~ QUAD II Amplifiers Modifying and Modernising ~

The concordant restoration on this site had a similar tufnol plate which was glued with epoxy for a clean finish an also filled the holes

Here the holes for the original speaker output connectors were filled with solder and rubbed flat ~ All external bolts including the KT66 sub chassis were changed to metric M4 or M3 hex button head with black finish

Remove the two plug posts and drill the holes in the insulation to fit 4mm posts ~ If the new posts have large heads like those shown they have to be fitted after the frame has been re-fitted to the chassis

The grey finish was obtained by first spraying with black Hammerite and then silver grey Hammerite from a distance so that the paint 'dust' fell onto the black and merged to give a mid grey ~ It requires a bit of practice to get the finish uniform across both amplifiers

This is how a new valve amp project should start but so many don't ~ working from the PSU to the input is the way to go

The chassis is is now connected to the mains earth for safety ~ The 0V of the supply connects to chassis via a 10Ω resistor and a 47nF capacitor which can be seen bottom right below the fuse

When a QUAD II is fully loaded with QUAD 22 pre–amp and FM and AM tuners the heavy core of the choke ~ which is not mechanically mounted like the output and Mains transformers ~ tends to drift down over the years as the tar that holds it in place melts and the internal connecting wires then rest on the backs of the tags waiting for you to resolder them one day

It also shows that the square block capacitor houses a more traditional cylindrical can 16µF+16µF electrolytic capacitor and not a paper in oil type

The choke was placed in a small cardboard box and put in the oven at about 150°C the tar melted and I managed to pull the core without damaging the paint then I removed the "E & I" pieces and washed them in white spirit to separate them ~ When the core had cooled down I removed the wire to see what had happened inside ~ It was not until the very end that I found the connection to the lead out was blown and had made a small hole in the bobbin ~ The other damaged coil had been subject to long term overload and the wire was burnt and shorted in many places

The picture right shows what I decided to do ~ I contacted a number of people including Mike Davis at Woodside Electronics (no longer trading) because he had wound the excellent replacement output transformers that I tested for Ade Clarke of QUAD World but a "commercial decision" led me to have Sowter Transformers fill the bobbins with a slightly thicker wire than originally used ~ The final inductance was around 29H when assembled with a 0.17mm shim (white card in picture)

Note the lead out wires are PTFE supplied by me to Sowter and are long enough to wire anywhere inside the chassis if required ~ By not wiring to the tags on the base plate of the choke these are now free to allow mounting of separate auto bias resistors for each KT66

Because the R12/C5 networks are no longer common to both output valve cathodes they have a greater effect on the low frequency response and are now used to deliberately reduce the LF response of the output stage

When one common resistor is used for both KT66 valves like R12 in the original QUAD11 circuit and the "push~pull" drive to their grids is well balanced [as it should be] then hardly any a.c. voltage will appear across R12 and hence C5 actually passes very little a.c. current

C5 need only be about 22µF ~ If it is any higher it will allow any low frequency imbalance current to saturate the output transformer ~ Making C5 higher than 100µF as some other QUADII modification sites advocate could be done but the low frequency response at the input should be tailored to about –3dB @ 30Hz ~ You cannot get more out of the little SPEC 1003A transformer than it is designed for ~ You need a better transformer

If separate bias resistors and capacitors for each KT66 were not used then I would have tailored the input low frequency response by making R1 100kΩ and placing a series capacitor of 56nF between the input connector and R1 ~ These amplifiers were not going to be driven by a QUAD 22 control Unit so the 100kΩ input impedance is fine ~ The input capacitor should be a polystyrene type which will not upset even the most discerning golden ears ~ Especially if you don't tell them it's there

Because the separate R12a/C5a and R12b/C5b do affect the low frequency response of their respective valves the CR time constants should be close and in this respect polypropylene capacitors 10% or better are a good choice especially as modern ones will be much better than the rated 10% ~ Also separate bias resistors are far more forgiving of unbalanced valves and individual valve faults so your output transformer has more chance of a longer life

It would have been nice to use the old carbon resistors and paper capacitors but KT66 valves ~ Output transformers and even EF86s are getting harder to find and more expensive as time goes by so losing an output valve due to a coupling capacitor or resistor failure is not worthwhile

It is also better to listen to clear music and not the inadequacies of aged and failing electronic components

