Re: What happens if a valve's cathode becomes electrically detached ?
The negative excursion on the grid was relatively low-amplitude - less than a volt - and lasted only a couple of seconds or so. With 220k grid leaks it would only have needed a few microamps. I agree that a completely isolated cathode would have charged up very quickly indeed. But perhaps Chris's suspected defective spot-weld had some small residual conductivity ? Some tens of kohm at that point would have allowed the cathode voltage to rise high enough to switch the valve off with around a milliamp of current flowing down through the cathode resistor. I'd have struggled to see that on the 200V range of the DMM I was using to monitor the cathode voltage. It'll be interesting to hear what the retailer finds.
Cheers,
GJ
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