[TenTec] AMP Question

[Robert & Linda McGraw K4TAX]

While running a Tentec Centurion at about 275 watts carrier output, the DC
input is some 2800 volts at about 300 ma plate current or about 840 watts or
so input.  I notice that the 3-500Z's show very little color on 75 M.
However, just noticed that with the same operating conditions on 40 M that
they show a good bit more color even though the DC input is the same and the
RF output is the same.  Both tubes show about same amount of color.

Any clues as to why?

73
Bob, K4TAX