These components along with new valve bases and better capacitor types for C2 and C3 will ensure that these amplifiers last for many more years to come and that the phase response of the pair is more coherent which is very important for good stereo imaging

The QUAD perspex name plates are back engraved and then paint filled but they can look much better if the paint is removed and the natural reflections are allowed to show through ~ I have done this to almost all the QUAD IIs I have modified since 1980 and there are many with this feature in Hong Kong ~ Japan ~ Australia ~ Canada and a few in the UK

The coupling capacitors C2 and C3 are mounted on the KT66 bases and clearly do not have the capacitance to ground that the metal bodied originals provided ~ This capacitance is required to level the response peak above 30kHz and is provided by the twisted Orange and Violet wires that now form a small capacitor between the anodes of the EF86 valves V1 and V2

It may not look as neat as the original laced wiring but the capacitor formed by the twisted wires is subject to the push–pull voltage swing at the anodes of V1 and V2 and so has twice the effect that a single equivalent capacitor to ground from each anode would provide ~ It also helps keep some of the loose wiring tidy(ish) and neater than some of the new 'QUAD' builds from IAG

The larger value smoothing capacitor 60µF + 60µF is also bypassed with a 4.7µF 630V polypropylene capacitor across section C6 this is mounted in the pipe clip which in turn is mounted on a long bolt of the rectifier valve base ~ This was an expensive addition but does appear to make the stereo image sharper ~ At present there is no poly–cap across the EF86 supply as here the impedances are much higher compared to that of C4 ~ In future I may add a poly bypass capacitor across C4 but for now I'll listen a while as it is

The input is a.c. coupled and had I not fitted the separate KT66 bias resistors the time constant of the input C and R1 would have been chosen to make the response about –3dB @ 30—40Hz ~ see above ~ With 390Ω KT66 bias resistors with only 10µF across them the input time constant needs to be bigger ~ R1 was made 100kΩ and the input capacitor is 0.68µF giving a time constant similar to the C2 / C3 coupling –3dB @ 2.3Hz

The output is now subject to overall negative feedback ~ Many people have tried to copy the QUAD II output transformer including myself but all too often unless built very close to the original way and used in the original circuit the result is an unstable amplifier

Note that the negative feedback is normally taken from only a third of the output winding (R11 connected to P) for the 15Ω tap with link S–R and and from half of the output winding for the 8Ω tap with links Q–R and S–T thus the feedback resistor does not need to be changed when the impedance links are changed

This being a QUAD amplifier there is most likely another reason why it is done this way and it is probably to keep the amplifier stable with load ~ Especially if the load is an ESL57 or its predecessor ~ With only part of the winding in the feedback path a low impedance cannot completely shunt the feedback

This is the exact opposite of the QUAD I where the feedback was taken from the 15Ω tap and when the 7Ω tap was used the unloaded section of 15Ω still provided the feedback same as the LEAK TL series ~ There is also rumour of a small value resistor in series with part of the QUAD II output winding

I wanted the output taps set to 8Ω so linked S–T and Q–R but then provided overall feedback by connecting R11 directly to the output terminal ~ I also wanted the sensitivity to be increased by applying less feedback ~ With a 2k7 resistor for R11 connected to the output terminal the sensitivity was about 0.5V for 10W and with these amplifiers clipping at over 20W due to the higher HT voltage from the GZ34s and the 60µF smoothing cap this was fine

The main effect of the feedback change is a reduction in output impedance or greater damping factor if you prefer ~ When coupled with ESL57s or ESL63s there was a noticeable lack of Bass most likely due to the lower resistance drive ~ With the normal feedback the output is more of a current source and the apparent extended Bass could well support the argument that "the best amplifier to drive ESL57s is the QUAD II"

Distortion with less but overall feedback was actually no worse than with the normal feedback arrangement and in both cases the second harmonic is higher than the third etc. ~ The harmonic distribution is unlike a normal push pull output stage where the even harmonics are usually much lower than the odd due to transformer cancellation

The QUAD 11 distortion characteristic is more like a Single Ended amplifier than a push-pull design and this may be it's appeal to those who listen to music rather than read specifications ~ The prominent 2nd harmonics are due to an imbalance in the drive to the KT66 grids which could be reduced by varying R8 but this would have to be done for each specific pair of EF86s and would probably ruin the "warm". . . "musical" . . . sound of this amplifier !