— QUAD II Amplifiers Modifying and Modernising —

Some battered old QUAD II valve amplifiers get a cosmetic and electrical make over — A pair in black are oversprayed with silver to give a grey similar to the original colour but with a light textured finish — A pair of Concordant modified QUAD IIs get restored back and a pair are finished in ALL black including both new output terminal posts
"Concordant" QUAD IIs get restored back to QUAD II In 2007 I was asked if I could "look at" a pair of QUAD IIs — When they arrive it turns out they have double triodes in place of the EF86s and capacitors where the GZ32s should be — Inside a label proudly states "concordant" and also a "repaired by . . ." label but I could not find much more information on them and every "concordant" QUAD I found appeared to be different ?
			Inside one of the concordant amplifiers Note both chokes have been removed and one has suffered from "internal meltdown" — At first I suspected the leads had shorted to the terminals [see below] but it looks like original square can TCC C4/C6 which had both sections paralleled had been arcing and the short circuit current had damaged the choke Due to the solid state rectifiers the HT was in excess of 470V at switch on until the valves warmed up and this — I suspect — had gradually damaged C4 and C6
Apart from the HT fault on both amplifiers they had been very badly wired and the paint had funny wavy lines so I agreed with the owner a complete strip down would be done and the Amplifiers resprayed in the original colour and returned to the QUAD design But first I wanted to repair both amplifiers and test them to see if they had anything more to offer than a standard QUAD II — I also sketched out the schematic but the layout around R5 and R6 does not make sense — it appears to be incorrectly copied from the LEAK topology — Other concordants have a valve regulator so what is a concordant ?
Testing with new HT capacitors and good valves showed the amplifiers produced just over 20W into 15Ω — The distortion at 15W was worse than any standard QUAD II I had ever tested but more notably the odd harmonics were much higher than the even harmonics so rather than being "concordant" this pair would have been "Dissonant" It was decided to fit modern input and output connectors but also leave the Jones plugs in service to allow connection to a QUAD 22 that I was also restoring and fitting the QUAD 22 RIAA PCB
			After stripping the paint from the chassis and Transformers the cause of the wavy lines was seen — The metal had started to rust as if little worms had burrowed under the paint Note the old paint looks green — This is the etching primer used which is normally very good but this time it had clearly not done it's job The metalwork was rubbed down and the extra holes made to mount the capacitor in the V5 position were filled with solder and smoothed flat — All the parts were primed and painted in a colour close to original
The finished chassis and transformers look good in metallic base coat without the clear top coat and will change colour over the years as the metallic fleck oxidises just like the original paint did If you look at the top left there is a green illuminated switch in the fuse position — The IEC inputs I fitted had 2 fuses and a voltage selector so something to fill the 19mm fuse holder hole was required
			On reflection the green illuminated switch did not look good and it was not needed to indicate that the amplifier was powered so a more subtle switch was fitted Note the voltage selector is in the IEC fused input — the selector had 4 positions for 2 primary windings so had to be modified to work with 3 taps of 240V 220V and 200V
At the input end the Jones socket is kept and an additional phono socket has been fitted in one of the old speaker terminal holes The other holes were filled with epoxy resin which was also used to fix the tufnell back plate to mount the new speaker terminals
			The concordant triode input and driver modification had badly damaged both tag boards — Some tags were already broken and others which must have been bent back and forth to remove and replace components broke off when touched with a soldering iron All the old tags were removed and replaced with ceramic posts with silver plated tags removed from a new old stock tag strip I had— the finished article looks nice and was much easier to work with than the original tags Because tags often break I decided to make a replacement PCB with copper tracks and turret tags to replace broken QUAD II tag boards and also a PCB to simplify the triode driver mod below
The modified tag board fitted and wired — Note C2 and C3 are also mounted on the tag board at the ends of R5—R7 and R6—R9 these are 0.1µF 400V polyester and clearly do not provide the same capacitance to ground that the metal bodied C2 and C3 of an original QUADII would

