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Old 21st Dec 2016, 10:05 pm   #1
Stylo N M
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Default What was the highest HT voltage you ever experienced?

Hi,

i just wondered, as there are such dangerously high voltages present in old televisions, both black and white, and colour, what was the most terrifing one you ever came accross?

Paul.
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Old 21st Dec 2016, 11:25 pm   #2
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Default Re: What was the highest HT voltage you ever experienced?

Not in a TV, but now vintage these days. Submarine systems Tat1 & Cantat1 & Anglo-Swedish systems used voltages above 5KV, at nasty currents, with high capacity cables, meaning that after switch off, lethal voltages could be present inside open cabinets. Not much in respect of TV ,but the currents were nastier.
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Old 21st Dec 2016, 11:26 pm   #3
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Default Re: What was the highest HT voltage you ever experienced?

Hi,

The highest voltage I've worked around is 25kv on colour sets which doesn't spook me but I am always aware that it is there if you get my drift but by far the most dangerous voltage I've worked on is only 3 kv derived from MOT's (microwave oven transformers) only 3kv which sounds weedy in comparison but because of the low impedance can deliver the full 3kv at nearly 500mA!!

One concentration lapse around these if powered (and mains derived EHT on old pre war TV's for that matter) and you'll definitely be knocking at the doors of the service department in the sky!

Cheers
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Old 21st Dec 2016, 11:28 pm   #4
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Default Re: What was the highest HT voltage you ever experienced?

Back in about 1980, I was cutting my teeth on CTV and was asked to look at a Korting CTV. The early one with a pair of PL509 valves in the EHT/Scan section. The picture was small and shimmering, and as I had a Marconi VTVM (1043C if memory serves) and a NEW Marconi 50 KV EHT probe (from Peter Thacker's ex Gov surplus sources) I decided a check was advisable. 33KV, so I switched off and decided this repair was not for me. I returned it to customer and suggested either a main Korting repair depot, or scrap it. It would be about 10 years old at that time. I had read too much worrying stuff in TV mag about X ray emission at 25KV and upwards, so decided discretion was better part of valour. I can't remember, but guess it had a PD500, but that did not seem able to tame it.
Les.
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Old 21st Dec 2016, 11:35 pm   #5
Craig Sawyers
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Default Re: What was the highest HT voltage you ever experienced?

Only 20kV, but at an entirely lethal 100mA. It was a 19" rack mounting monster from either Glassman or Spellman. Used as a charging supply to an equally lethal capacitor bank. Back in the early 80's. It is a wonder I am still alive.
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Old 21st Dec 2016, 11:36 pm   #6
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Default Re: What was the highest HT voltage you ever experienced?

radar transmitter pulse, 29kV at 26 Amps. Only a couple of µs long but enough to fry at a frequency of 180 Hz.
However, the most nerve tingling was for me, adjusting the Blue lateral magnet on the BRC 8000 chassis. It was mounted under the tube neck just above the PSU! Stupid place to put a live PSU heatsink!
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Old 21st Dec 2016, 11:45 pm   #7
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Default Re: What was the highest HT voltage you ever experienced?

Ditto here with the CTV 25kV, never had belt off that though, worse belt for me was 500VDC from a guitar/disco amp I built running a pair of 807's.

The most startling one was drawing a spark with a screwdriver from a tellies line output stage to light a fag with, normally it wasn't a problem as I used to draw the spark through a piece of paper first to ignite that then light the fag from that, anyways one day I thought I'd be a smart ass and do direct ignition....the roll up was a bit damp

Lawrence.
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Old 22nd Dec 2016, 12:57 am   #8
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Default Re: What was the highest HT voltage you ever experienced?

I used to have to repair 60kV 40mA (centre tapped at +/- 30kv to earth) power supplies for HV distribution testing for SWEB. They came with discharge wand because the item under test could stay charged.
The company I worked for didn't really have much in the way of test equipment so I would clip the test leads either side the plastic waste paper basket and use a string of antistatic bags as load resistors. They only stood a few seconds before they started overheating and catching fire
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Old 22nd Dec 2016, 1:19 am   #9
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Default Re: What was the highest HT voltage you ever experienced?

Around 6kV in a Racal TA1800 in transmitter test. Rather more than an amp behind it, it was a 10kW HF linear amplifier. I was very careful about not becoming its second victim

I've used insulation testers up to about 15kV but they weren't half as threatening.
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Old 22nd Dec 2016, 8:56 am   #10
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Default Re: What was the highest HT voltage you ever experienced?

Wimps!! I used to work for a power cable manufacturing company. We regularly tested cable test installations ups to 400KVac continuously, and up to 2.4MV for momentary lightning strikes. All safely behind safety interlocked metal barriers I might add.

