Early Cathode Bias Amplifiers - introduction



The earliest "thin-edged" amps


Early AC80/100s, distinguished by their "thin-edged" boxes (more on these below) and copper control panels, are rare birds. Probably only around 120 were made, beginning at serial number 101 (Paul McCartney's) and ending somewhere close to 220 - the production run, such as it was, extending from December 1963 to around October 1964. Serial number 221, which was sold on ebay in 2007, survives in its original thick-edged box. See the images on this page.


The amps were assembled by Westrex, contractors for Vox, in North West London and transported from there to the factory at Dartford for boxing up. Westrex at the time also produced AC30s. For further details, see this page.


"Cathode bias" designates the mode by which the operating point (the "control bias") of the main power valves - the four EL34s - is set: in the case of the AC80/100 a resistor and capacitor network run to ground from the cathode of each valve, R26, R27, R33, R34 and C13-C16 on the schematic below. "Fixed bias" might have produced more power, as it did later for the "100W Amplifier" and AC100 mark 2, and less heat, but the sound and responsiveness of the cathode biased amp is far superior. Denney's judgement was spot on.


80-100 Watt Amplifier schematic. The only really observable deviation from the schematic in production - a sort of minor revision/non-revision - lies in the values of the preamp filter capacitors: 32uf instead of the 50uf marked, and 16uf instead of 8uf. Further AC100 schematics are available on this page.



Quite how the process of production panned out is difficult to say. If one goes by simple averages, then the figures are: 120 amps over around 40 or so weeks = 3 or 4 per week. But perhaps assembly was more fitful, or at least irregular. Remember that at first the AC80/100 was a special order item; and remember too that a number of amps could have been made in the period immediately after the drawing up of the schematic on 26th September 1963. Paul Mc Cartney received his amp - probably serial no. 101 - in late December.


Above, the Westrex factory, 152 Coles Green Road, photographed in 1930. Picture from the britainfromabove website.





The boxes

The earliest boxes, as mentioned, were "thin-edged", the visible fore-edges being 1/4" thick compared to the 3/4" of the later "thick-edged" boxes. The cabinet maker was GlaRev, the company run by the flamboyant Henry Glass and his partner, Mr Revel.



Brown-fronted amps (ie. with brown grille cloth) were made for use with bass cabinets; and after August 1964, some black-fronted were produced, generally for use with guitar cabs. So far only one thin-edged black-fronted AC80/100 has come to light. However, the Beatles certainly had three, possibly four; the Rolling Stones had two; and it may be that other bands had them as well, though it is naturally difficult to tell from black and white photos.


On the left, Paul Mc Cartney's all-brown bass AC80/100 during the Washington concert, 11th Feb. 1964, from the Corbis images collection. On the right, John and George at Forest Hills (28th August 1964) with all-black AC80/100s and super deluxe cabinets.



The first few thin-edged boxes were apparently made without fixings along the top edge of the removable back panel; later ones for the most part had a central screw, but there is no absolute rule.


On the left, a shot of the back of Paul McCartney's amp. On the right, the rear of serial no. 150.





AC80/100s in thick-edged boxes

One hundred and fifty or so thick-edged box AC80/100s followed those in thin-edged, running in terms of serial numbers from 221 to approximately 420, and perhaps a few after that, the dates in view being from late 1964 into the second quarter of 1965. The last amps may have overlapped with the new fixed bias "100W Amplifier", the immediate precursor of the AC100 Mk2 - more of which in due course.


Grille cloth was brown at first, but changed to black somewhere in the mid 200s. Control panels were copper, then black, then grey.

Amps in thick-edged boxes. From left to right, serial numbers 221, 276 and 320.


In terms of contractors, Westrex continued up to early 1965 - around serial no. 300, still using green Woden transformers; while probably at much the same time, production started up at Burndept Electronics in Erith. Burndept-made amps have chassis numbers stamped below the input jacks and Woden transformers with plain metal shrouds. Burndept also introduced the smaller upright choke, replacing the larger "laydown" unit of later amps, and necessitating a slight redesign of the chassis plinth.


The new boxes, more suitable for the weight of the amp, were produced by P. A. Glock, though Glass evidently began making them again in 1966 and 1967 (some are stamped "G-R"). As ever, the best source of information on these firms and their work for Vox (GlaRev for instance later produced cabinets for Hiwatt and Vamp) is Jim Elyea, Vox Amplifiers, esp. pp. 136-137.





Note

Probably now, in 2014, fewer than two dozen thin-edged AC80/100s survive. Most stem from the end of the run - serial numbers 169 - 212. One possible explanation is that these were bought and paid for, and as such received more care and attention. A good number of those issued to bands must have come back to Vox, if they came back at all, in less than pristine condition.


In the video below, one can see what may be a thin-edged AC80/100 in Hessy's Music Shop in Liverpool in 1964:


Hessy's Music Shop in Liverpool seen in 1964, the amp visible in the background, esp. at around 0.47'