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

[Ian White]

Sorry to be arriving late to this discussion.

Gary's explanation is very nearly there... but only for readers who
already understand the need to re-peak the screen current with each
small change in loading. That point needs more emphasis, maybe something
like this {new wording in curly brackets}:

You adjust the loading control (less capacitance) to load the amplifier
heavier. {Every time you adjust the loading control, re-adjust the
tuning control for maximum screen current. The maximum in the screen
current} will decrease as you load the amplifier heavier.

Keep increasing loading (decreasing load capacitance) for maximum power
out,
{re-peaking the screen current at each step}.
Keep doing this until desired power output is reached {while making sure
that the tube manufacturer's maximum screen current rating} is not
exceeded. Keep increasing grid drive while tuning without
exceeding {the tube's maximum ratings for} grid current or screen
current.




73 from Ian GM3SEK


>-----Original Message-----
>From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of John
>Lyles
>Sent: 03 February 2015 06:07
>To: amps at contesting.com
>Subject: [Amps] {Collins} Understanding Screen Current in a Tetrode
>
>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
>
>--------------------------------------
>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
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