In my last job fixing tellies, 25kv, and 2 or 3 ''bites' of that over the years was plenty enough thank you. Only joshing with the 'wimp' comment!
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Old 22nd Dec 2016, 9:06 am   #11
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Default Re: What was the highest HT voltage you ever experienced?

Apart from colour television EHT supplies. As the opening question was for HT supplies, I have a Linear Amplifier that has about 1700 volts on the output valves anodes.

John
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Old 22nd Dec 2016, 9:23 am   #12
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Default Re: What was the highest HT voltage you ever experienced?

7kV, the EHT in a Type JDA radar display. I had one open in a darkened Ops Room on HMS Eskimo and managed to touch it! Didn't do that again! The muscles in my arm ached for ages afterwards.

It's had the effect of putting me off anything with a CRT so whilst I find the old 405 line televisions interesting, I don't want one!

A bit irrational I know.

John
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Old 22nd Dec 2016, 10:18 am   #13
Malcolm G6ANZ
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Default Re: What was the highest HT voltage you ever experienced?

The highest was 40Kv pulsed at ,almost, unlimited current used to fire a magnetron in a linear accelerator. One of the linacs I worked on had a lot lot of electronics isolated and floating at 25Kv. All signals went through opto isolators or long shaft motor/pot assemblies.

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Old 22nd Dec 2016, 11:29 am   #14
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Default Re: What was the highest HT voltage you ever experienced?

25kV at 8A for TV Klystrons.
 
Old 22nd Dec 2016, 11:36 am   #15
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Default Re: What was the highest HT voltage you ever experienced?

Quote:
Originally Posted by Malcolm G6ANZ View Post
The highest was 40Kv pulsed at ,almost, unlimited current used to fire a magnetron in a linear accelerator.
Where did you work, Malcolm?
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Old 22nd Dec 2016, 11:40 am   #16
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Default Re: What was the highest HT voltage you ever experienced?

Quote:
Originally Posted by merlinmaxwell View Post
25kV at 8A for TV Klystrons.
I remember as a teenager doing work experience at the Pye TVT factory in Cambridge, and seeing the long 'Christmas trees' of stud-mounted rectifiers on heatsinks on a plastic shaft connected in series for EHT rectification. I seem to remember the whole rectifier was a few feet long. Then again the klystron it supplied was pretty much as tall as I was at the time, so I guess everything was in proportion!

Chris
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Old 22nd Dec 2016, 12:10 pm   #17
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Default Re: What was the highest HT voltage you ever experienced?

There was a mains-derived EHT power supply for the first TV I repaired when I was 14. It was one of those cabinet-style jobs with a mirror. I worked out for myself that this was mortally dangerous - there was no internet and I had no-one to ask for advice.

The EHT power supply in this thread sources around 5.8kV
pulsed (perhaps 3.5kV under load), at high current.

It was a low source impedance and for a short period will supply an average current limited by the EHT rectifiers' current-carrying ability, which is now around 3A.

There is around 15kV at 20A in the tank circuit.

The plasma discharge at the antinode can be huge. It is RF, of course, and so travels on the surface of a conductor (or even what is normally an insulator) but it is possible to reach hundreds of Kilovolts. I don't want to speculate on the length of plasma we will see from mine, just yet.
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Old 22nd Dec 2016, 12:55 pm   #18
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Default Re: What was the highest HT voltage you ever experienced?

Terrifing? - The only time I have thought this could be the case was when I was on a visit to the Rugby shortwave transmitters. Some of the tales told really made me think - high voltage, current and RF mix. A discharge wand could be used on a feeder and dangerous voltages exist a short distance away on the same metal tube! A point I wish to remember and respect.

My highest that I can record is the 25kV or so on a CRT. I have put together various experimental circuits over the years and also did quite a bit with electronic ignition units on my bikes years ago so could not really put a figure on it, but all fairly low current.

I think the most dangerous was probably my trial circuitry for my high voltage PSU (final product designed for 650V at 250mA). On trial with an earlier design I had over 900V and similar current.
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Old 22nd Dec 2016, 2:00 pm   #19
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Default Re: What was the highest HT voltage you ever experienced?

400kV DC. The local converter station at Sellinge in Kent. It's a bi-directional power converter connected to France. The DC cables are buried beside the road and then go under the English channel. There is an enormous room full of three stacks of 1kV/4000A thyristor bricks, controlled by a big opto-isolator system. It's truly frightening to stand amongst it, listening to it and smelling the ozone. It sure beats the 35 kV belt I got off a Bahco projector.
Graham
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Old 22nd Dec 2016, 2:04 pm   #20
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Default Re: What was the highest HT voltage you ever experienced?

My personal experience is around 25KV on colour TV's but my most scary experience was as a student on a visit to a radio transmitter station, unfortunately I don't recall which but it was in the south east somewhere. When we were shown the output stages and told that the HT was about 100KV (from memory) and the effects of RF burn, I became a little spooked!
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