detail of the tone circuit of the AC50, and below it, the top-boost circuit of the AC30, designed in 1961 ("borrowed" by Vox from the Gibson GA-70).
The JMI top boost circuit clearly provided the model for the AC50 preamp. It is hard to believe that the latter was not created by Dick Denney at JMI (as has occasionally been claimed.)
Some great info from Graham Huggett, who worked at Triumph from 1963 to early 1966. Graham was "chassis beater", making the chassis for AC50s on fly-presses and hand guillotine.
The prototype for the AC50 was sent from Dartford to Croydon (Triumph was at 118 and 122 Brighton Road) for copying. Flat masters of the chassis were then made for production.
Around three people worked at Triumph full time. The wirers, mainly women, were part time and trained by Yvonne Andrews.
Control panels (bezels) were sent from JMI for the finished amps. Runs of around 50 chassis were made up on receipt of orders from Dartford.
Graham notes "assuming we had received the transformers from our manufacturer (took some weeks) the staff would easily wire up 15 chassis in a week." Testing was done with a meter, sine wave generator and scope, not with a guitar.
Triumph did not have boxes or serial number plates. The finished chassis were loaded into the back of Geoff Johnson's Ford Zephyr estate - Geoff Johnson was owner of Triumph - and driven by him, often at speed, to JMI.
There the AC50 chassis were given boxes and serial number plates added.
Graham had long-term loan of the prototype AC50 and used it for bass with his band for a couple of years.
The serial number plate of no. 1578 - fully hand stamped. Made by Triumph for Vox. Despite the relatively low number, the amp was made in October/November 1964. Two pots have the codes "HL" = August 1964. The others have "GL" = July '64.
Above, two adverts placed by Rudall, Carte and Co. in Crescendo Jazz magazine, the first June 1965, the second November. The first AC50 is a guitar amp, the other possibly a Foundation Bass. Rudall also had a Vox 80/100, perhaps an early one, in May '65. See this page.
Pics of serial number 3088, valve rectified, assembled at the Vox/Burndept works, now registered here. It seems likely that Burndept was simply using up available chassis as they came to hand - and this one, in company with serial no. 3226, happened to have been made for the old valve rectified circuit.
A shot of the Goodmans catalogue page for the Audiom 90 - the first British 18" driver produced - used by Vox in the earliest Foundation Bass cabs (in its slightly later 15 ohm version). £28 in early 1964 is equivalent to around £520 in today's money. These were not cheap.
Three new AC50s from the third quarter of 1964 added on this page, along with some updates to the intro. The amps in question have serial numbers 1729, 1746 and 1763. None preserves its original voltage selector, however.
22nd April (2)
Pics of serial no. 2031, late 1964, twin channel, valve rectified, now registered on this page. Further pics here.
Been a little while since the last series of updates - a good amount of new material to be added now over the coming days.
Serial number 118x - small thick edged box, diamond input - on sale here.
The band "Daybreak" with a small box Foundation bass set and a large box AC50 amp.
"The Citations", an all AC50 band.
Click for a larger image. The Vox Solid State catalogue from mid 1967 - still the AC30 and AC50. The AC100 is not depicted, though it does appear in the contemporary pricelist.
28th January (2)
An early Thomas Organ flyer for its new solid state range, the "Berkeley" clearly styled after the large box AC50. The format was later changed. The blurb reads:
"High quality amplifier incorporating variable length reverberation and multi-speed variable depth vibrato. Two inputs, separate tone and volume controls. Amplifier and loudspeaker units are in separate cabinets to give greater power with feed back."
28th January (1)
"The Others and Me" with their large-box AC50 guitar amp, and thin-edged Mk1 Foundation Bass. The Mk1 amp is visible behind the bassist, Gary Thain, in the pic. below.
"Country Pie" in the early seventies with a thick-edged small-box Foundation rig, a large box AC50 and Vox Conqueror.
21st January (2)
A late Vox Sound Ltd advert for the AC50, still the Foundation Bass, the guitar version with a new cab (closed back and deeper than normal) and trolley, which might have benefitted from better styling.
21st January (1)
Above, "The Rapiers", from East Anglia, on stage with a small box AC50 and T60 cab. Picture originally posted here. Note the position of the logo on the amp - middle left, rather than top left, as in the amp below.
The amp from this set is now in Brighton, the cab in London.