[Amps] Shorted turns reduce harmonics?

[Mark Marsden]

Hi Amp'ers

The harmonics from my homebrew amplifier, on some bands, are much lower
than expected, and I've been wondering why.
 
The output Pi tank coil is made up of 3 orthogonal coils for 10-12m,
15-80m & 160m, mounted at 90 degrees to each other, see
http://granta.digital-crocus.com/linear/Mvc-078x.jpg

The 15-80m coil is tapped in 3 places and turns are shorted out by the
band switch. The other 2 coils, 10-12m & 160m, are not tapped. 

Now a Pi-tank, transforming 1400R to 50R with a working Q of 12, has a
2nd harmonic attenuation of about 30dB and an amplifier running in class
B has an anode current that is a half sine. So the resultant 2nd
harmonic level at the output can reasonably expected to be about 35-45
dB down.  

However there's more, on the bands where there are shorted
mutually-coupled turns, the harmonics are attenuated much more.  

The level of the second harmonic relative to 1kW on each band is shown
below. 

160m -45.3 dBc  no shorted mutual turns
80m  -47.0 dBc  no shorted mutual turns
40m  -58.5 dBc  7 shorted mutual turns
30m  -56.3 dBc  7 shorted mutual turns
20m  -62.9 dBc  10 shorted mutual turns
17m  -70.0 dBc  13 shorted mutual turns
15m  < noise    13 shorted mutual turns
12m  -45.3 dBc  no shorted mutual turns
10m  -54.2 dBc  no shorted mutual turns

See http://granta.digital-crocus.com/Linear_design_notes.php3

It seems that the shorted turns are absorbing harmonic energy. Has
anyone seen this effect before?

73, Mark, G4AXX