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Excellent post!..

Posted by Paul Joppa (P) on March 31, 2014 at 21:15:21

In Reply to: RE: High transconductance tubes sound as transistors... posted by Tre' on March 30, 2014 at 20:59:49:

Thanks, man. Yeah, I was expressing some frustration of my own - I apologize if the flak hit you accidentally. :^)

But I must say, you brought up a serious and important question, which it was my intention to further address - what (exactly, measurably) is a good definition of linear operation?

My own rule of thumb for power triodes - I can't call it a definition - is to design for a load line that gives twice the quiescent current, at zero grid-cathode volts. With a really linear tube (see below) that gives about 3% second harmonic at full output. Less linear tubes will have more distortion, I've calculated as much as 10% based on published curves, but those tubes will distort at practically any operating point! The reason I chose this is that even with huge negative feedback the tube will remain in Class A throughout a full sinewave cycle.

My model of tube linearity is based on a distribution of mu along the cathode. If the mu is everywhere the same, then cutoff (zero plate current) happens at the same grid voltage everywhere. I call such a triode "linear" because it follows the 3/2 power law exactly. That law is of course not a linear function, but you can't in theory do any better with a triode.

Of course, if the load impedance is much greater than that which produces the most power - which is the case for drivers if they have a high-impedance plate load such as a choke or current source - then it is easier to operate the tube in a region where the mu does not vary significantly, and you can this way approach truly linear operation.

This leads to the idea that there can be a region of nearly constant mu, within which tube operation is actually quite linear. But you still need a number - perhaps a 10% variation of mu is acceptable? Open for comments!

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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