Use this form to submit comments directly to the Asylum moderators for this forum. We're particularly interested in truly outstanding posts that might be added to our FAQs.
You may also use this form to provide feedback or to call attention to messages that may be in violation of our content rules.
RE: OK Tube Guru's, Question
Posted by Chris O on July 4, 2012 at 12:30:46:
Just be aware, that the amp does not know it has different tubes in it and it will try to use them like KT90s (which KT66s are not).
Your bias setting is the Class A bias (and perhaps tube balance as well). Your amp is A/B which means that when it runs out of Class A it moves into Class B. Class A bias has no effect on Class B and setting the bias lower only helps to keep the tubes from melting at idle. The 90 watts for the KT90 is all Class B.
So if your amp has a 450/500 volt upgraded power supply and your driver section has the AC swing (which it does), you can turn the amp up so the KT66s will try to play at 90 watts per channel. Either the distortion will make you stop or the little tubes will glow cherry red and melt if asked to pump out the jams for any length of time.
Your amp will still try to play at 90 watts if you tell it to. If you want to use these lower rated tubes, the only thing that will protect them and the amp is if you are pretty careful with setting the volume control and are not trying to play them into unfriendly speaker impedance. I would say that you will be in the 20 to 25 watt area with reasonable distortion numbers. Your amp won't simply "not go any louder in a harmless, carefree sort of way" when the KT66 tubes run out of gas, they will commit suicide trying to please you.
(Note: tube failure can cause a short in the tube which can make the tube look like a short to ground for the output transformer, shorting the output transformer primary can arc and melt the insulating varnish, thus ruining the transformer.)