Fixed Bias Vox AC100s - introduction

The AC100 in 1965

The new fixed bias AC100 - first the "100W Amplifier", then the AC100 Mark 2 - was designed to fulfil a primary, almost primitive need: POWER! Anyone who has listened to recordings of the Beatles live in late '64 and 1965 will know just how stupendous the jet-engine roar of fans in multi-thousand seat venues could be. Sometimes quite unbelievable.

Views of The Beatles' Concert at Shea Stadium, taken by a member of the audience.

So the new design entered the picture, delivering a true 100 watts against the 70-80 watts of its predecessor, the AC80/100. That some of the sweetness of the latter had to be sacrificed was perhaps inevitable, but the AC100 Mark 2 is nonetheless a fine amplifier, and something of an engineering triumph - louder, tighter, and in many ways cleaner than the AC80/100. As numerous commentators have noted, in terms of sound pressure level (SPL) - sheer whoomph - the Mark 2 has no rival.

What gave the new amps legs in these respects was the introduction of what is termed "fixed bias" for the output valves - - a network of components generating a fixed voltage to regulate and stabilise the EL34s, instead of resistors and capacitors grounding the cathode. One cannot help think too that the devising of the new circuit was prompted by the heat troubles of its forerunner.

The first schematic (0S/036), entitled simply "100 Watt Amplifier", is dated 30th May, 1965. The amps were assembled by , as the later AC80/100s had been. For further pictures of the "100W" amps, .

On the 17th June the sheet was redrawn as the "AC100/2", and on the 28th, a brimistor (a means of delaying inrush current) was brought into the circuit.

On the left, a detail from the "100 Watt Amplifier" schematic. On the right, a detail from the standard AC100/2 sheet.

The chief differences from the AC80/100, aside from the fixed bias circuit itself, which used among other things the voltage-clamping properties of zener diodes to generate the 35v needed to keep the EL34s within reasonable operating limits, were:

(1) a change in the voltages applied to the preamp valves - lower voltages for the first ECC82 and the ECC83, much higher though for the second ECC82 (the valve that drives the EL34s).

(2) a higher B+ voltage (the main driving voltage). This last was made possible by the new transformers that were sourced for the AC100 Mark 2, probably by Burndept Electronics. These units are instantly identifiable by their black shrouds, as the pictures below will show. They seem to have been employed in the fixed bias amps from the first.

One of the earliest surviving fixed bias AC100s - one of the first of around two hundred "100W Amplifiers" (the precursor of the Mark 2).

The secondaries of these new mains transformers supply 360v against the 350v produced by the Wodens employed in the AC80/100s. The desire from the outset seems to have been to drive the plates and screens of the EL34s much harder - which goes hand in hand with a fixed bias zener network. Higher screen and plate voltages would have caused colossal difficulties in the AC80/100.

Brimistors

Soon after the first schematic had been drawn up, a significant change was made to the circuit. By 28th June, when the revision was recorded, a brimistor had been brought in (along with a second voltage-dropping resistor - R7 - for the preamp).

On the left, the brimistor in a late AC100, just visible by the choke (click for a larger pic). On the right, a replacement brimistor/thermistor, in other words not the "CZ4" of the schematic. This is fairly unusual. Normally one finds 10K seven or ten watt resistors as replacements, or simply a straight piece of wire. A good original, still in position, is visible in amp no. 444, below.

The inclusion of the brimistor, a sort of temperature dependent resistor - "thermistor" - made by the company Brimar (Brimar + thermistor = brimistor), was an attempt on the part of Vox at giving a "slow start" to the main voltages inside the amp. When an amp is switched on, the cathodes of the valves, which operate on a supply voltage of 6.3V, need a while to warm up - this is why valve amps don't come on straight away.

In amps such as the AC30, all is well, as the valve rectifer, which produces the main voltages, takes a while to warm up too. But in an amp with solid state rectifiers, as the AC100 had, the main voltages are present from the moment the switch is thrown, which puts great stress on the EL34 until such time as the cathodes have had a chance to reach operating conditions.

The brimistor, which has a high resistance at cold, and a minimal resistance after it had warmed up, was an attempt therefore to remedy the situation by stopping the the main voltages from developing too quickly. That at any rate was the intention. for a copy of the brimistor specificaton sheet.

Fixtures, fittings and components

In terms of fixtures and fittings the AC100 of 1965 has: a grey control panel, domed voltage selector, black bat on/off switch, two-line serial plate, and, occasionally, a fuse holder with a drum top and FUSE marked three times around the circumference.

