VTTC modulation problem

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Original poster: herwig.roscher-at-gmx.de 


VTTC coilers,

I am trying to audio modulate a 4-125 tube. As I was not satisfied
with the audio quality from control grid modulation, I am using
screen grid modulation now. Plate voltage is 2500 V and screen grid
voltage is 175 V +- 175 V (i.e. the screen grid swings between 0 V
and 350 V).

To my surprise there is a small brush of about 1/4 " height without a
screen grid voltage at all. Does somebody have an explanation for
this phenomenon?

Another strange behavior: Even with full swing screen voltage it is
not possible to increase the modulation factor to more than about 15
%. Could there be a connection to the upper problem?

Cheers,

Herwig