[Amps] Question for the tetrode experts

[Roger (K8RI)]

On 11/7/2013 6:14 PM, Vic K2VCO wrote:
>
>     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.

Many possibilities. Are you tuning for maximum screen current and noy 
maximum output.

There should be no grid current with a 4CX1000


>     The screen circuit is well-regulated at 325v, so this isn't case of runaway
>     caused by increasing voltage.

There is the possibility of a component going in the screen supply.

The plate voltage stays up?

  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 take it the screen and plate voltages are the recommended?

>     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?

I'm inclined to think it's a component, but no guarantee.

Tuning up with carrier, you should see a negative scree current until 
you are putting out a substantial output. At that point upu start tuning 
for maximum screen current (with in the tube ratings) and increasing 
drive. The 4CX1000 should be capable of the legal limit, but needs 
plenty of air to do that.  I run a 4CX1500B and it puts out well in 
excess of the legal limit.

73 and good luck,

Roger (K8RI)

>