[Amps] Ep Sweet Spot

[sccook1 at cox.net]

Team,

Playing around with my TL-922A this weekend (pair of 3-500Zs), I noticed that the amp was significantly more efficient with the plate voltage set to the lower voltage (2.2KV vs 3.2KV).  

For a given power output -- say 500 Watts, the low voltage position required less drive, and resulted in lower Ip and Eg.

Just curious -- does this mean that for a given tube / power level, there is a Sweet Spot where efficiency is optimal?  Or, was I seeing the effects of a tank circuit not being loaded at the design 'Q'?

I always assumed more was better with respect to Plate Voltage (at least to a point).  But now -- I don't know...

Ep Curious...

Steve (WG7K)
Tucson, AZ