Output tube plate resistance and OT Impedance matching

[Deke609]

In Doc B's Blues Master article in Valve Vol. 1 1998 at p. 9, Doc notes that "the rule thumb for SE amps is that the impedance of the output transformer primary should be a minimum of 3 times the output tube’s plate resistance."

What is the reason for this? Not just the approx. target ratio (3+:1), but the need for a ratio at all? 

Many thanks in advance,

Derek

[Paul Birkeland]

Valve 1998 volumes 3 and 4 cover this pretty well.  You can also look at the giant table of operating points from Western Electric for the 300B.

[Deke609]

Thanks PB. I see that PJ's 3-part article deals with my question in great detail. This is gonna take me some serious time to make sense of.