So far as one can judge, most AC80/100s and AC100s were fitted with Mullard XF2s made at the Blackburn Factory. On the XF2, see the wonderful history of the EL34 by Pasquale Russo, available for purchase through his website.
Click on the image above to watch a great film about the Mullard factory at Blackburn.
The codes are normally printed on the glass (be careful, they can sometimes wipe off all too easily):
The first letter is the code for the Factory. "B" signifies the Blackburn Factory in Lancashire.
The number following the initial letter is the year. So "B5" is Blackburn, 1965.
The third and fourth elements of the code, a letter and number, are the month ("A" = January, and so on); the number is the week of the month, e.g. "B5C5" = Blackburn, 1965, fifth week of March.
The general public and smaller companies will normally have bought individually-boxed EL34s. And in spite of what one sometimes reads about "matched pairs" only coming into being when production standards began to slip in the 1970s, sets were in fact available in the 1960s.
Above, a couple of XF2s from October 1965 - unopened for fifty years and unused.
But for larger companies, the contractors that assembled amps for Vox (see below) doubtless included, bulk cartons of 50 (enough for twelve AC100s) must have been preferable.
An astonishing five cases of new-old-stock Mullard XF2s - picture from this thread.
Mullard was really the only viable supplier. The Brimar valve company at Foots Cray in Sidcup - a few miles from Erith and Dartford, and within easy reach of London - does not seem to have produced EL34s in the early or mid 1960s (despite claims, no-one has come up with an example - see this thread). In the late sixties and early seventies, it simply rebranded Mullards, Teslas and German valves - RFTs and so on. The Brimar Factory at Rochester, a little way down the Kent coast, mainly dealt with military contracts.
Brimar preamp valves - ECC82s and ECC83s - do, however, regularly pop up in Burndept-made Vox amps (along with GZ34 rectifiers in the case of AC30s). The closeness of Brimar and Burndept can be seen on this map.
Note that Brimar made the brimistor, used in later AC100s.
If replacement EL34s were needed in the USA, options were available. Amperex, GE, RCA and a number of other companies provided matched pairs or quads of Mullard XF2s through to the 1970s.
A set of five RCA branded XF2s from 1969
Original valves in Vox AC100s
Cathode Biased Amps
In 1964, Vox set in motion production of two amplifiers that were based around EL34 output valves: the AC50 and the AC80/100. Triumph Electronics in Purley produced the former under contract, and Westrex in North London the latter. Each had its own supply line, presumably extending to stocks of EL34s.
Serial number 177
The amp survives complete with its speaker cab, covers and original valves. The codes are B3K4 = fourth week of November 1963.
AC80/100 number 177 was probably produced in July/August 1964 so its EL34s spent around seven or eight months in store at Mullard and then Westrex before being fitted. Westrex were the contractors that produced the amp for Vox.
Serial number 392
By the time no. 392 came to be produced, production of the AC80/100 had been shifted from Westrex to the Burndept Factory at Erith. Date codes of the EL34s are B4L3 (one), B4L4 (three) = Blackburn Factory, 1964, third week and last week of December 1964. All are XF2s, first version: small dark brown base, eight metal pins, hole in the central stem, double getter rings. The print has faded from yellow to white in a couple of cases. The surviving Mullard yellow-print ECC82s have the code B4C4 (fourth week of March 1964). The EL34s are too far gone to be usable unfortunately.
AC80/100 serial number 392 was made in late April 1965, so its EL34s had been in store for around five months at Mullard, and then Erith (the Burndept Factory) before being fitted.
The 100W Amplifier
Serial number 531
Three original valves and one early replacement. As above, Mullard XF2s. Codes of the the original three valves are B5C5, B5E4 and B5E5 = fifth week of March 1965 and fourth and fifth week of May. The replacement is a GE-branded Mullard, code B5H3 = Blackburn 1965, third week of August.
AC100 serial number 531 was produced in early July 1965 so its valves had been in store for three or four months before being fitted.