Resistors are the white carbon composites made by Erie - for the most part 1W in the preamp, but 2W for voltage dropping duties and for the plates (underside, on the power board), and 1/4W grid stoppers (attached directly to the output valve sockets).

On the left, a standard AC80/100 preamp. On the right, the new style (with some more recent replacement components).


TRANSFORMERS: as mentioned above, are the units with black shrouds, probably sourced, by Burndept. Now and again one finds examples of unpainted shrouds, but these are rare. Towards the end of summer 1965 part numbers appear, stamped on the underside insulation: the output is no. 66776 and the mains no. 66775. By late 1965 AC50s also had transformers made by the same company - a rationalisation of the supply line.

SERIAL NUMBER PLATES: in 1965 were generally of the two-line variety, and the cut-off point for amps pictured on this page is the appearance on the scene of the three-line serial number plate later in the year.

From left to right: numbers 825; 1099; what appears to be 1375 (certainly not an AC10, as the numerical sequence for AC10s began at 4000); and 1297. The move to the new style of three-line plate was by no means abrupt; and as Jim Elyea notes, two-line plates resurface from time to time in early 1966, e.g. serial numbers 1168 and 1375.

INPUT AND OUTPUT SOCKETS: were at first Cannon XLRs, which had a relatively flat profile. Later, during the course of 1965, Amphenol sockets were brought in. These are the ones with a doughnut-like appearance.

WARNING PLAQUES: were generally the new red type. In 1966 and 1967, however, some white ones turn up again - presumably a stash of old stock had been unearthed.

Explosions

The Vox AC100 has often been charged with being temperamental. Part of that reputation doubtless rests on the experience of The Who. One fan relates

"I distinctly remember the occasion when Pete first used his VOX A.C.100 on stage. It literally self-destructed during the second number and real ozone started to smoke from the back. Pete nonchalantly disconnected the head and chucked the smoking missile in the direction of the band room and proceeded to set up his old stack while the rest of the band kept playing. In keeping with tradition, the show must go on!"

See the excellent for the original context of this quote. Probably the amp was a late cathode biased model. Townsend later said:

"Then we used Vox amps for a little while. We did a deal with them but they were really rubbish. The AC30 is the only good amplifier they ever made."

Jeff Beck in the clip, below, from the film "Blow Up" (1966) is clearly acting out Pete's frustrations.

Jimmy Page is later reported as saying that he had "blown up" four himself. But for all that, the fixed bias AC100 when used in the UK - in other words at standard UK voltage (240V) - was no more unreliable than any of its contemporaries. A modern designer would no doubt have made the output transformer larger, but the size that was chosen by Vox/Triumph - the largest that would fit on the chassis - was of exactly the same specifications in all respects as the transformers that went into many 100W Marshalls in 1967. Indeed, many assert that having a transformer that is slightly on the small side is a positive asset.

The one thing that Vox should perhaps have done is produced cabs with an impedance of 16ohms. Running the AC100 on the 16ohm tap of the output transformer would have ensured that the whole of the secondary winding was brought into play. As it was, the large 4 x 12" Vox cabs all had an impedance of 8ohms, and running the OT on the 8ohm tap necessarily meant that only half of the secondary windings were used - stressing the output section when the amp was driven hard. This clearly was a result of a basic choice - to use Celestion 8ohm T1088 alnico speakers (the "silver bells"). The 16ohm variety - the T1096 - which were designed in 1964 seem only to have been employed in 1967 for 4 x 12"s that were to be paired with both the AC100 and the new solid state Supreme.

The first three images are of a silver 16ohm Celestion T1096 with date code JM (September 1967). The last image is of a pair of 8ohm T1088s, as used in AC100 cabinets.

But Vox was not alone in advocating 8ohm cabs for amps that might perhaps have been better suited to driving a 16ohm load.

Using the amp in the USA was and is a different matter, however, and as Jim Elyea has pointed out, the 115V mains tap on the AC100 actually only really gives the desired running voltage if 110V is supplied. Perhaps in places in the 1960s, 110V was the actual voltage supplied by wall outlets. But any voltage above 110V causes the internal voltages of the amp to rise well above specification, thereby stressing both valves and filter capacitors.

Evidently Paul McCartney had trouble at the first concert at the Hollywood Bowl in 1965. In the clip below, one can hear John Lennon pointing out Mal Evans, who had come on stage at the close of "A Hard Day's Night" to sort the amp out once and for all - it had been playing up consistently throughout the earlier portions of the concert.

Click to play

But for all that has been said, the Vox AC100 is a great amp, and inherently no more unstable than its contemporary 100W Marshall brethren.

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