Goodmans Midax Horns

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GOODMANS MIDAX HORNS

An early promotional picture.

Page from the Goodmans catalogue of 1958.

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:

(2) the pairs of Celestion blues (T530) used in the Beatles' cabs could jointly handle only 35-40W at most (taking into consideration their semi-closed backs). In normal, "open-back" conditions, the T530 was rated at around 15W. The Midax, with its 25W handling, served 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. Small box AC50 amplifiers at 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 The silvers were in turn superceded in AC50 cabs by ceramic drivers - Fane 122/17s (25W), Celestion T1217s (25W), which further increased the power handling.

Goodmans specifications

The salient characteristics of the horn, as given in the brochure illustrated below, are:

Frequency Range

Crossover Frequency

System handling capacity

Impedance

Baffle cut-out

Fixing holes

Overall length

Width of horn mouth

Depth of horn mouth
650-8000 c/s

950-5000 c/s

25W (50W USA)

15ohms

5 5/8" x 2 9/16"

6 holes 0.191"

9 15/16"

6 1/2"

3 1/2"

A rare thing - Midax horns in their original boxes. Thanks to Guy for the pictures.

Above, the booklet supplied with the horns.

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.

And the horns became even more critical to the AC100 cab (from mid 1965), as later fixed biased amps were made to kick out a true 100W.

By mid 1965 and into 1966, increasing numbers of 12" speakers with ceramic magnets and greater power handling potential were of course becoming available. Vox, however, stuck to the Celestion silver alnicos, and for very good reasons: they are one of the best guitar speakers ever made. Their high efficiency tends to render the occasional sharpness of the horns, especially at lower volume levels, less evident.

The picture is different however with ceramic speakers, fitted in some later AC100 cabs. The lower efficiency of the ceramics tends to make the horns sound more prominent and the overall sound less balanced. This goes for AC50s with ceramic speakers too.

Midaxes are evidently not to everyone's taste, though - some say they are "shrill", "ice-picky", and so on, especially for indoor playing. There are a couple of caveats about taking the Midaxes out of circuit:

One, the value of your vintage cab may be decreased, especially if its wiring is original. Going at it with a soldering iron is not the answer. Two, if the horns are taken out of circuit, the power handling of your cab will be diminished. Stressing fifty-year old speakers to the max is clearly not the greatest of ideas at the best of times - i.e. putting an AC100 through them at full whack. Reducing the cab's ability to handle the power of an amp it was designed to accompany is a serious consideration. The last thing one wants to have to do is find replacements for blown T1088s (at over £200 each these days).....

If detaching the horns is a necessity, it is sometimes possible, depending on how the cab is wired, simply to pull out the banana plug on the negative side of the Midax - generally the side with a yellow wire leading from it - see the schematic on this page.

The circuit and impedance

Low frequencies are prevented from reaching the horns by the capacitors - 2uf, made by the Telegraph Condenser Company (TCC) and painted black. The impedance of the cab for bass notes/bass chords is therefore 8ohms - ie. simply the impedance of the Celestion T1088s. The horns are "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 combined impedance of the two units is around 8ohms (two lots of 15ohms in parallel).

The total impedance of the cab therefore changes, transiently, to 4ohms.

The formula is a version of Ohm's Law: R1 x R2 / R1 + R2 = Total.

8 x 8 / 8 + 8 = 64/16 = 4ohms.

Early Midaxes

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.

Midax horns in Vox AC100 cabs

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.