[Amps] Tank Coil spacing

[Peter Chadwick]

Steve asked:
>It seems most of the homebrew and Handbook amps I see are Pi tanks.  Is the Pi-L really necessary?<
To meet the current requirements of the FCC and the Radio Regulations on spurious emissions, yes. The Class AB stage produces a harmonic content in the plate current that is 6 dB down on the second harmonic, and if you do the sums, getting down the extrs 37dB to meet FCC or 44dB to meet the Radio Regs limits is pretty darn difficult in just a pi network- in fact, I reckon impossible, although there may be somebody who can come up with values that in a particular case manage to do it, at least in theory. It can even be a bit difficult in a Pi-L under some circumstances. 
However, using a pi and an external low pass filter selected by band, or a tuner,  is another approach

73
Peter G3RZP