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Re: Citation II's with KT90's

Posted by Jim McShane (D) on July 29, 2006 at 12:37:54

In Reply to: Citation II's with KT90's posted by Apogee on July 29, 2006 at 12:06:56:

Steve,

It is not a problem with the type III per se. Pre-NATO campaign type IIIs are fine, if you can find them then use 'em! However the current production KT-90 tube has misaligned screens causing significant screen overheating which causes the red on the plate. I've personally seen screens so badly misaligned that the tube glowed like a regular 25 watt light bulb when B+ was applied. It was all the white hot screen wires all glowing!

Many of the KT-90s being produced now show red on the plates at 30 watts dissipation, that's 60% of normal spec. To turn down the bias in a Cit II that far to limit dissipation would completely change the character of the amp. The KT-90 and the amp work best when the bias is at least 75 ma. 75 ma. with 460 volts on the plate would mean dissipation of about 35 watts. That's not safe with many of the current KT-90s.

Misaligned screens also mean higher distortion and accelerated wear to certain areas of the cathode. So the curent tube will likely have a shorter lifespan.

Please take Eli's advice and stay away from them - you're asking for trouble if you don't.

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