2nd Harmonic
Network is a pi-L set up for 3800 Ohms at the plates of the 4-400A's.  It has 40dB suppression at 2nd harmonic and nearly 60dB at third harmonic.  This insures the output is greater than 60dB down at second and third harmonic.  For 375 watts the plate voltage should be 2100V and the plate current (total, 2 tubes) should be 275mA.