I've just assembled a HVDC test station to test a whole large whack
of 50 pF -at- 20,000 volt vacuum caps that I acquired surplus, and plan to
employ in my HV divider probe. The tester is essentially a 0-50 kV DC variable
power supply feeding through a 40 megohm resistor to limit fault current to
the capacitor under test. An Electrostatic voltmeter measures the
applied DC voltage at the capacitor. The negative return from the
capacitor to system ground goes through a 0-50 uA d'arsonvall type
moving coil meter.
This test set has been working well and I'm disappointed to find that
some 40 % of my caps are failing this test showing breakdowns of
excessive leakage current.
What is new to my experience is that on some of those leaky vacuum
caps where I sustain several microamps of leakage current at say 23
kV DC, the cold field emission involved produces a suprisingly high
amount of x-rays! I am using a GM type civil defence radiation
counter (Anton Electronic Labs item CD V-700 model 5), which has a meter and
headphones. With the shield open on the GM tube head I can bury the
instrument's needle on the most sensitive range (0-0.5 MR/HR) with the probe
held 15 inches from the vacuum capacitor with 5 uA -at- 23 kV applied.
The point is, put used vacuum capacitors on your list, along with
6BK4 receiving tubes as a possible x-ray tube obtainable to the
hobbyist for x-ray experiments.
This realization of the cold field emission effect in a vacuum tube
has me wondering now of the probable x-ray hazard one would be
exposed to when hand holding some types of tubes into the very high
voltage field produced by a Tesla coil as us coilers are known to do from
time to time. The accelerating potentials available in the Tesla
coil based experiment are many times greater than the 23 kilovolts
which causes such active emission from my vacuum capacitor as just
The tubes which I often hold near an operating TC are not vacuum tubes but
glass envelopes filled with mercury vapour or neon, argon, etc., at
low pressures. I think some experiments are in order here to attempt
to measure these lamps for x-ray emission with very high Tesla coils
type voltages applied.
Has anyone out there any experience or information to share along