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PJL · 17th Oct 2019, 9:47 am

Just one other check before we can conclude it is most likely the UCL82, with the UCL82 removed, measure the resistance from the pentode grid (pin 3) to chassis, it should be about 480K.

What you are seeing is typical thermal runaway, the valve runs at too high current, it overheats, extra heat makes it conduct more.... This is often caused by impurities that have condensed on the inside of the valve causing leakage to the grid. Try monitoring the voltage from chassis to the grid (pin 3), it should be <0.5V but I suspect it will slowly rise as the valve gets warm.

17th Oct 2019, 9:47 am	#25
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Westminster S470M power supply problem? Just one other check before we can conclude it is most likely the UCL82, with the UCL82 removed, measure the resistance from the pentode grid (pin 3) to chassis, it should be about 480K. What you are seeing is typical thermal runaway, the valve runs at too high current, it overheats, extra heat makes it conduct more.... This is often caused by impurities that have condensed on the inside of the valve causing leakage to the grid. Try monitoring the voltage from chassis to the grid (pin 3), it should be <0.5V but I suspect it will slowly rise as the valve gets warm.