[Amps] Question for the tetrode experts

[Carl]

>   I've finally applied full drive to my 4CX1000A amplifier. Here is what 
> I'm
>   seeing:
>   I have tried two tubes, both pulls. In one case I can run about 1kW out, 
> and
>   with the other, maybe 1200. What happens when I try to push past this 
> point
>   is this: when I hold the key down for 10 seconds or so, the screen 
> current
>   starts to drop, then goes negative, and when it hits -35 mA the screen 
> trip
>   circuit kicks in. I haven't seen even a flicker of grid current.
>   The screen circuit is well-regulated at 325v, so this isn't case of 
> runaway
>   caused by increasing voltage. What it seems like to me is that the 
> screen is
>   heating up and the secondary emission is rapidly increasing. Screen 
> current
>   before this happens is reasonable, 20-25 mA, so I am not exceeding its
>   rating.
>   I have a couple more tubes to try, but I have a bad feeling that I am 
> going
>   to end up buying a new tube if I want 1500 watts from this.
>   Do the experts think my diagnosis is correct?
> -- 
> Vic, K2VCO


It could also be a damaged screen from overcurrent in the past.

Both are rather common tetrode problems where screen current isnt constantly 
monitored as a seemingly simple QSY can drastically affect it.

Also running the screen voltage at or close to maximum rated is not a good 
idea and I would also give up the idea of 1500W from one 
tube...reliably...even Alpha couldnt do it (-;

Carl
KM1H