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Nah

Sun May 29 2011, 01:48PM

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

All right, how about a VTVM?

With a 11 + Mohm impendence and impervious to most EMR, lets see how you can beat my mighty HP with your wimpy baklite meter!

Conundrum

Sun May 29 2011, 02:10PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

I have some nice antique moving coil meters here, anyone interested?
Mainly voltmeters and ammeters, some work and some do not.
Possibly fixable, might just need wires soldering back on.

-A

Steve Conner

Sun May 29 2011, 02:42PM

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

That's easy Nah, say I wanted to measure some current. Oh dear, VTVMs don't have current ranges.

Or I wanted to go test my car battery. Damn, there is no outlet to plug the VTVM in.

For some reason, "Valve voltmeters" never caught on in the UK, they were mainly an American thing. As far as I'm concerned they have the same advantages and disadvantages as a digital meter. I've got an old ITT Metrix multimeter that has both a digital display and an analog needle, but it somehow manages to combine the worst characteristics of both instruments.

Ash Small

Sun May 29 2011, 03:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Nah wrote ...

All right, how about a VTVM?

With a 11 + Mohm impendence and impervious to most EMR, lets see how you can beat my mighty HP with your wimpy baklite meter!

That's pretty interesting. I might pick up a 12AU7/ECC82.

There is a circuit diagram here:

Nah

Sun May 29 2011, 03:15PM

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

How about this...

I need to measure .5 ohms exactly. Oh dear, my 260 and my DVM can't measure low resistances! I want to read 20 kvolts at 100 microamps. Oh dear, my 260 and my DVM can't measure that high, and will load the output! What do I do? I use my HP VTVM with a 30kv high voltage probe! When I connect the leads of my fluke together what do I get? 0.01 ohms! With my handy RCA, I can use the ohms adjust to get 0 ohms! Also, what's this? MY meter is picking up EMF and showing 0.01 ac volts? With my EICO, I can use the volts adjust to get 0.00 volts!

Also, lets say that I buy a $5 VTVM and a $5 DVM. Oh dear, I can't find a VOM fr $5! Then, lets put them on the 10 volt range and input 200 volts. VTVM- nothing DVM- BOOM, fizz, fizz

Right back atch ya!

Proud Mary

Sun May 29 2011, 03:21PM

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

Steve McConner wrote ...

For some reason, "Valve voltmeters" never caught on in the UK, they were mainly an American thing.

Not wanting to put your nose out of joint, squire, but here is yer actual Avo Electronic Testmeter, aka the Avo valve voltmeter, full of meaty goodness as they say in dog food commercials:

1306681071 543 FT0 Avo Electronic Testmeter

There was also a later Electronic Avometer type EA113 which had a high Zin - compared to your analogue 20 kÎ©/volt, that is - but which had little of the Herculean industrial design of the Electronic Testmeter, being stylistically similar to the rather nasty Marconi Instruments TF2600 Sensitive Valve Voltmeter:

1306681944 543 FT0 Marconi Valve Voltmeter

Steve Conner

Sun May 29 2011, 04:54PM

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

Ash Small wrote ...

That's pretty interesting. I might pick up a 12AU7/ECC82.

There is a circuit diagram here:

If you're going to build one in the 21st century, at least consider replacing the tubes with JFETs. See the circuit on page 3 here.

They recommend the 2N4340, but any JFET will be better than a tube.

Nah

Sun May 29 2011, 10:40PM

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

It will be MUCH cheaper just to buy a ready built VTVM from the 50s. You can pick one up at a Hamfest for $5, or build one for $50.

What happens when you over voltage the transistors or over current them? With tubes they'll be fine for some time, transistors, BOOM.

No, I am not as old as dirt, only as old as hydrogen.

Ash Small

Mon May 30 2011, 12:05AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think you have a valid point, Nah, but I'll let Steve answer. I hope Chris doesn't lock this thread, it's starting to get interesting

Steve Conner

Mon May 30 2011, 06:59AM

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

Well, granted, the FETs will go boom far easier than tubes. And I guess if Avo made a VTVM, they can't be all bad.

But, VTVMs aren't as common over here, nor are hamfests for that matter. (if you're looking for one, the British name is "Radio rally".)

And, I'm thinking of the hassle of getting a really good 12AU7. The VTVM is very demanding of the tube's DC performance, and some random one pulled from a radio might give disappointing results.

For some reason, while I've managed to scavenge about 2 dozen 12AX7s, I always struggled to find the T and Us. I needed a couple of good 12AU7s recently and ended up paying nearly Â£20 each for JAN Philips 5814s at Watford Valve.

A 6SN7 or its Russian equivalent can also be used.

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