An early promotional picture.
VOX AND THE MIDAX
The Goodmans Midax mid-range horn, which was normally used in hifi applications, as one can see from the literature further below on this page, was first used by Vox in late 1963 in the new "small-box" AC50 speaker cabinets issued to the Beatles, the Dave Clark Five, and perhaps others. Only a very few cabs were made. For surviving early amps, see this page.
Click for a larger image. One can see the ends of the Midax horns peeping out through circular holes in the backs of John and George's cabs.
The reasons for the adoption of the Midax were twofold:
(1) the EL34 valves in the new AC50s were judged to produce less top end "sparkle" than the EL84s of the AC30s, and so the horns were brought in as a means of enhancing the treble. This is "the story" regularly repeated. However (and also):
(2) the pairs of Celestion blues (T530) used in the Beatles' semi-closed-back cabs could on their own really only handle 35-40 watts at most. The Midax, which was rated at 25W, served, importantly, to increase the amount of power the cabs could stand. This was noted by Dave Petersen in 1998 - D. Petersen and D. Denney, "The Vox Story" (1993), p. 47. In 1964, small box AC50 amplifiers on test at Triumph Electronics, the contractors that made the chassis for Vox, generally made 46W - not the 50 of the name, but certainly enough to destroy a pair of Celestion blues.
When, in mid 1964, large-box AC50s came into production, Celestion silver alnicos (doped to give power handling of 17W) were used instead of the blues, along with a Midax. The silvers were in turn superceded in AC50 cabs by ceramic drivers - Fane 122/17s (25W), Celestion T1217s (25W), which further increased the power handling.
The salient characteristics of the horn, as given in the brochure illustrated below, are:
System handling capacity
Width of horn mouth
Depth of horn mouth
25W (50W USA)
5 5/8" x 2 9/16"
6 holes 0.191"
A rare thing - Midax horns in their original boxes. Thanks to Guy for the pictures.
Above, the booklet supplied with the horns.
The circuit and impedance
The capacitor in this case is mounted directly above the horn. One sometimes finds it mounted on the upper internal face of the cab.
Low frequencies are prevented from reaching the horns by a 2uf capacitor, positioned in circuit between the speakers. In other words, the impedance of the cab for bass notes/bass chords is therefore the impedance of the speakers - ie. simply the impedance of the two Celestion T1088s or two Fane 122/17s (wired in series). The horn is effectively "invisible".
When the horns begin to pass mid range and high frequencies (including harmonics when distortion sets in), they become "visible" in the circuit.
The formula to express their general effect in terms of impedance is a version of Ohm's Law: R1 x R2 / R1 + R2 = Total.
Taking the example of an early cab with silver alnicos, the sum is: 8 x 8 / 8 + 8 = 64/16 = 4ohms. So depending on the frequencies being driven, the cab can present a load in the range (varying moment by moment) of 4-15ohms.
One of the first generation of Midaxes - note the label. Probably a lower power handling than later horns - the diaphragm is visibly smaller.
An early Midax - smooth finish, no serial number on the label.
An early Midax with its box and brochure.
The AC100 cab
When Vox came to design the large AC100 cab in mid 1964, several considerations evidently played a part. It had to handle the power, sound good, and be as compact as possible. The solution arrived at was effectively a double small-box AC50 cab - two compartments, each with two 12" Celestion silver alnico (T1088) drivers, and one Midax horn.
The Midaxes, rated at 25W each, were an important part of the circuit: four silver alnicos, with their rated handling of around 17W apiece, would only give 68W, or perhaps with luck, a little more. Cathode biased AC80/100s, as their name suggests, when properly set up, produced 80-100W.
For more on the AC100, see the AC100 website - https://www.voxac100.org.uk.
Long-throw horn with Midax 100 driver
Early Trebax horns
Later Midax horns
The section that follows is arranged by serial number (as a rough though not infallible guide to date).
From early 1966 large batches of horns went straight from Goodmans to the USA for Super Beatle and Royal Guardsman cabs. "The Vox Story", p. 144, gives a short account of the testing process, part of a larger report of Dick Denney's visit to the States in late 1965:
"Tweeters for Use in Super Beatle Type Speakers. Samples of a domestic horn-type tweeter were compared briefly to the Goodman-type used in Jennings speaker cabinet[s]. Initial observations indicated that the two horns performed quite differently, with the Goodman unit providing more contribution in the mid-treble range, and the domestic horn perhaps slightly more brilliant in the highest treble range. Additional listening tests will be required to make a determination of the suitability of this particular sample or to develop information for the speaker manufacturer to use in making additional samples."
Thomas Organ evidently opted for Goodmans. It is not clear, at present, who the manufacturer of the "domestic" horn might have been - very probably Klipsch, however.
For the reasons why Vox/JMI used Midax horns in the first place, see further below, where some additional technical info. is given.
Serial no. 34467 in a Supreme cab from mid 1967.
An example from late 1967 / early 1968 - serial no. 36490.
An American Super Beatle amp (V1143). Midax serial nos 36612 and 38880.