As with many of my other rebuilds or mods using poly caps for C2 and C3 the wires from the tag board to V3 and V4 grids are twisted to provide some balanced C giving the pole required for closed loop stability — The yellow twisted wires were not sufficient to make the amplifier response flat above 20kHz and additional capacitors were fitted across R5 and R6 — 4.7kΩ "grid stopper" resistors were fitted on the valve bases to further assists stability with some valves like the 5881 The IEC input earth connection is direct to chassis on a tag fitted under a mains transformer bolt and the 0V of the circuit is connected to chassis via the 10Ω resistor and parallel capacitor mounted from E of the transformer — This helps prevent earth hum loops while ensuring that the metalwork of the amplifier is safe to handle

Capacitors C4 and C6 are low ESR 68µF 450V SMPS types which work well in valve amps — Using 2 separate small capacitors allows them to be connected directly where they need to be — Direct short wiring of the valve heaters and greater isolation of the mains wiring gave the amplifiers a hum level better than -100dB referred to 15W whereas original QUAD11s rarely achieve more than -85dB
The circuit is essentially the original design — the only difference apart from larger value C4 and C6 is that the phono input is capacitor coupled — The output power was over 20W into 15Ω but this was now due to the larger better C6 not a much higher HT supply The distortion and noise were within spec and more importantly the harmonic distribution was once again "Concordant" even if the layout wasn't On the right — Looking good but more important sounding as they should
The "Grey" Pair ( 2000 ) updated 2009 Download a pdf of the original QUAD II schematic
			The input end of the the "Grey" QUAD IIs — An insulated tufnell plate about 3mm thick was cut to fit inside the chassis and this is held in place with M4 black button head hex bolts — The concordant restoration above had the tufnell plate glued with epoxy for a clean finish The holes for the original output connectors were filled with solder and rubbed flat — All external bolts including the KT66 sub chassis have been changed to metric hex button head M4 or M3
The plate holding the KT66s was re-taped to M4 which works for 4BA if the depth of thread is enough — The transformers were refitted with the original BA bolts and the input and and output connectors only are at this end — The mains connector is now fitted the other end in place of the voltage selector
	An alternative way to mount output sockets using the mains socket I did this for some around 1980-83 but having spare mains sockets proved useful so the tufnell plate above has been used since — All you have to do is remove the two plug posts and drill the holes to just fit the new 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 other end of the grey QUAD11 fitted with a new safer IEC-6A mans socket in place of the voltage selector 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 and careful positioning of all the parts so that the finish is uniform across both amplifiers
		New 60µF + 60µF PSU capacitor which is isolated from the chassis by insulated "terry" clips 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 capacitor which can be seen bottom right below the fuse
The only changes from the original design for this pair was going to be bigger PSU capacitors but when I powered up the first grey amp PSU stage I found the previously OK choke was now open circuit ! — Investigation inside showed that where I had unsoldered and cleaned the tags of the choke the plastic insulation on the wire inside had melted and touched one of the studs that mount R12 and as this was wired to ground the choke fused 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
				This neat X-ray of a QUAD II (found on the Web so please claim the credit if it's yours) shows the core of the choke "up" where it should be.
Luckily I had a spare good choke and also a damaged one which I had previously unwound to find the cause of failure but having spent hours trying to match the paint effect on all the parts I did not fancy doing it again for one choke so I decided to see if I could remove the core without damaging the paint 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.
All very interesting! — but should I remove the good core from my spare choke and put it in the painted case or get the two empty bobbins rewound and make two new chokes ? The picture right shows what I decided to do — I contacted a number of people including Mike Davis at Woodside Electronics (no longer trading) as he had made 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 which is close to the original when assembled with a 0.17mm shim (white card in picture)
	Note that the E & I pieces are NOT interleaved and a shim is required to prevent the d.c. current from saturating the core and lowering the inductance — A shim is also used in single ended output transformers to prevent them saturating whereas in push-pull transformers the magnetism due to the d.c. current is cancelled so the E & I pieces are generally interleaved for better magnetic coupling — The choke cases were cleaned with white spirit and the assembled cores were clamped up tight , taped then potted in electrolube polyurethane resin — The good core of the choke from the other grey amp was "melted" back into the remaining old case leaving me with two original spares and two new chokes.
The 2 new choke cores potted in polyurethane resin which is a very good electrical and mechanical sound insulator and unlike the original Tar should remain in place over the years even as the heat rises 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
			A new choke fitted and wired directly to the smoothing capacitor — using the long PTFE leads — leaving the 4 studs free to mount individual 390Ω bias resistors each bypassed with a 10µF polypropylene capacitor Because the R12/C5 networks are no longer common to both output valve cathodes they now have an affect on the low frequency response
The 10µF capacitors across 390Ω resistors give the output valves an open loop high pass response about -3dB @ 41Hz which with overall feedback (see below) becomes about -2dB @ 20Hz which the output transformer can handle reasonably well When one common resistor is used for both KT66 valves like R12 in the QUAD11 and the "push—pull" drive to their grids is 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 — If there is any out of balance signal at low frequencies then you need new valves or the amplifier repaired not a 1000µF C5 ! C5 need only be about 22µF — If it is any higher it will allow low frequency current to saturate the output transformer — Making C5 higher than 47µF as some other QUADII modification sites advocate could be done but the low frequency response at the input should be tailored to fall below 30Hz — You cannot get more out of the little SPEC 1003A transformer than it is designed for If I had not used separate bias resistors and capacitors for each KT66 for this mod 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 are 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 R12"A" and R12"B" 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 — 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 — I fitted an input capacitor of 0.68µF with R1 100k to stop d.c. and give a time constant similar to the grid coupling of the KT66s
		An original tag board from a QUAD 11 power amp It would have been nice to use 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 to listen to clear music and not the inadequacies of aged and failing electronic components
A QUAD II tag strip with the QUAD component values fitted but using modern 0.75W high voltage resistors and a 160V extended foil polystyrene capacitor for C1 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
A nice finishing touch is to remove the paint from the logo boilerplate using hot soapy water and a tooth brush 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 — See some examples

