The Mkll (or Quad II) was designed by Peter Walker and released in 1952 - its innovative design is still considered to be one of his best ever and almost 140,000 were made until production finally ceased in the late 1960s. The Quad II was small by comparison with many of its contemporaries and underneath the chassis the layout was extremely neat and tidy. (see picture, lower right).

The heart of the circuit was the output stage and transformer, the latter was wound in thirteen sections, tightly coupled and connected so that portions of the load were applied to cathode, screen and anode circuits. the KT66 output valves appeared as triodes but had less than half the non-linearity of conventional push-pull triode circuits. A self-balancing push-pull signal was developed in the EF86 push-pull stage which was RC coupled to the output stage. because of the extremely low intermodulation, 12dB of feedback was needed, although more could be applied if desired (up to 30dB).

Because the Quad II only had two stages, 1.4V was required to drive it to 12 watts output. Total maximum distortion was 0.25% (-52dB) with higher order distortion 0.05% (-66dB). Frequency response was 20 - 20,000 cps within 0.3dB

In a July 1998 issue of Hi-Fi World, Haden Boardman wrote that the Achilles Heel of the Quad II was its power transformer. The primary supply was run up to its limit when the Quad 22 control unit (which derived its power from the main amplifier) was attached and if the matching AM and FM tuners were also in use, power transformer failure was almost inevitable!

Originally priced in the UK at around £15, today these classic mono amplifiers can reach prices well in excess of £500 for a nice pair. On the other side of the Atlantic, a pair in excellent condition (both cosmetically and electrically) recently sold on internet auction for $USD 1850 and $2,200 was paid for a pair of power amps with matching 22 preamplifier and tuner, all in 100% original working condition.

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