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Old 5th Feb 2020, 8:23 pm   #41
Luxman1050
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Default Re: Making an RF probe for VTVM

just found this bit of info on EA50.
This valve was used in the British Army AA 3Mk7 radar during the early post WW11 years. A favourite trick played by R.E.M.E. training instructors at Arborfield, during fault finding exercises, was to replace one of these valves, having first cut off the centre pin.

This little diode was used in a probe for VTVMs to rectify high frequency AC without loss in the cable.
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Old 5th Feb 2020, 9:45 pm   #42
David G4EBT
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Default Re: Making an RF probe for VTVM

Quote:
Originally Posted by ms660 View Post
The heater current for the EA50 is 0.15 Amp not 0.1 Amp:
Oops - thanks Lawrence for spotting the typo in my text! (Correct on the sketch).
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Old 5th Feb 2020, 10:04 pm   #43
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Default Re: Making an RF probe for VTVM

No problem, the HP410b manual shows the double screened cable method of connection, also shows the probe construction etc:

http://hparchive.com/Manuals/HP-410B...prefix-024.pdf

Lawrence.
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Old 5th Feb 2020, 11:38 pm   #44
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Default Re: Making an RF probe for VTVM

One thing I would be looking out for Chris!! The lil piddly transformer used in the VTVM is already at maximum, I dont think it will live long with another valve heater hanging off it.
This is not like a 50 watt transformer in a mantle radio which would hardly notice another 150 mA in heater current.
Its just a friendly warning.

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Old 6th Feb 2020, 11:00 am   #45
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Default Re: Making an RF probe for VTVM

Good point Joe just throwing things in the pot quite alot of information coming in it's brilliant. Who would have thought there's so many variations and the complexity too boot. Mind you just goes to show how versatile these VTVM meters where. No such luck today.
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Old 6th Feb 2020, 11:06 am   #46
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Default Re: Making an RF probe for VTVM

Did you sort a probe body?

Looking for something entirely different and came across these. Can't remember what they are from but plastic, so unlikely to be screened.
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Old 6th Feb 2020, 11:17 am   #47
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Default Re: Making an RF probe for VTVM

Blimey looks like a pneumatic drill! What on earth was that used for? Body looks big.
Yes I have some tubing although it does say it needs to be in a metal can not plastic which is what I have.
Am I right in saying this diode can be directly heated via a battery? Since thinking about it all that needs heating up is the anode the cathode is intended to be cold so electrons move up to the anode but not back down to the cathode as it's cold. So since the anode and cathode are built into the circuit an external source of power is all that's needed I.e. a battery. Just a thought.
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Old 6th Feb 2020, 12:09 pm   #48
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Default Re: Making an RF probe for VTVM

I'll be gentle, you need to brush up on valve theory.

http://www.r-type.org/articles/art-010b.htm

Other articles are available.

That aside, the heater in the EA50 is best supplied from a mains derived source rather than a battery.

Lawrence.

Last edited by ms660; 6th Feb 2020 at 12:16 pm. Reason: Link added
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Old 6th Feb 2020, 12:27 pm   #49
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Default Re: Making an RF probe for VTVM

Ha ha thanks Lawrence. I need to brush up on more than that. I need another 20 years plus to catch up with you guys.
I don't think year and half without proper theory and practical skills is near enough.
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Old 6th Feb 2020, 12:31 pm   #50
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Default Re: Making an RF probe for VTVM

Em not far off after reading that.
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Old 6th Feb 2020, 1:19 pm   #51
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Default Re: Making an RF probe for VTVM

Okay guys looking at threads 22# 28# 33# 35# and 38# all that is required is power source. So going over those posts then the EA50 needs to be hard wired into the existing heater circuit of the meter? So feeds could be taken off the 6AL5 connected to two Jack's installed on side of metre but would this draw too much power for the transformer since its only supplying two valves and in essence we would be adding a third into the equation. All components as the original schematic for this unit are supposed to fit in a single can now this must hold true otherwise it's a design flaw. Which I doubt. Obviously the hardest thing in relation to this design is they forgot to put in where the power for the heaters came from. So I would assume if your an experienced engineer then you would know straight off. I assuming this is what the designer had in mind hence leaving out the voltage supply as he/she felt did not need to explain.
I don't think hard wiring into the unit removing existing RF double diode is viable option for me.
I think looking at post 33# Packard 410b is worth investigating.
But we still have not been able using the diagram supplied with the EA50 make our minds up where it fits into the heater supply. We cannot use transformer as Joe said draw too much power, we cannot use a battery supply, we could tap into the 6AL5 heater supply but that would mean again that the transformer is supplying a third valve which was this particular unit designed to do.
So really it's trial and error while hopefully causing no damage to the meter.
Any thoughts?
Cheers Chris
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Old 6th Feb 2020, 7:27 pm   #52
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Default Re: Making an RF probe for VTVM

A conundrum for you all.
Off subject but can you tell me what's wrong with this electrolytic capacitor?
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Old 6th Feb 2020, 7:49 pm   #53
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Default Re: Making an RF probe for VTVM

The metal can of most electrolytic capacitors is usually negative or isolated.

This type of capacitor would be useful in circuits that have a negative bias supply.
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Old 6th Feb 2020, 8:29 pm   #54
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Default Re: Making an RF probe for VTVM

If you look at the label it's the wrong way round. The +ve is at the -ve end as opposed to being at the positive rubber grommet end.
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Old 6th Feb 2020, 9:31 pm   #55
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Default Re: Making an RF probe for VTVM

There have been cases in the past where capacitor manufacturers loaded parts in their forming rigs the wrong way round and made exactly that arrangement, with the outer can positive. Of course, they didn't notice this and the sleeve merrily declared the seal end positive. They didn't last long in service!

So, there isn't necessarily anything wrong in that photo. Such a part could be made and would work. From the valve days it seemed sensible to make cans the negative end as they usually got connected to chassis as power rails were almost exclusively positive ones.

I wonder if anyone made positive can electrolytics back when negative rails started to be used?

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Old 6th Feb 2020, 10:11 pm   #56
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Default Re: Making an RF probe for VTVM

Okay I should have said can you see the mistake on the can. Yes still usuable but I got in touch with supplier as someone just starting out would no doubt think that's the right way round I would have and pop or worse still they have there head over it and it decides to go bang.
So there re checking their stock plus I received some compensation.
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Old 6th Feb 2020, 11:06 pm   #57
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Default Re: Making an RF probe for VTVM

Thast green probe in #46 is an oscilloscope divider probe. X1, X10 and x 100 as I recall. I have (or had) two of them. THEY WORK! Yes, a bit on the BIG size, but they still work.
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Old 7th Feb 2020, 4:04 pm   #58
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Default Re: Making an RF probe for VTVM

A scope probe!

Looking more closely I see that the switch is marked 5kV peak, 1000V peak and 500V peak.

Could even be useful checking the mains with my scope.

Still can't think what they came off.
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Old 7th Feb 2020, 4:10 pm   #59
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Default Re: Making an RF probe for VTVM

I think there for an oscilloscope not a VTVM from what I've read could be wrong.
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Old 7th Feb 2020, 4:40 pm   #60
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Default Re: Making an RF probe for VTVM

Do a Google image search for.....vintage MX-2681/UP probe.

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