shouldn't a VTVM have near infinite input impeadance?

[ELS122]

I'm looking at VTVM shematics and noticed that all of them have a clear path to ground. like range selectors. it's high (~10M) but it's still there.
to my knowladge a VTVM should have basically no connection between ground and the test probe, appart from some capacitance.
is that wrong. or is there some specific term I need to look for to find that kind of VTVM?

yeah but if it wouldn't have a path to ground than the tube would go into all kinds of oscillation?

[gbaddeley]

Where did you get the idea from that a VTVM would have infinite input resistance?

[ELS122]

some guy said that and many people in the same discord server agreed
I guess they're all total idiots

so a VTVM is better just because it uses much higher input resistors that DVM's?

[David Hess]

so a VTVM is better just because it uses much higher input resistors that DVM's?

Vacuum tube voltmeters had higher input resistance which distinguished them from *analog* meters which might be only 400 kilohms on their 20 volt scale.  They share the same input structures and performance with digital multimeters.

In most cases the input resistance will be 10 megohms because of the input dividers or shunt resistance however like DVMs, there are exceptions for special applications.

[bd139]

VTVMs are almost universally 11 meg-ish. There's usually a 10M resistive divider inside the meter and a 1M series resistor in the probe tip.

Some ammo to throw back at the claimants originally which smells awfully like QRZ lot...

VTVM advantages:

1. Because there is a 1M resistor in the probe tip then it isolates the circuit under test from capacitive loading. To a degree.
2. Doesn't inject much noise into the circuit under test
3. Analogue movement has some damping.
4. If you blow it up you can probably fix it.
5. They go for lots of money on ebay with a bit of turd polish which can be spent on better meters.

VTVM disadvantages:

1. The things are usually floating mains devices which are not double insulated.
2. The tubes wear out over time
3. They are mechanically fragile.
4. You can do the same trick with a resistor in the probe with a DVM.
5. Resolution is terrible.
6. Accuracy across all ranges is terrible.
7. Need to be plugged in
8. Usually have a leaked ohms range battery hiding somewhere.
9. Easy to blow up the ohms range.
10. The chassis is usually grounded to the DUT so you can kill yourself with one in two seconds flat if you do something stupid.
11. Full of shitty old wax capacitors which dissolve.
12. They're huge!

So I'll keep my Fluke thanks

[David Hess]

Another disadvantage at least compared to a semiconductor input stage is that most VTVM have a higher input bias current which contributes to error.  Solid state high impedance input stages can have much lower input bias current.  The exception will be VTVMs which use "electrometer" input tubes but offhand I do not know of any and these are much more difficult to maintain anyway.

2. Doesn't inject much noise into the circuit under test

Most DVMs and all modern ones suffer from some level of charge injection however some old DVMs use a linear input stage and do not.  They are effectively a solid state implementation of a VTVM followed by a digital readout.

[T3sl4co1l]

"Infinite" impedance ranges are only possible on the lowest settings, and if designed in properly.  (A lot of DMMs do this, say on or below the 2 or 5 or 10V ranges.)  The main advantage you'd have with tubes is, because the operating voltages are higher, the inf-Z range could be higher as well.

Indeed, old school electrometers were made this way, using special low-emission tubes that draw an absolute minimum of grid current (while delivering hardly any plate current, and almost no transconductance, but what do you expect? ).  The operating range might be a few hundred volts.  Nothing you'll see in a bog standard VTVM though.

Tim

[Tom45]

Back when VTVMs were a thing, the alternative was analog meters with a typical 50 uAmp movement. That is a current meter and to measure volts, a series resistor was added to the meter movement. For a 1 volt range, use 20,000 ohms. For 10 volt range use 200,000 ohms, etc. So analog meters were specified at so many ohms / volt. Or 20,000 ohms/volt for a 50 uAmp movement.

The VTVM input impedance was indeed nearly infinite compared to those analog meters.

[ELS122]

well yeah I know vacuum tube stuff usually is much worse than solid state, but there are 3 advantages that will never make me change my mind that tube's still rock:
they're SUPER tough,
they're pretty simple,
they just look cool.
and I am too dumb to switch to solid state anyway 

but measuring even relatively sensitive circuits like RF, or many audio circuits. even a meter with 1M input impedance will be a really big problem, and I've experienced that, for example measuring the voltages in an LTP PI, my meter loaded down the circuit a LOT, and I spent like 3 hours troubleshooting before I came to the conclusion that my meter is the problem.

but yeah they're huge, so I would only use one if I really needed it. sadly, I don't have one 
I am thinking about building one from the RCA WV77 circuit, but instead using Russian 6H2P and 6N1P tubes. just gotta find what rotary switches am I gonna use.

[PartialDischarge]

well yeah I know vacuum tube stuff usually is much worse than solid state, but there are 3 advantages that will never make me change my mind that tube's still rock:
they're SUPER tough,
they're pretty simple,
they just look cool.
and I am too dumb to switch to solid state anyway 

You will like Mr Carlson Lab videos then

[ELS122]

well yeah I know vacuum tube stuff usually is much worse than solid state, but there are 3 advantages that will never make me change my mind that tube's still rock:
they're SUPER tough,
they're pretty simple,
they just look cool.
and I am too dumb to switch to solid state anyway 

You will like Mr Carlson Lab videos then

well appart from the multiple hour long videos, I do like em xD.
but my jam is shango066, smoke has always been cool for me