Genalex KT66 valves for Quad II mono amps (original version)

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I recently fulfilled a lifetime dream of getting hold of some original Quad II mono amps. I first put hands on a pair in 1966, as my early teenage exposure to audio and this is a nostalgia buy, to go with my new Stirling V3 ls3/5a.

The output valves are all brand new Genalex Gold Lion KT66. My question: can (original) QII users running these particular valves give me some idea of the length of their operating life? (and any tips!) Cheers.

 

[lilolee]

This from Joe Rosen on Audio Asylum when I used to run Quad II's back in 2002

You will get good results with the Sovtek 5881WXT. I recommend that you replace the cathode resistor & condenser. Replace the baked-out 180 ohm too-small jobbie with a 5 or 10 Watt Dale CW or RS series resistor, or an equivalent that uses a glazed or black coating.
You want one that's all welded, not those crummy white or beige rectangular cement-filled things. Increase the cathode condenser to at least 50uf, up to a maximum of 220uf. Use a 50 Volt unit.
The new resistor should be 220-250 ohms.
The improvement in the cathode resistor/capacitor will more than offset any minor difference in sound caused by using the 5881. The new values are fully compatible with the original Genalex valves, and should be done by anyone with a QUAD II, and the sooner the better.

Then I would use 5881 for every day use and keep the KT66 for Sunday best.

You should also check all of the other resistors and capacitors. Keith Snook has a nice page http://keith-snook.info/quad-ii-valve-power-amplifier.html

 

[brecklandboy]

The vintage GEC / MOV KT66s fitted in the original Quad 2 power amplifiers were run very conservatively and should last for many thousands of hours before requiring replacement. Quite how long modern production / re-issue KT66s will last is a guesstimate but longevity should of course be greater if any newly installed valves are in excellent health in terms of high emissions, low C / H leakage etc. Incidentally, according to Quad, close ' matching ' of pairs of output valves is not critical to the operation of these amplifiers.

In passing, i should mention that the only Quad authorised replacement for the original KT66s when supply of these tubes became scarce in the 1990s was the Philips or GE (USA) 7581A. These are also very long lived and reliable in my experience and give a noticeably 'cleaner' and more precise / detailed / articulate sound than the original KT66s. Well worth a listen if you can borrow some for a 'tube rolling' experience.

 

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Incidentally, according to Quad, close ' matching ' of pairs of output valves is not critical to the operation of these amplifiers.

That's of interest. But should individual valves in each pair be closely matched? I was wondering what to do if one KT66 goes belly-up.

 

[ampedup]

In passing, i should mention that the only Quad authorised replacement for the original KT66s when supply of these tubes became scarce in the 1990s was the Philips or GE (USA) 7581A. These are also very long lived and reliable in my experience and give a noticeably 'cleaner' and more precise / detailed / articulate sound than the original KT66s. Well worth a listen if you can borrow some for a 'tube rolling' experience.

Good to know.
The Genalex KT66 seem to be universally available. I notice that Watford Valves is selling a "KT66 Harma Retro" based on an Osram tube.

 

[Arkless Electronics]

Modern valves are likely to fail for random reasons long before they actually wear out when used in a Quad II which runs the valves very gently. They are particularly uncritical due to the way they are used in the Quad and I've even seen 6L6 and KT66 in the same amp (not a good idea!) working "adequately".

 

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The only valve amp I owned before was a modern amp using KT120 valves. Even when working correctly within spec, it would blow two or three tubes per year. An expensive choice for amplification.

 

[brecklandboy]

That's of interest. But should individual valves in each pair be closely matched? I was wondering what to do if one KT66 goes belly-up.

Quoting from a period Quad sales / marketing booklet for the Quad 2 power amp dated 1964 . " The performance specification is fully maintained with random valve replacements from standard commercially tested valves without matching or alignment of any kind. "

 

[Arkless Electronics]

The only valve amp I owned before was a modern amp using KT120 valves. Even when working correctly within spec, it would blow two or three tubes per year. An expensive choice for amplification.

Unfortunately this seems to be the way it is with modern valves... output valves specifically as they are under vastly greater stress than small signal valves. To state the obvious somewhat, back when the valve was king and no solid state option existed, all power electronics from hi fi amps to guitar to PA, industrial servos, shaker beds, medical and military applications all needed "output valves" and with what was at stake in industrial, medical and military applications they had to be reliable as they could make them and it was really big business. Just Mullards Blackburn factory employed thousands of people and a whole industry existed to provide hyper purity chemicals and metal alloys used in valve production. Add in the valves used in every TV set and radio and the scale of the industry can be seen to be huge!
In comparison, today's valve makers are a few blokes in sheds getting by with whatever materiel's they can purloin from other industries leftovers that happen to be "near enough ish".... Output valves tend to work well enough when new but to succumb to any of many potential ailments over a few months to a couple of years which can cause sudden failure well before they actually wear out in the normal sense of the phrase.

 

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Quoting from a period Quad sales / marketing booklet for the Quad 2 power amp dated 1964 . " The performance specification is fully maintained with random valve replacements from standard commercially tested valves without matching or alignment of any kind. "

That's encouraging, thanks.