[www.keith-snook.info]

~ QUAD II Buying ~

If you are thinking of buying a QUAD II ~ or 2 ~ be patient ~ take your time ~ there are still shed loads (and lofts full) of them out there but there is also a lot of junk and 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 90,000 'domestic' versions ~ and the last of these made around 1971 were little different from the first 1953 "mono" units some of which had metal nameplates ~ Full production "mono" units and most "stereo" domestic units had back engraved and paint filled perspex nameplates

Metal 'black and silver' name plates may indicate 'commercial' models like 100V line PA amps where the customers serial was stamped on the plates at the time of manufacture ~ I would avoid these unless you need them for parts or you intend to replace the output transformers ~ There were also some 'domestic' models with these name plates usually serial numbers >90,000 and these can be a good find

Perspex (Clear plastic) nameplates used on the majority of domestic QUAD IIs were engraved with serial numbers then back filled with white paint ~ Because they were made in batches and were wrapped to protect them before use a manufacturing run could have widely varying serial numbers as nameplates were pulled at random from the parts bins

The mains voltage selector annotation changed for the "STEREO" versions with the introduction of the QC22 stereo controller or pre-amp in 1959


QUAD Hi-Fi amplifier mains voltage selectors


The unit above on the left is an "early" model originally supplied as a MONO amplifier up to serial number ≈35,000 ~ That on the right is one of a STEREO pair

Note the shroud around the fuse holder in the right hand picture ~ The tops of the early fuse holders shown on the left would often get knocked off exposing dangerous mains voltage but neither of these type could be considered safe nowadays


QUAD-II-HiFi-Mono-Stereo


The amplifier 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 would say 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 the older ones ~ probably due to oil from the tar soaking into the paper insulation and core material changes ~ The need to get two units with close serial numbers is therefore justified for stereo operation even if they are both "MONO" types but with so many repaired units this may not be easy


~ The "Odd Couple" ~


QUAD II amplifiers 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 the original output transformers

The perspex name plates were made in batches and wrapped in paper to protect them in the storage bins so serial numbers of units made on the same day could vary as the name plates were picked at random from stores ~ I know many people who have pairs of QUAD IIs purchased from QUAD with serial numbers several thousand apart

The production period was almost 20 years and the "early models" are now over 60 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


Underside of the QUAD Hi Fi Amplifiers
Looking inside the "Odd Couple" it is clear that C2 and C3 have been changed in both amplifiers

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


The capacitance to ground is intended to reduce the high frequency open loop response of the driver stage so that frequencies outside the output transformer response are reduced ~ The design of the output stage and the way the negative feedback is applied make the QUAD II design more stable than other valve amplifiers and the reduction in hf response helps keep TID low

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 ~

Component changes like those shown above can be easily corrected or even put back to original state if you can get the parts ~ Where components have been changed look out for broken tags on the tag board ~ A common fault where the valves do not glow at all is often the mains voltage selector with its common rivet burnt or wires not soldered

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 may not have worked if required

QUAD Hi-Fi Amplifier mains selector
QUAD-Hi-Fi-Original
The picture left shows how an original QUAD II should look inside ~ You can expect a small amount of tar around the transformer bases

Note the black/silver transfer label on the choke beneath R12 ~ On early models the wound components had these labels but on later units the bases were embossed with white paint ~ Most of the original QUAD II 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 grey potting compound but you can only check for this when they are removed from the chassis

Transformers can be easily swapped as can the nameplates which makes identifying the true age of a QUAD II difficult ~ One subtle feature of model serial <20,000 is that the chassis had 2BA clearance holes (5.4mm) for the 4BA screws securing the output transformer (OPT) so these bolts had large washers to close the gap as seen above

The chokes of models serial <20,000 were fitted with "dome head" 4BA screws and large washers although the holes were 4BA clearance ~ "later" models had 4BA (3.8mm) holes for the OPT and used "cheese head" screws ~ "later" stereo models used no washers on the transformers

The early wound component cans had obvious spot weld marks at the joins whereas later models had a smooth look ~ Where transformers have been re-sprayed this may not be so clear ~ difficult to cover with paint and difficult to check without removing ~ is the thickness of "early" cans which was about 1mm ~ the later ones were about 0.9mm

The baseplates of serial numbers <8,000 had 2 hexagonal steel 2BA bushes and the KT66 sub chassis had 4 hexagonal steel 4BA bushes whereas later models used round brass bushes

"Stereo" versions of the QUAD II appeared around serial 35,000 and were fitted with red and black output terminals ~ prior to this at about serial >30,000 Resistors R10 Rll were changed from ±10% tolerance to ±5% high stability types to give better gain consistency ~ These resistors with an additional pink band marking are shorter than the others as seen in the pictures above

The only sure way to check an OPT is to measure it in a good amp ~ checking low frequency output clipping does not cause parasitic oscillation ~ Distortion at 10W mid band between 500~1500Hz should be better than 0.1% ~ Distortion at 20kHz 10W is less than 2% ~ The first series resonant peak due to leakage L and primary C is greater than 60kHz

Whatever you buy ~ provided it is in reasonable good condition ~ it should out perform the QUAD classic models made by IAG in the name of QUAD and many other "new" valve amplifiers where the output transformers are made at a price ~ The OPT is the main "character" affecting component of a (non faulty) valve amplifier followed by the valves

~ Valves ~

If you are buying a pair of QUAD IIs you will most likely expect the valves to come with it ~ The valves are a most important part of the amplifier but if you don't have a valve tester how can you tell if they are in reasonable condition ?

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 okay" they will change as they heat up and will certainly not last for many more years

QUAD-HiFi-QUAD II-Under
QUAD-Hf-Fi-Amplifier-boxed
A good original example like those shown above may even come with the original packaging and handbooks

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


QUAD Hi Fi Valve data Sheets QUAD Hi-Fi Buy Beer Button Valve Audio Articles Valves for Sale Component Colour Codes

"Son you're playing with fire ~ the kid will live and learn as he watches his bridges burn from the point of no return"