Should I get a VTVM?

[Electron Tornado]

Looking for some advice on whether I should get a VTVM. I have an old Radio Shack solid state analog multi-meter that's still in great shape, and is my primary meter.

Any pros, cons, or pitfalls regarding VTVMs?

[merlinb]

Does anyone use VTVMs anymore (vacuum-tube volt meter)? Get a digital multimeter. Even a cheap one is more than adequate for hobby electronics.

[GibsonGM]

I believe the only thing to gain with a VTVM would be a very high input impedance that was not otherwise obtainable back then...these days, people build essentially a buffer to place ahead of their DMM and increase the impedance to insane levels....thus making the ol' tube job an obsolete shelf ornament.   

Nice to look at up there, tho 

[amptramp]

I have several VTVM's and although they are nice and evoke an era when a lot of people were into electronics, modern FET-based analog meters and DMM's are better.  I use a VTVM for things like alignment of radio receivers where you have to spot a tuning peak that is broad and well within one digit of a digital meter.  Even though you power them off the wall outlet, they usually have a 1.5 volt battery inside for the ohms range supply.

If you get a DMM, watch for the ohms range - some only go up to 2 megohms and if you have to diagnose an LFO or any other high-resistance circuit, it may show a good resistor as open because it can't tell the difference between a 2.2 megohm resistor and an open circuit.

[PRR]

A needle-meter beats heck out of a numeric display for reading moving voltages. Trem/LFO for one. Power supply rise glitches. (And mistakes: my needle meter shot up faster than most DMM could follow when my "400V" supply was mis-wired for 800V.)

The classic analog ohm-meter can (with insight) read semiconductor junctions more interestingly than DMMs. They are usually a 1.5V battery plus a range resistor. The range-resistor is equal to the half-scale value of that Ohms range. Put a diode across the probes. Read on the 1.5 VDC scale, this is the diode forward voltage at that current. What current? Subtract Vd from 1.5V, divide by range resistor value. Neat feature: change to the next range. Vd changes very little. If the ranges are 10X different, you ideally get a 0.060V change of Vd. For most diodes and meters this will be true over most ranges. On the 1XOhms range (typical 10r mid-scale) the jump from the lower range will be larger. ~~0.8V/10r is 80mA, and most small junctions will deviate from the exponential due to internal resistance. But this also means you "can" read junctions in-circuit. Unless the parallel resistance is very low, there will be some ranges showing the 60mV/decade trend.

There are real advantages in having a DMM.

There is IMHO advantage in having *three* meters. B+, Vk, output AC all at once. Also to cross-check when one seems to be goofy. (Just two meters, hard to prove which is the goof.)

I acquired a fine VTVM before FET meters were reliable, so that is what I know. VTVMs *must* be wall-grounded for safety, and one of the first things I did with my VTVM was blow-out power to the whole office wing. Later FET meters are battery power (but beware of odd-size batts!) and can be used "floating" with less concern.

VTVMs can be found easily but WILL have trouble with tarnish on the switches. Almost always there was a C-cell battery leak and clean/replacement needed. (I finally fudged a DC rail off the 6VAC heater to power a 1.5V regulator.) B+ filtering is quite un-critical (a few had none!) but replace the 10uFd 150V cap with a 22u 200V part. Do NOT change the 12AU7 or 6AL6 unless it is downright dead. A well-aged tube is a treasure in a VTVM. I changed my 1971 tube in 1990 and then changed it back. Warm-up and idle drift was better on the "old" tube. Also if I was using it every week I left it on 24/7.

FET meters are newer but I guess they are already getting tarnish and old-cap troubles.

"High input impedance" is not an immediate advantage. VTVMs were typically 11Megs and DMMs are typically 10Megs. What "can" matter is that IF you get the original probes, many VTVMs put 1Meg IN the probe for DCV work, which isolates the circuit being probes from the AC loading of the probe cable. DMM suppliers have lost this wisdom. You could add your own 1Meg, but then you have to multiply readings by 11/10, which now is not just a calibration trim.

[thermionix]

I have a 1960s US Navy VTVM (VTMM?).  It worked pretty well when I first got it, but after years in the closet it needed a cap job, some tubes, oddball neon bulbs, and lots of wire insulation was dry rotting.  So I basically gutted it except for the milliammeter function, which is totally passive.  I swapped the fixed probe leads for banana jacks, and now it's what I use to bias amps, since the Fluke I had been using died.

[armillary]

I have recapped several VTVMs just because they are a cheap project and it's fun to see them working again. But I use a digital meter for day to day work.

Analog meters will let you see variables that a digital meter might not be quick enough to keep up with. But an oscilloscope will show them even better.

[Electron Tornado]

Thanks everyone for the feedback. It sounds like if I want a spare analog meter I don't need to spring for a VTVM, and I should be fine with a FET based meter. The next question is, how do I know if an analog meter is FET based?

My DMM is a Southwire 21010N that I bought to help with some furnace repair last year. Works OK on the work bench as well.

Here is a long, but detailed refurb and re-cal of an old VTVM:

[garcho]

+1 for oscilloscopes, other than i find myself wasting time staring at the screen like a caveman impressed with shiny rocks

[PRR]

> how do I know if an analog meter is FET based?

I eBayed a bit, and there does not seem to be a universal names for these.

Aside from the fact that the majority of sellers have very little clue what they are selling. (Goes with "unable to test"?)

I might try looking for "VTVM" and then finding ones that are smaller and do not have a line-cord. Of course the cord may have got cut-off. And some FET meters used 22V batteries or other oddities.

"FET VOM" is another more specific search term, but may not be well known. And it brings up all the passive VOMs.

Here is a tip. A passive Ohm-meter has "Infinite Ohms" at the same side as "Zero Volts". The basic VTVM (and FET clone) has Infinite Ohms over on the 1.5V side of the scale.

[armillary]

Interesting video on the VTVM restoration. It looked vaguely familiar, so I looked in the basement and brought this up:
https://www.dropbox.com/s/ajroevhrsr7usae/DSCN5927.JPG?dl=0

You can email me if you have any interest. 

[Electron Tornado]

armillary, PM sent.