I have removed the resistor/varistor combinations that go between the poles of the target and tube heaters, and between those two supplies. I tested all the components to make sure they were still working after the sparks - for the record the varistors clamp at 23V, not 33V as implied by the schematic.

Tuesday, March 06, 2018, 10:48  BIAS/EE/EZL = 59.5/-1000/29.5  There was some kind of spark, the current on the bias supply went up to 400uA ( normally at 40 +/- 5 ) and the vacuum jumped a bit ( but still in the 10e-7 range ) but I didn't hear anything. It took out TP6 again. See first attachment.

Tuesday, March 06, 2018, 10:54  BIAS/EE/EZL = 60/-1/30

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Problems

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Tuesday, March 06, 2018, 12:00 BIAS/EE/EZL = 60/-1.5/29

Tuesday, March 06, 2018, 12:37 BIAS/EE/EZL = 62/-1.5/30

Tuesday, March 06, 2018, 14:11 BIAS/EE/EZL = 64/-1.5/31 No sparks but occasional ticking sounds from either the EZL or behind where the EE is... see second screenshot

Tuesday, March 06, 2018, 14:40 BIAS/EE/EZL = 65/-1.5/33

Test stand has been sitting at 65kV for about an hour now. Some small increases in bias power supply current but nothing significant (40uA). Some small sparks on the way to this voltage, and I ramped up very slowly. The corona discharges seen before seem to be related to the resistor/varistor combos since there are none now that I have removed them. Will redesign and reinstall.