Haden Boardman delves into the fascinating history of the Acoustical Manufacturing Company and their classic Quad II amplifier.

Of all vintage valve amplifiers, the Quad II has to be the most famous and the most written about. It became so important to its maker that it actually changed the company's name! The Acoustical Manufacturing Company had been set up before World War 2 to build high-quality public address equipment. Post-war, this slowly turned to hi-fi, Acoustical launching their first domestic amplifier, the Quality Unit - Amplifier Domestic (or Quad for short) in 1948.

But the real tale begins with the launch of the Mkll version in 1952. Here was an amplifier destined to remain in production for nearly 20 years, with almost 140,000 of the things made!

One thing that immediately strikes you about a Quad II compared to its early 1950s counterparts is its compact size. Everything is carefully engineered to fit within the confines of a tiny footprint of minimum volume. 

Inside the casework, the layout is very neat and tidy (something modern valve amplifier manufacturers should take note ofl. Under the two EF86 drive pentodes hides a small tag strip holding the majority of the signal components. The output valve coupling capacitors are located on the base of the output valves, and the shared cathode resistor on the base of the choke.

The power supply uses the aforementioned choke with a paper-in-oil capacitor block and a GZ32 rectifier (later a GZ34). Input is via a six-pin 'Jones' plug, and AC mains is fed in on a two-pin mains connector. At the other end of the case there are voltage adjustment taps for the primary of the mains transformer. This amp, in common with most vintage power amplifiers, has no power switch.

If you must modify the connectors and fit phono sockets, don't drill the chassis. Instead, remove the Jones plug carefully and fit a small piece of Veroboard in the hole and mount the phono socket on that. If you have the original matching pre-amp, then remove its plug and use that - it is a better audio connector than the phono socket. However, do be careful. 

The High and Low Tension power supplies for the pre-amp are also on the connector. The audio signal goes through the bottom pair of connectors, earth on the left, signal on the right.

The Quad lI's exterior is painted in a 'slate grey' finish and carries a little perspex name tag with the unit's serial number. When looking to buy a pair of Quad lls, try to get the closest matched serial numbers you can, and steer clear of all really early versions (unless you are a mad Quad collector!) made before serial number 8000 because they are near impossible to pair up.

The Quad lI's Achilles heel is its mains transformer. The primary is run right up to its limit when powering the associated pre-amps and tuners. The matching Quad 22 stereo control unit scavenges power from just one poor old Quad II to enable it to be totally mono compatible, and if you have both the AM and FM tuners connected, then mains transformer failure is liable to occur.

Another thing to watch out for are modded Quad lls. A few years ago, voltage doublers, valve regulators, EL34s and all kinds of other tweaks were thrust upon the amp. Unfortunately, they A) sound worse, and B) if they haven't already killed the mains transformer, they soon will! 

Another point to look out for are original KT66 valves. These are very thin on the ground, and a pair of Quad lls are always worth more when fitted with original GEC examples. Avoid like the plague any so-called direct Chinese replacements. GEC KT66s have a 0.9amp heater filament; some of the Chinese valves draw up to 1.8amps. This is guaranteed to fry the mains transformer. Fitting EL34s is a no-no for the same reason (1 .5amp heater), as is the KT88. If you cannot find original KT66s, then fit 6L6s or 5881s, but make sure they only consume 0.9amp of filament current.

Originality is everything with a set of Quad lls. If you have a pair which have never been touched, have all their original components, have never been damaged and still possess the original valves, then you can ask virtually what you want for them; certainly up to £500 would not be out of the question. If, on the other hand, they have been 'improved' with modern components, you will be lucky if a dealer offers you £100 for the set.

The Quad lI's circuit is quite interesting. The amplifier has only two stages, hence it requires 1.4volts of input signal to produce its 12watt output, and so must be used with some kind of pre-amplifier. A quick mod for a little more gain is to use a 470hm resistor instead of the 100ohm one located on the cathodes of both the EF86s. This results in a dramatic increase in sensitivity, with no real compromise in sound quality. You will still need to find 600millivolts (0.6volt), so a pre- amp is called for.

The output transformer is unusual in that it incorporates a cathode winding as well as the usual anode windings. This applies an 'out of phase' signal to the transformer and helps it produce triode-style impedance from two tetrodes, with the advantage of only needing the same drive voltage. The disadvantage is that the output valves are running quite a bit harder than the 12watt power rating would suggest.

If you were being cruel to a Quad II, you could point out that a Leak Stereo 20 has a larger output transformer, and to be honest, the basic 'guts' of the Leak are way better. But thanks to some clever trick with resistors within the transformer case, the Quad II gets away with

Sound-wise, the Quad is one of the finest amps from its period. It is clean, sweet and very load tolerant (the ideal partner for the Quad Electrostatic Loudspeaker). The original pre-amp works fairly well with CD sources and the tone controls are de-activated when the filter switch is in the 'Cancel' position. The phono stage though has too much hiss and hum for modern-day use, and to be honest, compared to a Leak Troughline, the Acoustical FM tuner is a bit of a disappointment.

It is very hard to describe the Quad sound - it is what the amp doesn't do that is more noticeable. When all original, it has a magic, open sound that very few amplifiers can compete with. On the flip side, the bass is a bit warm, and when driven into overload, it can bounce about all over the place. 

But then this is classic valve-amp behaviour. Because of this soft, bouncy limit, stick with efficient loudspeakers - Klipsch and Lowthers are ideal matches (Quad lls with Klipsch La Scalas are awesome!), and don't forget a good pre-amp.