If you are thinking of buying a QUAD II — or Two — Be patient — take your time — there are still shed loads of them out there but there is also a lot of junk and many badly repaired or modified units — Try to view as many as possible to get a feel for what looks right

There were thousands of QUAD IIs made — Quad state over 90,000 — and the last units made around 1971 were little different from the early 1953 "mono" units some of which had metal nameplates — All later "mono" units and all "stereo" units had back engraved and paint filled perspex nameplates

The mains voltage selector annotation changed slightly with the advent of the "stereo" versions in 1958 although I have been told the transformers were the same specification throughout production ?

The unit above left is an early type originally supplied as a MONO amplifier — That on the right is one of a later STEREO pair — Note the shroud around the fuse holder — The tops of the early fuse holders shown on the left would often get knocked off exposing dangerous mains voltage but neither type can be considered safe nowadays — The voltage selector can also be dangerous and often fails due to its riveted construction this is why one of my modifications is to replace it although I don't tend to change the later style fuse holders if they are OK.

Once again above left would have been supplied as a mono amplifier — Note the speaker outputs are not colour coded although the phase of the output was maintained with the lower socket being connected to chassis ground — QUAD Hi-Fi service department will tell you that the specification of the output transformer has always been the same although earlier units are marked "SPEC 1003" and later ones "SPEC 1003A"

Transformers can show variation with age with later units both testing and sounding better (or different if you prefer) than earlier ones — The need to get two units with close serial numbers is therefore justified for stereo operation of a pair of QUAD 11 even if they are both "mono" types

— Beware of the "Odd Couple" —

The QUAD IIs were made in their thousands and the build changed very little over the years — The most significant thing to affect the stereo performance — apart from a fault — will be the output transformer's age so try to get two units from about the same period i.e. with close serial numbers and original output transformers

It is likely that the perspex name plates were made in batches and wrapped in paper to protect them in the storage bins and serial numbers of units made on the day would vary greatly as the name plates were picked at random from stores — I know many people who have pairs of QUAD IIs purchased directly from QUAD with serial numbers several thousand apart

The production period was almost 20 years and the early units are now over 50 years old giving the possibility of a lot of mismatched partners — Pictured below are the undersides of the mono and stereo units shown above — They were sold as a "pair" although clearly from different generations with widely spaced serial numbers

The yellow capacitors are a good quality metalised polypropylene type which are ideal for audio except in this case they may cause a peak in the treble due to the fact that the metal canned paper/oil types that they replaced provided a capacitance to ground

Above — R12 has also been changed in both amplifiers probably due to it failing and although the two resistors are odd styles they look like they are rated higher than the original 3W types which is good as they actually dissipate more than 3W under the designed conditions

The type of capacitor fitted for C5 in the lower amplifier is not good it is far too small to take the heat and a.c. component of the output stage current and will most likely fail — Also too small are R5 and R6 not just for the power but their voltage rating cannot be correct for this position — C5 in the top amplifier is a good size but could benefit from the clamp that is fitted on the bottom unit doing nothing

— What to look out for —

The picture right shows a mains selector with both faults — Note the two left hand tags although not used in this case were dry joint and would not have worked if required

Note the transfer label on the choke beneath R12 — On early models the wound components had these labels like "SPEC No. 1002" but on later units the bases were embossed with white paint — Most of the original QUAD 11 wound components were filled with tar which tends to leak a little over the years — Some of the last units made and most replacement parts were filled with a light grey potting compound but you can only check for this when they are removed from the chassis

Look at the KT66 output valves in low light — There should be no red glow on the outside of the anode plates and no blue glow inside the anode — If only one anode has an obvious hot spot it could be a low emission valve taking more current than the other due to the common cathode biasing or it may have a leaky coupling capacitor — If the valves are swapped and the same anode glows suspect the valve — If the same position has the fault then it is most likely a capacitor fault (C2 or C3)

A blue glow seen through the rectangular windows in the anode or looking down on the valve is a sign of gas in the tube — Other signs of output valve ageing (best seen on clear glass KT66s) are black/silver marks on the glass opposite the rectangular windows in the anode and transparency of the getter — The getter is the silver metal film fired down from the two rings at the base of the tube — it turns powdery grey starting at the edges as it "eats up" any gas in the tube — On a good valve the getter should be bright silver and not be transparent

A design problem with the KT66 was the glass pinch seal where the connections leave the tube to connect to the base you can often see black deposits around the wires in the pinch, this is not a problem as such but has been known to cause leakage between the elements specifically cathode/heater which is not easy to measure without a tester. On the physical side check that the glass tube and base are secured together and that each KT66 base has a locating pin

The EF86s show no external signs of wear but can be microphonic or cause hum, listen carefully for hum with no input in SK1 and tap the EF86s carefully with a pencil or similar light but hard material to check for microphony

If you feel comfortable doing so and the seller allows it you should measure the voltages as shown on the circuit diagram

Nowadays eBay appears to dictate the price of classic items like the QUAD II and many people are buying junk without viewing or testing the amplifiers and often without asking questions about things that look suspicious like odd looking "pairs"

Occasionally eBay has some very clean original items like those shown opposite but they will be very expensive and the components will be over 40 years old and although they may "test OK" they will change as they heat up and will certainly not last for many more years

It is likely that these have been well cared for — Or if your really lucky you have found an unused QUAD II or better a pair of QUAD11 hidden away in the original box for years