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Old 12th Sep 2006, 8:19 am   #4
Mike Phelan
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Join Date: Nov 2003
Location: Near Leeds, West Yorkshire, UK
Posts: 4,259
Default Operation of a valve line output stage

OK - here we go, Chris (and Jim ). This will be a bit long, so you may fall asleep. Keep away from the PY top cap.

Assuming that the LOP stage is already running, and the spot is in the RH side of the raster, and the combined scan coils/width control/LOPTx are treated as a single inductor.
  • The PL is turned hard on due to the grid drive, and its anode voltage is about 40. The current through the LOPTx is increasing linearly - the sawtooth for the forward scan. The PY is turned on and feeds the HT to the PL via the LOPTx. CBoost receives a charge of about twice HT with normal autotransformer action.
  • At the end of the scan, the PL is cut off by the grid drive so the current in the LOPTx is cut off.
  • The LOPTx and Cstray start to go into oscillation - the LOPTx energy is passed to CStray, whose voltage rises to several Kv; autotransformer action means that the PY cathode and the EHT overwind follow. CBoost is effectively earth. We are halfway through the flyback, and the PY is cut off by the high voltage on its cathode.
  • This is an unstable state - CStray discharges into the LOPTx in the opposite direction - the current is reaching the end of the flyback. Left to its own devices, this would all keep oscillating until losses intervened.
  • However, as the voltage spike falls, when the PY cathode reaches HT, the PY turns on and effectively puts CBoost across it and passes current through the LOPTx from CBoost but in the opposite direction, starting the forward scan.
  • This current would only last for about half the scan, but the PL is turned on at about 40% before the current from CBoost becomes non-linear.
  • So, there is a saving because the LOP stage is operating at about 600v and the first part of the scan is provided by CBoost charged during flyback.
Make sense? I shall be asking questions!
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Last edited by Mike Phelan; 2nd Apr 2011 at 5:01 pm. Reason: Changed flickr graphic to JPG.
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