tetrode output

[Lane Zeitler]

I have a homebrew tetrode HF amp. Single 4cx1500B. I am not the builder. It
covers 160 to 15 with a tapped L. Nice large vacuum variables with turns
counters, etc. With about 10 watts of drive it is delivering 1050 to 1150
watts out depending on the band. The screen voltage is 250 vdc and is
regulated quite well. The documentation is piece meal and sparse and I have
only a partial master schematic but the builder (SK) was an engineer and did
a very professional job.

Questions:

1. The screen voltage has taps for 150, 250, or 350 vdc. I am using it on
the 250vdc tap. When I go to the 350vdc tap the ZSAC rises to about 1000mA.

Why?

2. I can not reduce the output out of my TS-850 any lower than 10 watts,
even with alc. What kind of variable attenuator network can I build on the
input side of the amp so I can vary the drive that the tube receives from
the exciter? There are times when 400 watts is all that is needed.

3. With ten watts in and 1100 out or so is this a typical reading for a
4cx1500B? There is zero grid current or screen current with this drive
level. If I increase drive to 15 or 20 watts then it starts drawing grid
current. I understand this is a no no with tetrodes. I am trying to
determine if this tube is full out. Plate voltage under load is 3500 vdc and
Ip is about 550 mA with 1100 watts output.

I would like to raise the screen voltage to the 350vdc tap but I need to
determine why the ZSAC goes through the roof.

Any comments appreciated.

Lane
Ku7i