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~ "Concordant" QUAD IIs get restored back to Peter Walker circuit ~

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 schematic I found appeared to be different and none matched these amps ?

Inside one of the concordant amplifiers

Note both chokes have been removed and one has suffered from "internal melt-down" ~ At first I suspected the leads had shorted to the terminals but it looks like original square can TCC C4/C6 which had both sections wired in parallel 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 did not make sense because with the direct coupling from the EF86 V1 the values R5 and R6 should be identical ~ I can only think this was a mistake made during repair ~ Other concordants are similar(ish) to my 1980s QUAD II triode mod but with a valve HT regulator ~ So what is a concordant QUAD II ?

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 higher than the even harmonics despite the drive imbalance due to the different values for R5 and R6 ~ Rather than being "concordant" this pair were very "Dissonant"

It was decided to fit modern input and output connectors but leave the Jones plugs in service to allow connection to a QUAD 22 that I was also restoring and fitting with the QUAD 22 RIAA PCB and a new back panel

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 to mount the capacitor in the V5 position were filled with solder and smoothed flat ~ All the parts were etch primed and painted in a colour close to original but a little darker

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 tufnol back plate to mount the new speaker terminals

The concordant triode input and driver modification had badly damaged both tag boards

Some tags had already been broken and soldered back to look okay and others must have been bent back and forth to remove and replace components as they broke off imediately when touched with a soldering iron

This time the old tags were removed and replaced with ceramic post silver plated tags removed from a new old stock tag strip ~ The finished article looks good and was much easier to work with than the original

The tags fail close to the board where they were formed during manufacture and they break due to movement as they are un-soldered ~ Using a solder sucker to clear the tags can "kick" them loose so I fit a silicone rubber sleeve over the sucker end to reduce the kick and I cut the old component leads

Tags can be soldered back in place but this is awkward as component and tag have to be held in place while soldering ~ The 180kΩ resistor pictured right has had its long lead shaped to look like a tag and has been soldered directly to the base of the broken tag

The picture below shows 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 once did
As with many of my 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 to "round off" the peak in frequency response above 20kHz

The yellow twisted wires were not sufficient to make the response completely flat this time and additional capacitors were fitted across R5 and R6 and 4.7kΩ "grid stoppers" were fitted on the valve bases which further assists stability with some valves like the 5881 which may suffer parasitic oscillation bursts on low frequency cycles

The IEC input socket earth is connected direct to the chassis on a solder tag fitted under the mains transformer 2BA bolt ~ The 0V ground of the circuit is connected to chassis via a 10Ω 3W resistor and parallel 47nF capacitor mounted between E on the transformer and the chassis solder tag

This helps prevent earth hum loops while ensuring that the metalwork of the amplifier is safe to handle ~ Because of this arrangement the capacitance to chassis of the original C2 C3 if fitted would be less than desirable and any required C to ground now needs to be connected direct to the circuit ground ~ either across R5 and R6 or to 0V wiring but not to the chassis

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 required leaving the area of the original block capacitor free for the large IEC voltage selector ~ Direct short wiring of the valve heaters and greater isolation between the mains and signal wiring gave the amplifiers a hum level better than 100dB below 15W whereas original QUAD11s rarely achieve more than -85dB
The circuit is essentially original ~ the only difference apart from larger value C4 and C6 is the phono input which is capacitor coupled ~ The output power was over 20W into 15Ω before clipping but this was now due to the larger better C6 not a higher HT supply

The distortion and noise were well within the original 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

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