Quad 2 Concordant

[onewatt]

Hi Folks,
Can anyone throw any light on the concordant modifications carried out on the quads?
I have a couple that have been got at and I'd like to return them to the best version of quad 2/KT66, but before I do I'd like a better understanding of what and why?
Is there a concordant ct for comparison?
Mine have 6L6's fitted and ss rectifiers.
Reduced loading on the tx? and I'm told a modified input for the pre amp.
The controller has a reduced valve count by 2 as well.

KR
Chris

[Ted Kendall]

Keith Snook goes into this in some detail on his website. The best thing, as you have already surmised, is to put them back to standard...

[ahankinson]

... and get rid of those old Hunts coupling caps.

[GrimJosef]

Concordant was a one-man band, the man being the late Doug Dunlop. Doug was an experimenter, so while Concordant Quad IIs tend to have some features in common they're by no means all identical.

One of the claims made for them is that they will deliver more output power than the stock Quad IIs and the use of solid-state rectifiers, which give higher HT voltage, along with changed output stage biasing circuitry means that this is true. My pair will deliver 25 W/ch if I remember rightly.

However in mine the HT is passed through a 6550 in what was the GZ32 socket, so the heater loading on the mains transformer isn't much reduced and the overall loading is actually increased by the increased current drawn from the HT winding. I once ran them as a display pair at a show, operating with the music playing quietly, and after an hour or two the amps' metalwork was way too hot to touch ! This is not a great place to be, particularly given that the HT winding is the most fragile of all of the mains TX ones.

That said, with 6L6s and no 6550 it may be that the current drawn in yours is less than with the EL34s and 6550 in mine. I note that your output stage cathode resistors look quite weedy, so perhaps the output stage is running fairly gently.

I'd agree that going back to the stock circuit is the best thing to do, and also that the Hunts inter-stage couplers age very badly, becoming dangerously prone to leakage. This can easily see off output valves (bad) or mains and/or output transformers (disastrously expensive, and any replacement won't be a colour match). Renew them a.s.a.p.

Cheers,

GJ

[Craig Sawyers]

Totally agree that the Hunts paper capacitors of death should be instantly replaced. They will be leaky, pretty much guaranteed.

If you want it to look original, hollow out the innards of the Hunts caps, and hide a polypropylene axial capacitor inside.

Craig

[Nickthedentist]

...unless those silver Hunts have been re-stuffed?

[ahankinson]

That would take a special kind of someone to go to the effort to restuff old caps to preserve the look, while simultaneously re-doing the whole circuit around it!

[wd40addict]

I remember a review of these, halfway through the reviewing one of them failed! Back in the day (mid 80s?) when KT66s became hard to obtain various mods were touted. One of the Electronics magazines even published an article about how to convert them to use EL34s!

What all these mods had in common was a desire to push the power output up from the original's 15W and I'm not sure whether much thought was given to the poor transformers. To put it in context: at the time these amplifiers would have been regarded as a bit old hat and a good candidate for experiments and the valve revival was yet to happen.

Now KT66s are available again (even if not quite as good as the original GECs) it may well be better to return the amp to stock.

[onewatt]

Thanks All,
Yes, my plan is to replace all the Hunts caps and all resistors.

Its been pointed out to me that the caps used at the time of manufacture were most likely chosen based on cost rather than audio quality, so I plan to use the best replacements available to me and my pocket without going ott, and which are most likely better than the original parts fitted 30 odd years ago.
My ears are working class!
I’m not going to re-stuff in this instance although I can understand why some peeps like to do it.

Keith’s page shows the con mod with KT66’s rather than 6L6 but I just want to get a handle on what’s been changed/altered so just going through the original Q2 circuit against what I actually have.

Question regarding pre-amp/controller.
Mine appears to have 2 valves missing and Keith’s page shows a board with the pick up mod fitted where I have gaps! Am I missing 2 valves in mine and is the pick up pcb available?

KR
Chris

[GrimJosef]

Originally Posted by onewatt

... Question regarding pre-amp/controller.
Mine appears to have 2 valves missing ...

Sorry if this is too obvious, but is your controller the QC II or the Quad 22 ? They look similar from the outside (except the QC II has round buttons on the front whereas the 22 has rectangular ones) but the QC II is mono whereas the 22 is stereo. If you have a QC II that would explain the two 'missing' valves.

Cheers,

GJ

[Nickthedentist]

Originally Posted by ahankinson

That would take a special kind of someone to go to the effort to restuff old caps to preserve the look, while simultaneously re-doing the whole circuit around it!

