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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 16th Jun 2018, 9:16 am   #1
Diabolical Artificer
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Default Testing 813's.

I knocked up a 1kV PSU to test some 813's I had, video here - https://www.youtube.com/watch?v=XybEOt6XzXI . I got six good valves out of 7, not bad, as I bought them untested.

One thing I was concerned about is the DC blocking capability of the bridge rectifiers I used and diodes in general. The bridges were GBPC2508's at the high voltage end with jerry rigged bridges made out of 1N4007's at the lower voltage end ( tfmr secs in series, stacked as in attached. As you see he uses 1000V bridges here). The datasheet for the 08's says 800V PIV and DC blocking. I measured 20V ish DC on the winding that fed the "top" bridge, whether this was actual DC or a misreading by my DMM I'm not sure.

If it is DC leakage I'm reading, would putting an EHT diode in series after the bridge and smoothing cap be beneficial? I intend to build a 2500V PSU eventually, this baby HV PSU was by way of a test.

Andy.
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Old 16th Jun 2018, 2:10 pm   #2
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Default Re: Testing 813's.

The transformer does not have windings in series, there are several supplies with the DC outputs connected in series.
Each supply will have to do the work of any other so there is no difference between high voltage end and low voltage end other than insulation capability. It seems you have some leakage. Show us where you got this measurement.

What is the voltage of each output winding? 400 or 500V should be OK to feed into a 800V bridge but the higher the diode voltage the better. I have some 1200V diodes and higher voltage diodes are available.

Make sure the balancing resistors are of good quality.
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Old 16th Jun 2018, 3:09 pm   #3
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Default Re: Testing 813's.

Pity you aren't closer Andy or I might have tried sweet talking you into testing mine. I've had some 813s for years and never proved they are OK, other than using the heaters as loads for stabilised PSU testing!

Mike
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Old 17th Jun 2018, 7:07 am   #4
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Default Re: Testing 813's.

"The transformer does not have windings in series, there are several supplies with the DC outputs connected in series." Slip of the tongue.

" there is no difference between high voltage end and low voltage end" Except there is a higher DC potential and thus the bridge or diode sees more than it's rated voltage. I connected a DMM across the winding, I'll take a pic to clarify.

Regarding diode leakage I've always tested diodes with a DMM on the DC resistance setting. A reading of 20v as you say indicates the diodes in the bridge are leaking. A good diode should show no reading or a high one yes? Not sure how valid this test is. Again I'll do some tests and report back.

Winding voltages are 195v, 137v, 113v, 113v and 180v . I have diodes with higher PIV's, but if you take a look at bridges on Farnell say, there are a few 1.2kv, but none higher than that.

I had mine 813's sitting about for a while too Mike, it's took me a while to get round to testing them. I only tested Ia, though this is a pretty good indication of a valves health. All my 813's came through the post, if you can post mike I'll happily test them for you.

Andy.
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Old 17th Jun 2018, 10:25 am   #5
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Default Re: Testing 813's.

That's very kind of you Andy. I'm always reticent to entrust anything like this to the post. IIRC, I have three new looking 813s, two Canadian Marconi and one that's JAN US marked mil spec. I think I have a fourth somewhere where the soldered top cap has fallen off the main pin coming out of the top of the valve! Anyway, let me see if I can even find them.

Mike
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Old 17th Jun 2018, 10:35 am   #6
TrevorG3VLF
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Default Re: Testing 813's.

With winding voltages up to 195, then 1N4007 diodes will be plenty.

I tested high voltage diodes by connecting then to a 1kV supply and then measuring the leakage with an analog meter set to the 1kV range. I subtract reading from 1kV to measure the voltage that the diode can withstand. This seemed to work well with a fistful of surplus diodes bought many years ago. I now use modern diodes as they are so cheap and work without testing.

My limited experience is that possibly faulty diodes are either good or bad as measured with low voltage.
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Old 17th Jun 2018, 3:53 pm   #7
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Default Re: Testing 813's.

Only had one busted valve when posting etc Mike, but that was down to bad packing. If you pack them right you should be ok, but will leave it up to you. The last lot of 813's I got came in plastic PVC pipe, cracking packing.

"With winding voltages up to 195, then 1N4007 diodes will be plenty." But it's the 1kv DC present I'm worrying about. Thing is how is over voltage measured in regards to the datasheet spec for a diodes DC breakdown voltage? The +1kv isn't present across the two diodes of the positive side of the bridge. The highest potential I guess is across two diodes, which would mean DC breakdown voltage is doubled, as the DC across the cap, here a tad less than 270v is all that par of the bridge sees.

Probably tending to over think things here, it's easy to fret though when such high voltages at quite high current are present.

Anyhoo, I'll do some testing, been out today so not had chance.

Andy.
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Old 17th Jun 2018, 9:49 pm   #8
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Default Re: Testing 813's.

As I see it, there are several power supplies connected in series. Each power supply will provide <270V and will feed into the string. There will be no very high voltage on the power supply. There is the possibility of reverse voltage with a fault but the bridge rectifier will take care of that.

The winding at the 'top' will have a very high voltage to the core so must be insulated very well.
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Old 18th Jun 2018, 4:13 pm   #9
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Default Re: Testing 813's.

Good work Andy. I've been thinking of doing something similar to test some high power RF valves I've got and you have provided me with some inspiration.

Looked at your video and I'm sure I spotted a TEK 500 series scope and the big transformer you are using must originate in such a scope. I've got one too and it would be ideal for supplying a high power amp. They are superb and solid transformers with lots of secondary windings, HT and LT. Heavy of course but that's what is needed. Interesting that the TEK 500 power supplies build up to the needed voltages by adding up lower HTs in series to achieve around 600 volts in the 585 so I'm sure TEK will have built in sufficient isolation between windings and core. I should add that I do have a working 544 that I restored and it will remain so!

Ian
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