On the Left a nameplate as fitted by QUAD Hi-Fi and on the Right cleaned of its paint and catching the light
The intention was to build this pair to the original schematic — But the failure of the choke led to fitting the separate KT66 bias resistors which was a good move — Other changes or additions are outlined in summary around the picture below

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 required for loop control 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 — The capacitor so formed 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 the loose wiring tidy 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 is 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 and half of the output winding for the 8Ω tap thus the feedback resistor does not need to be changed when the impedance taps 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 the output winding! I wanted the output taps set to 8Ω so linked S-T and Q-R but provided overall feedback by connecting R11 directly to the output terminal — I also wanted the sensitivity to be increased by applying less feedback which would help maintain stability — 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 would be a reduced output impedance or greater damping factor if you prefer but this has not been seen or heard to be a problem Distortion with the feedback changed as above was actually no worse than with the normal feedback arrangement and incidently in both cases the second harmonic is higher than the third etc. and the THD was about 0.2% at 20W — The harmonic distribution is unlike a normal push pull output stage where the even harmonics are usually much lower than the odd due to cancellation — It appears that the QUAD 11 characteristic could be more like a Single Ended Triode amplifier than a push-pull and this may be it's appeal to those who listen to music rather than read Hi-Fi magazines I can't "show" you how they sound but you can see what they look like in service here

The "Black Pair" Sounds a bit dramatic but the Black pair are two amplifiers finished in black Hammerite paint that I have owned for many years now — They are the chassis that I used to make my early triode driver mods which featured on the first 1992 d.c.~daylight website — Ade Clarke of QUAD World saw them when I tested some output transformers for him but following discussion about the pros and cons of triode mods he chose to re-build his amplifiers using the QUAD EF86 driver stage Over the years there have been several attempts at triode drivers for the QUAD KT66 output stage and since the early 1980s I have tried a few my self — In every case there were problems that are already solved by the QUAD EF86 driver so there has to be a very good reason to change to a triode input / driver The design changes will often require 2 double triodes — If you elect to use ECCnn the heater current will increase but not actually double — EF86 has 200mA heater current @ 6.3 and ECC81 or ECC83 300mA — ECC88 and some others require over 350mA per valve — Some mods like fitting a 6080 regulator overload the mains transformer before connecting a 22 control unit or tuners To get enough gain to provide the correct voltage swing at the KT66 input grids often requires a 3 stage amplifier which will be less stable than a 2 stage with feedback applied across all stages like the Concordant above or other "designs" simply copied from the LEAK series or application data sheets without a though for what is required

"While the music played you worked by candle light"