[Amps] Why hasn't solid state replaced tubes?

[af4mp at mindspring.com (Zyg Skrobanski)]

>What do you mean?
>asymmetrical distortion?

Whenever a situation occurs that causes the IMD products, above and below a
center frequency, to be uncorrelated.

>But the only way you can get IMD upper an lower sidebands of different 
>amplitudes is to have frequency modulation or phase modulation on top  
>of amplitude modulation.
>All non-linearities will produce equal upper and lower sidebands.

Please explain then, why the ARRL testing of IMD in apparently healthy
transmitters shows otherwise?

--
Zyg AF4MP
Roswell GA USA