[Amps] {Collins} Understanding Screen Current in a Tetrode

[John Lyles]

Last week I read with interest the discussion here. There were numerous 
replies, each with a bit of good information. Gary, K4FMX, nailed it 
spot on, with this explanation reproduced here. This agrees with my 
experience with tetrodes, where the plate dip is difficult to measure. 
Screen current peak always seems to stand out.

73
John
K5PRO

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A little clarification is in order here:
You tune the PLATE tune control for a peak in screen current the same 
way as you would dip the plate current. The screen current is much more 
sensitive than the plate current.

You adjust the loading control (less capacitance) to load the amplifier
heavier. The screen current will decrease as you load the amplifier 
heavier. Again adjust the plate tune control for a peak in screen current.

Keep increasing loading (decreasing load capacitance) for maximum power 
out. Keep doing this until desired power output is reached and maximum 
screen current is not exceeded. Keep increasing grid drive while tuning 
with out exceeding maximum grid current or maximum screen current.

Too much drive when the amp is not loaded heavily enough will cause the
screen current to go very high.

With a tube like a 4CX1000A it is rated at zero grid current but slight 
grid current of around 1 Ma is allowed while tuning.

When you are done tuning the screen current may be negative with tubes 
like the 4CX1000A. you always peak the screen current in the positive 
direction with the plate tune capacitor even if the screen current is 
negative.
73
Gary  K4FMX