Output tube plate resistance and OT Impedance matching

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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,


Offline Paul Birkeland

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Reply #1 on: June 30, 2019, 06:31:38 AM
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.

Paul "PB" Birkeland

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Reply #2 on: June 30, 2019, 08:12:31 AM
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.