[Amps] Non-linear linear

[Paul Baldock]

I have a problem with my home brew HF linear being non linear on one band.

On 80, 40, 20, 12 and 10 with 75W drive my linear will give about 
1500W output. If I reduce the input power by 9.5dB the output goes 
down by 9.5dB. So all is good.

On 15 with 75W drive my linear will give about 1500W output. If I 
reduce the input power by 9.5dB the output goes down by only 5.2dB. That's bad.

On 17 with 75W drive my linear will give about 1500W output. If I 
reduce the input power by 9.5dB the output goes down by only 7.6dB. 
That's not too good but not as bad as 15.

Some info on the amp:

Single GS35B, about 3.7KV, Idle current 210mA. No parasitic choke. 
Ameritron plate choke.
Shared PI output for 10 and 12. Shared PI output for15 and 17.
Shared PI input coil for 10/12 and 15/17 but with padder caps 
switched in for the lower freq.

One anomaly I notice is that at the lower power input on 15 the plate 
current is about 390mA, where as on 80/40/20/12/10 its about 300mA.

At the high power input Plate Current is about 650mA on all bands and 
grid current is around 350mA. At the low power input grid current is 
zero on all bands.

Any thoughts.

- Paul   KW7Y