Cathode biased AC80/100s (the AC100 Mk1)

Designed in 1963 by Dick Denney in a continuing quest to provide the Beatles with more power (Paul McCartney first, John and George a little later), the AC80/100 was perhaps one of the loudest, if not the loudest amplifier of its day, producing something in the region of 80 watts clean, with a little more to spare when really pushed. Its only major rivals in terms of volume were two Fender amps: the high powered Twin, and the Dual Showman, both of which were difficult to obtain in England until the later 1960s. Even by today's standards, it is a great amp.

And its sound is like that of no other - full, sweet and slightly gritty - it has been described as having "a big, heavy low end, full mids, plus tons of harmonics." Perhaps its most notable characteristic, however, is its "swell". Hit a chord, and the chord resonates, recedes, and then returns with a huge percussive "whoomph". Listen for instance to the live recording of the Beatles at the Hollywood Bowl in 1964 - three AC80/100s played hard. Denney's design was a good one.

Anyone wanting to know more about Denney and all things Vox - the amps, the legions of bands who used them - can do no better than consult Jim Elyea's excellent, beautifully-illustrated book: Vox Amplifiers. The JMI Years (California, 2009), and to visit Gary Hahlbeck's fabulous Vox Showroom site.

Not to forget either the hosts of authors of and contributors to threads in the Vox section of the Plexi Palace bulletin board, and similarly to the Beatgearcavern, and now defunct VoxTalks. The archives of these sites are a mine of wonderfully recondite and useful information; and it doesn't matter a jot that some of the authors are anonymous (using screen names/aliases/avatars), while others are themselves.

As the first schematic indicates (OS 136), the process of designing, testing and prototyping the AC80/100 seems effectively to have been finished by 26th September 1963. Three months later, between the 21st and 24th of December, Paul McCartney received his amp (an early Christmas present from Vox?). Presumably Vox spent September to December setting up the supply lines for production at Westrex, to whom assembly was contracted out. On Westrex, see this page.

Quite whether Paul's amp was a pre-production unit or part of a more concerted production run is impossible to say. Very probably however it bore the serial number 101 - the first of a rare breed.

The only obvious difference from standard production amps is the fuse holder on the back panel underneath the mains socket. The holder in production amps was on the chassis, inside.

Left, the first advert - "Beat Monthly" magazine, April 1964; and right, the first appearance in a catalogue (of much the same date. Up to April 1964 the amp was probably a special order item. Numbers were probably loaned to bands though.

Prices: 1964 - 1967

May 1964 - see the pricelist on the Vox Showroom.

A.C.100 Model: £195 - note that no speaker cab is specified.

A.C.100 Amp. Section only. 100 watts: £105.

September 1964 - pricelist

A.C.100 (Cabinet housing two 15" loudspeakers): £205 16s.

A.C.100 Super De Luxe with Beatles type Speaker Cabinet (Housing four 12" and 2 Midax loudspeakers): £252.

A.C.100 Amp Section only, 100 watts: £105.

May 1965 - pricelist.

A.C.100 (Cabinet housing two 15" loudspeakers): £205 16s.

A.C.100 Super De Luxe with Beatles type Speaker Cabinet (Housing four 12" and 2 Midax loudspeakers): £252.

Amplifier Stand; A.C.100 Speaker Cabinet Type: £22 5s.

November 1965 - pricelist.

A.C.100 (Cabinet housing two 15" loudspeakers): £205 16s.

A.C.100 Super De Luxe with Beatles type Speaker Cabinet (Housing four 12" and 2 Midax loudspeakers): £252.

Amplifier Stand; A.C.100 Speaker Cabinet Type: £22 5s.

April 1967 - pricelist.

A.C.100 Bass: £205 16s.

A.C.100 Super de Luxe with stand: £252.

A.C.100 Amp Section: £105.

A.C.100 de Luxe Cabinet (4 x 12" and 2 horns) and stand: £147.

Covers: A.C.100 Amplifier Section: £1 13s.

For further details on the amps used by the Beatles and other bands in 1964, see the pages on this index page.

In terms of electronic design, the AC80/100 was vanishingly simple. The preamp, with a few tweaks, was essentially the "treble" channel of the Vox AC30, and the power amp, four EL34s run with cathode bias. Controls were: volume, bass and treble. But this is not to diminish the technical accomplishment of the Vox engineers, which was considerable - the chassis is a masterpiece of compactness. The brief to make a powerful valve amp that was no wider than a T60 speaker cabinet (19 inches), the cabinet with which the AC80/100 was initially paired (as in the "jumping Beatles" advert above), was not an easy one to fulfil.

As far as fixtures and fittings are concerned, the earliest amps had brown grille cloth (as mentioned above), thin-edged wooden cabinets at front; a 3- or 4-pin XLR socket for the mains supply; normally a single 3-pin XLR speaker output; and white warning plaques. Sometimes a small bass flag was provided on the front grille, lower right.

Pictures showing the difference between an early thin-edged box (3/8") and the later thick-edge version (5/8").

For early amps in thin-edged boxes (serial numbers 101 - 220) see these pages. Amps in thick-edged boxes are represented there too (starting with serial number 221).

At least three different types of cabinets were paired with the amps. The "Foundation Bass" unit, with an 18 inch speaker (Bill Wyman normally used a pair); the 2 x 15, as used by McCartney; and from September 1964, the iconic 4 x 12 and trolley - the AC100 SDL (Super Deluxe).

The only cautionary note to sound about AC80/100s is that they run hot. The low value of the cathode resistors (270 ohms), which regulate the dissipation of the power valves, is the principal culprit. The EL34s - Mullard XF2s - were pushed to their limits.

In wintry Britain ('63-'64 was particularly cold), this was probably regarded simply as the consequence of being at the bleeding edge of technology. But when the amps came to be exported to America in mid 1965, problems are reckoned to have arisen in voltage regulation, causing potential (and real) instabilities in the amp.

But by the third quarter of 1965 the time for a new version had come in any case.

AC100 Mark 2 - fixed bias amps (1965 - 1967)

The revised amp that Vox produced looked pretty much like the old one. From an electronic standpoint, however, key elements were updated. A fixed bias arrangement - a network of diodes and resistors - now regulated the valves, which in tandem with certain other changes, resulted in an AC100 that not only ran cooler than its predecessor, but produced more power. A true 100W (RMS, British Standard) was easily achieved.

For further images and discussion, see the pages here and here.

Testimony to its popularity, somewhere around 1600 new AC100s were made in a two year period; and even the Beach Boys, for whom Fenders were the norm, fell sway to their charm.

An ever-growing collection of images is gathered together on the Bands 1965 - 1970 pages.

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