Good point!

[Radio Wrangler]

But on the other hand there are some very special people around. I wouldn't rule anything out on the grounds of 'no-one would be daft enough to...'

David

[GrimJosef]

Originally Posted by GrimJosef

... I note that your output stage cathode resistors look quite weedy, so perhaps the output stage is running fairly gently ...

Looking more carefully I see that the centre tap of the output transformer secondary winding which feeds the 6L6s' cathodes has actually been grounded, and the weedy resistor and large electrolytic which are positioned where the cathode bias resistor/electrolytic used to be are actually there to make a fixed negative bias supply for the 6L6s' grids. So they may not be running gently after all.

Cheers,

GJ

[onewatt]

Hi GJ,
I have no problem with obvious at all.
It’s got Square knobs on the pb’s, and mono/stereo labelled on l/h pair so I am assuming it to be quad 22 I didn’t know whether this was another mod tied in with the conc mod.
The amps no longer have ef86’s fitted and appear to have a double triode nearest the op valve.
The pre amp has been prepared for 2 more valves, but perhaps an option when new? although Istr seeing a similar chassis somewhere when surfing. Will do some more!
Kind Thanks
Chris

[onewatt]

Duly noted. I’ll do some more work on it tonightwhen I get home from work. I did notice there’s a resistor that’s changing colour to a nice even brown! So either underrated or the hunts are doing what there good at!
Chris

[snowman_al]

Originally Posted by wd40addict

I remember a review of these, halfway through the reviewing one of them failed! Back in the day (mid 80s?) when KT66s became hard to obtain various mods were touted. One of the Electronics magazines even published an article about how to convert them to use EL34s!

What all these mods had in common was a desire to push the power output up from the original's 15W and I'm not sure whether much thought was given to the poor transformers. To put it in context: at the time these amplifiers would have been regarded as a bit old hat and a good candidate for experiments and the valve revival was yet to happen.

Now KT66s are available again (even if not quite as good as the original GECs) it may well be better to return the amp to stock.

There was such an article in 'Everyday and Practical Electronics', dated Feb 1994. Rather sensible for the time, keeping the Quad circuit as is, apart from splitting the cathode resistor into two.

[GrimJosef]

In the Feb '94 E&PE article the splitting of the output stage cathode resistor is suggested as an alternative to leaving it as it originally was. The author (Jake Rothman) says the split resistor option "is useful to allow tweaking in cases of severe valve mis-matching". Of course the big difference in his piece is the switch from KT66s to EL34s (remembering to connect the EL34 suppressor grids to the cathodes, and adding 1k screen grid stoppers too). I've never tried just a straight KT66/EL34 swap like this. Given how carefully tweaked the original output transformer was I'd be surprised if the amp would work quite as well with such a big change - pentode for beam tetrode and twice the gm - but I don't know for certain that it doesn't.

Quite by coincidence, I'm sure, in the very same month Haden Boardman also wrote a short piece on "Reviving the Quad 22/II Valve Amplifier" for the supplement to Hi-Fi World. He rails against people modifying the circuit

Needless to say you must avoid 'modified' QIIs like the plague. A handful of companies in the past have persuaded people to fit voltage doubler power supplies, EL34 output pentodes, anything they can do to boost the power output. I'm afraid it's all a load of cobblers. Watch these vandals. Mr Walker knows best. QUAD IIs were designed for 15 watts output - not fifty.

But he then proceeds to recommend splitting the KT66 cathode resistor too . It seems the temptation to 'put PJW right' is very hard to resist.

Cheers,

GJ

[Leon Crampin]

MOV specifically recommend separate cathode resistors for push-pull KT66s. As in other cases (Quad 405 op amp feed resistors for example) the circuit design of these amplifiers is poor in some areas. Maybe Ohms Law never reached Huntington....

Leon.

[GrimJosef]

Originally Posted by Leon Crampin

... Maybe Ohms Law never reached Huntington....

On the contrary. In his obituary of Peter Walker https://www.stereophile.com/news/121...ker/index.html John Atkinson says

...Peter reiterated something that he first told me on the 1982 Huntingdon visit, and that has invariably proved true: that any problem in audio engineering can be successfully addressed with "an equal mix of Ohm's Law and common sense, hmmm?"

Just for clarity where, specifically, does the Quad II's design indicate an ignorance of Ohm's law ?

Cheers,

GJ

[Leon Crampin]

Cathode resistor dissipation.

Leon.