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Current production KT66 for Quad ii amps

Discussion in 'classic' started by _webwasher, Jun 30, 2019.

  1. _webwasher

    _webwasher pfm Member

    my carefully restored Quad ii are running with it’s original set of gec kt66 tubes and I really like the sound.
    As these tubes are getting rare and expensive nowadays I’m asking for your experience with now available kt’s. I was told that max heater current is to watch out for and golden lion are a good match. But how do they sound and what about other brands?

    Thanks for sharing your experience with Quad and new power tubes.
  2. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Russian mil spec 5881 equivalents with the "button base" are about the best around.
  3. George J

    George J Herefordshire member

    In a follow up question for the Q II Forty, what would be the best KT 88 valves today? I have the official Quad labelled ones, but though I think they work well for sonic, they certainly are not long lived in that application.

    I wish amps could be designed a step back from requiring an almost red-line level of performance from the output valves to give them a bit longer to live. I'd trade a bit more distortion for long life if that is how it works!

    Best wishes from George
  4. _webwasher

    _webwasher pfm Member

    But where to get these and any changes in circuit necessary?
    An easily available drop in solution would be great :)
  5. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    No really good KT88's available now. If you can get NOS Sylvania or GE 6550's then they would likely be the best and again NOS the Svetlana "winged C" ones.
  6. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    They are drop in and easily available.
  7. Zombie

    Zombie pfm Member

    Used Saratov KT66 (bought from kwtubes on the bay) in my QIIs, but NOS matched GEC KT66s were fabulous
  8. Tony L

    Tony L Administrator

    Given how gently they are run and how long they last I’d be tempted to stick with GEC KT66s. Try and land a set just like the ones in there at present and that way you can mix and match as they gradually die (if they ever do). As I understand it the Quad II just asking 15 Watts from a pair runs them way, way inside their performance envelope hence so many still being fine to this day. I know they are expensive, but even just landing a pair, or a couple of singles will expand the reach of what you have. Access to a valve tester is useful obviously.
  9. Old Valve Guy

    Old Valve Guy Active Member

    I use Gold Lion brand KT66 in two amps and I'm quite happy with them . Have had GEC in the past , but as they age , the comparisons with new unused replacements become difficult .
  10. _webwasher

    _webwasher pfm Member

    Thanks all for sharing, very appreciated :)
  11. lilolee

    lilolee pfm Member

    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.

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