Making a High Voltage probe for EHT supplies

[WME_bill]

The current thread about checking the EHT on a Cossor CDU150 brings this subject up again.
An EHT probe for your voltmeter is very easy to make. String a few 10M resistors in series in a PVC tube, with a probe tip cemented in a one end, and with a 1M at the bottom. Connect your DVM across the bottom 1M. This will be alright up to say 5Kv or 10Kv.
For my Avo8, I just use the string as a series step down to the 1.0kv range, at 50ua full scale.
Calculate what the rough step down ratio is, and adjust the top resistor selection to give you a convenient ratio: say 20:1 or whatever. Never mind the exact value or the type of resistors, or the voltage rating (if you use it a few minutes every year, the resistance will not alter under voltage stress, though it will by age).
BUT the secret is to re-calibrate before each session you use it. Measure a highish voltage line elsewhere in the scope (probably the X or Y output amplifier feed) with and without the probe, and note the exact ratio. Then when you make your measurements at HV, calculate the corrected value.
This recalibration tip came years ago from a Tektronix or HP manual. It was I think intended for their professional ultra stable HV probes (? Tek P6015 x1000 to 40Kv).
WARNING. Use it to measure the cathode and focus voltages around the tube, but even with this high impedance to a DVM, best not the grid terminal directly. The current flow will swing the grid positive, and destroy the tube.
When you put the probe on the EHT rails, it will fizz in a most impressive manner, and that will make you very quick and careful. I usually wear kitchen rubber gloves which gives me confidence, but I am not sure whether it really helps.
Obviously be conscious that these voltages can be uncomfortable on modern scopes, and dangerous on old mains driven EHT supplies (as Telequipment D43 or Solartron CD1400) which need extra care.
This subject comes up about every 6months, and I wonder if the Moderators would want to make it more permanent.
What do others members do for measuring this. wme_bill

[maninashed]

You will need to ground the other end of the 1 meg or it will float at full eht !
If you are unsure, leave this to more experienced techs.
Bill

[GrimJosef]

Originally Posted by maninashed

You will need to ground the other end of the 1 meg or it will float at full eht !
If you are unsure, leave this to more experienced techs.
Bill

Very, very true. If in any doubt check with someone who has experience of these things.

I used to work with large high-voltage systems and right at the start of my career my supervisor pointed out to me that an important element of acquiring any skill was to learn from mistakes. In most fields they could be your own mistakes. But in HV Eng they had to be other people's mistakes. You could get the details from the coroner's report. I thought it was funny too until I heard that an American chap who I'd actually met had been killed while trying to track down corona on an EHT system. He'd turned the lights out to see if he could see it and in the dark he'd tripped and put his hand into the equipment.

Incidentally at a few kV corona probably won't be a significant issue. But in the 10kV+ region any sharp points towards the top of the resistor stack can bleed significant current out into the surrounding air. This effect can't be corrected by calibrating at a lower voltage of course. So try to envelope any joints in nice smooth solder blobs. Or (pro tip) little blobs of Plasticene also work.

Cheers,

GJ