Quote:
Originally Posted by VintagePC
Electronic M - In retrospect, I'm intrigued by your comment... One would think sets would be designed so that typical failure of any arbitrary tube would result in minimal or no damage. What is it about the circuit design that makes the HO tube red-plate if there is no oscillator? Are there other parts of the circuit that are similarly affected if another single tube fails?
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There were a couple of chassis I'm aware of (IIRC the CTC-2 was one) with fail safes, but most neglected them. In most output stage designs the output tube is biased by DC, but in the case of TV Horizontal outputs the grid is normally biased by grid-cathode rectification of the horizontal osc drive waveform....No waveform no bias...Or more accurately the grid in absence of the waveform pulling it negative instead drifts positive in lieu of it's bias source and ceases to limit the cathode current. The cathode current skyrockets well beyond it's design ratings, the emissive cathode material rapidly burns off, the plate and grids get more energy than they are designed to handle and sometimes melt, and the fly is subjected to DC current well in excess of it's ratings and begins to heat...Sometimes until burning open.
It ain't pretty seeing a Horizontal output destroy it's self...Well maybe it is, but it ceases to be once you know what your seeing.
