Prototype set is here

[miniman82]

Originally Posted by ChrisW6ATV

Nick, how about trying a 25 meg on both ends of the convergence pot followed by a 50 meg to ground? Mouser has 1-watt 25M HV-rated resistors, but none with higher power ratings. I used two of their 50M 5-watt ones in my CT-100 and they work well, even fitting the cup on the HV rectifier socket.

I was going to try that, but Ohm's Law tells me it's a bad idea. Reducing the resistance on the low potential side does a couple unwanted things. #1 it drags the low potential side of the pot down, which is counter productive (I need higher voltage). #2 it increases the total current drain of the string, placing added stress on the first resistor causing it to heat up and drift more than it already is. With a 100 meg as the second one, the low potential side of the pot is pushed higher. Total current drain is reduced, taking stress away from the other ones in the string. Here's a comparison chart, with 25 meg for both and on the right the way things sit right now.




In practice there's less voltage available at either end of the convergence pot, because of the first resistor drifting. About 10kv and 8kv respectively, by the time I measure it. So as you can see, it's beneficial to dissipate the majority of the voltage after the convergence pot.