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Nah

Tue Jan 04 2011, 08:27PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Hi,

I was wondering, why don't any of you use vtvms for tesla coil work? They are tougher than digital meters, and are less expensive than your prized fluke. I Also see 2 advantages over the fluke, one, they can measure higher voltages. Using a $5 eico and a high voltage probe, you can easily measure up to 30,000 volts. Also, if you add more resistance to the dc probe, you could get even more! Also, I would like to see you $400 hand held fluke to measure more than 5 mhz. A good heath kit can measure up to 10 mhz!

Nah

Steve Conner

Tue Jan 04 2011, 09:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, I've heard that VTVMs exist, but I've never seen one. Before my time.

I don't put my (well, my employers') prize Fluke near anything dangerous either. I use an old analog meter. Over here the standard kind is an "AVO meter"

but in America it would be a Simpson.

GluD

Tue Jan 04 2011, 09:18PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

Hi,

I think most people dont use vacuum tube volt meters because they dont know they exist, I didnt before I saw them at a hamfest last year.

Bought this one

It was damn cheap like you say, but the seller didnt have a probe for it so I havnt used it yet, I just thought it looked nice. Shame they put stickers and draw numbers on it though.
Maybe I'll improvise some sort of probe for it sometime.
Of course its also alot easier to just buy a cheap DMM and use that for the risky bussiness instead of puting the fluke on the line. I think most people use the cheapo DMM's.

Nah

Tue Jan 04 2011, 11:07PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

It's VERY easy to make a probe

Look up the model number and you will most likely find the schematic on a site called BAMA. There, it's easy to figure out how to make theem. It's basicly o ohms on the ohms and ac probe, and around 1 mohm on the dc. The vtvms also have a high inpendence, around 25 mohms!

BTW what model is that? Is it a War time HP?

Steve, I didn't know that you were that old! The first vtvms were in the 20s and 30s. The bridge circuit that makes all vtvms possible was invented in 1916!

Nah

GluD

Tue Jan 04 2011, 11:19PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

Its a BrÃ¼el & KjÃ¦r, type 2407.

I really got alot of other projects so making a probe probably wont be soon.
Switched it on sometime ago and the tubes light up, thats pretty much all I've done with it thus far. Aperantly they used the type 2407 number for a new instrument so searching doesnt apear to do any good.

radiotech

Wed Jan 05 2011, 07:17PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The meter you have is an Audio Vacuum Tube voltmeter.
It is for use with amplifiers, microphones. It does not
measure DC. Bruell & Kjaer is the top line of
European audio instruments, and there's lots of data
out there.

The output jacks will allow you to use it as a preamplifier
to drive a chart recorder. It would be quite happy if
you had the connector cord like this:

Nah

Wed Jan 05 2011, 08:56PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

I didn't know that!

I thougth it was something for the army. Thanks for seting me up right. I'm afraid I know squat about European tube stuff, ony stuf in the good old USA I think HP and Heath kit and RCA made audio vtvms, I don't think they are very useful, I think they are a vtvm with only a ac function, as any vtvm can read up to the mhz range.

Nah

GluD

Wed Jan 05 2011, 09:24PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

Maybe theres lots of data somewhere, I only bothered to do a quick google which informed me that they aperantly use the type 2407 number for another, new, instrument.
I dont know what to use it for so I dont see the point in researching any further until I got a use for it. Just bought it because I think its pretty.
I appreciate your input Radiotech, thank you.

Sulaiman

Thu Jan 06 2011, 05:13AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Why I don't use a vtvm;
- a new one is unobtainable
- old ones have reliability issues
- I like my meter to be portable
- a vtvm won't survive a drop from my workbench
- a 1% error dmm costs less than GBP10 with RS232.
- anything better than 1% error I'm unable to calibrate

I do sometimes use moving coil meters when there's rfi around

I'd rather risk a dmm than an AVO meter which costs more.

P.S.
Going from vtvm to fet vom allowed us to have battery powered meters
going from fet vom to ic dmm allowed small battery powered devices of great accuracy and versatility.
I also don't listen to valve radios !

Proud Mary

Thu Jan 06 2011, 10:31AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Nah wrote ...

I was wondering, why don't any of you use vtvms for tesla coil work? They are tougher than digital meters, and are less expensive than your prized fluke. I Also see 2 advantages over the fluke, one, they can measure higher voltages. Using a $5 eico and a high voltage probe, you can easily measure up to 30,000 volts. Also, if you add more resistance to the dc probe, you could get even more! Also, I would like to see you $400 hand held fluke to measure more than 5 mhz. A good heath kit can measure up to 10 mhz!

Valve voltmeters became obsolete not out of a perverse dislike of having nothing but a vacuum between the ears, but because of their relatively poor performance and need for frequent re-calibration when compared with well designed solid state devices. Short term thermal drift/cycling due to valve heaters, warm resistors etc, and long term drift due to valve ageing could be compensated for, but never wholly eliminated. Calibration standard valve voltmeters were usually left running for their entire working lives to maintain a workable thermal equilibrium. Some contained 'ovens' - temperature controlled, dessicated compartments. Only very modest bumps and bangs could change an instrument sufficiently to require recalibration.

Fluke produce a wide range of products, so I'm not at all clear what you mean by "a Fluke." I have an elderly solid state Fluke Differential Voltmeter whose long term stability exceeds that of the best of the calibration standard valve voltmeters, and whose theoretically infinite DC input impedance is hard to match.

If you can't find a TV-type EHT probe to measure 30kV in conjunction with a cheap and cheerful 10MÎ© input impedance DVM, you are clearly not looking in the right place.

Is there any role at all for the vacuum tube voltmeter? There may be a very small niche for electrometer valve inputs, when radiation 'hardening' is an issue. Some Russian designers think so.

Stella

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