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Old 17th Jan 2008, 12:10 am   #9
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Quad 2 Valve power amp questions

If everything else in the amp is working OK then your transformer should be happy too. The trouble is it's one of the weakest links, so if some kind of fault develops then it can be the first thing to go. And replacing it can be eye-wateringly expensive. And the paint colour never seems to match.

So the trick is to keep an eye on all the other stuff. In fact the components most vulnerable to heat can be the capacitors. If the 0.1uF coupling caps go leaky or (less likely) if the resistance of the R or C in the KT66 cathode circuit should go low then the KT66s can draw enough current to see the transformer off. I've replaced all these components in my Quad IIs using modern, reliable alternatives. Not very authentic I'm afraid but I like to think Peter Walker would've used polyprops instead of paper-in-oils and polyesters instead of electrolytics if only he'd had them available. A check of the KT66 cathode and grid voltages will tell you if the old caps are going west. Watch out also for second hand KT66s which have higher-than-usual emission. The old ones are rare now and people will sell "hot" ones claiming that their high current means they've got loads of life left. Maybe they have, but if you put them in your Quad II then maybe it won't have ...

Another problem, apparently, is that the HT choke can migrate inside its can until it shorts to the pins holding the cathode components. Unless you've got the sort of kit that was used to take the X-ray on Keith Snook's page http://www.dc-daylight.ltd.uk/Valve-...D-II-Mods.html you don't get any warning of this (maybe tar oozing from the choke ?).

Sorry if all this sounds a bit gloomy. As Nick Ross says, don't have nightmares ! Once they're up and running these amps are pretty reliable. The best thing for them is to keep enjoying them

Cheers,

GJ
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