[Amps] Glow, gettering, and voltage

[Catherine James]

I've noticed an interesting effect with the 3-500Z tubes in my SB-220. 

When I run a kilowatt SSB on the low voltage setting on low HF bands, there is very little glow. When I run the same power on the high voltage setting (using less exciter drive), there is a noticeable dull orange glow.

I cannot think of a reason why higher voltage operation would be less efficient. Thoughts? 

73,
Cathy 
N5WVR