[Amps] Tune cap at minimum

[Dave Hachadorian]

I have an amplifier that tunes up on 21 MHz with the tune 
capacitor at minimum value.   It acts like the capacitance should 
be even lower for max efficiency and output on 21 MHz.  All of 
the other bands, 10 through 160 are fine.  The output network is 
a pi-L, and the tubes are a pair of GU-74b's.

I was thinking of squeezing or spreading the coil turns on the 21 
MHz inductor closest to the tube, but I can't find the dang 
formulas to figure out which way to go.

Any ideas?

Many thanks.

Dave Hachadorian, K6LL
Yuma, Arizona