Quote:
Originally Posted by cbenham
This sounds like a Barkhausen oscillation in the HOT. I have seen ION traps installed on the HO tube and rotated until the vertical line disappears to cure this problem.
Hope someone else can chime in with more details...
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A while back someone wrote a very good explanation. IIRC it has to do with screen grid conduction causing oscillation. When the plate becomes less positive than the screen grid the screen begins to conduct. IIRC at those transition points the tube will go into high frequency oscillation changing the plate current slightly for a fraction of each line.