Necessary DVM features

[ricach]

I've come to realize my radio shack analog meter is not going to be good enough to work with tube amps. What features should I look for in a digital meter? - I don't want to spend more than necessary, but I also don't want to find out down the line of a convenient feature mine is lacking.

Any specific suggestions would be appreciated as well. Thanks.

[cbarrow7625]

Make sure that the DVM works over the full audio bandwidth!!! (20Hz - 20kHz). This is overlooked very often. Most DVM's are designed to measure power lines & not audio. They have a bandwith that only extends up to about 400Hz. Any readings above 400Hz will be erroneous (i.e., since the frequency response of the meter is falling off, anything above that will display lower on the meter than it actually is). That will lead you to make erroneous conclusions about when & why your amp is clipping. You measure 1.8VDC accurately between the grid & cathode and then inject a 1kHz tone into the amp and see only 1.7VAC and you assume that you are not overdriving the tube but in reality the real voltage is more like 2.1VAC @ 1kHz but your 400Hz meter is just giving you bogus readings. Never mind that your meter is also reading RMS and not peaks (it's the peaks that overdrive first).

Oh yeah, that's another thing (or two) you should have: 1.) Make sure it is a "True RMS" meter and 2.) Make sure you can also measure Peak AC voltage as well as RMS (that will save you from breaking out the calculator all the time to multiply the RMS by 1.414).

I'm a big fan of autoranging & auto shutoff too (that saves me more money in batteries than I will ever know).

This will not be a $30 - 40 meter. Good equipment always costs pretty good money.

The most important thing is the full audio bandwidth for measuring. I know fluke makes a few. You mayhave to go to their site to research them. I don't know that places like Mouser readily list that kind of data right in the catalog.

Good Luck.

Chris

[ricach]

Thanks for your input. Appears something like that is in the $300+ range (for Fluke anyway). I should mention that I do have a good Tektronix O-scope. How much of what you suggested can be covered by the scope?

[cbarrow7625]

It can all be covered by the scope. If you are only using your meter to measure DC than none of what I mentioned matters (except the auto-ranging & shutoff, of course).

I find it way quicker & easier to use a DVM for most things & use the scope only when I need to see a waveform.

Chris

[Alex R]

Well that's all very handy but you can dispense with audio range measurement (your scope can do that job) and even true RMS (I use an old VTVM to measure output VAC and work out wattage) and still have quite a useful tool for a lot less money. My list of absolute essentials in a meter would be shorter:

up to 1000Vdc
diode test function
up to 10Aac, preferably
accurate low-ohm measurement

There are some cheap unused ex-military Flukes around that are pretty good. I got off Ebay a new-in-box Fluke 25 for - I forget - about 20 UK pounds, and it is extremely useful and very robust. Even cheaper was on old racal-decca DMM which needs a mains supply - ok if it's on your bench all the time - with nice red LED display. So don't despair... unless you're designing and selling equipment you don't need all that much accuracy.

[Don Moose]

Be sure the PROBES are good to at least 600V (1kV is better).

Sometimes the meter is good to that voltage, but the probes are only good to 300V.

Hope this helps!

[Alex R]

Yeah absolutely - and in general it is I think worth paying the premium for a quality make like a Fluke, as they will have not have handy features like cheapo probes, too-easily-destroyed ammeter functions and so on. It's just that you don't need most of the fancy stuff they put on the new models if you're just looking at audio equipment, so you can save some cash that way.

[Enzo]

My everyday FLuke 75 hand meter is older than quite a few of our members here, and it serves me reliably day after day, fall after fall, abuse after abuse. I buy nice Fluke or other good leads when mine break. The $3 leads suck.

I want to enter a word of caution. COnsider your level when buying a meter. I don't need a laboratory instrument in my repair shop. If I need to know the onset of clipping, I use my scope, not my meter. My meter is rated up to about 400Hz, but it has always measured pretty close up to 1kHz, and I don't recall needing to meter something over that in any recent year.

What you need is a good quality meter with the basics. We do see voltages over 600 now and then, not very often though. 1000 volts will cover most needs, and frankly 600 volts covers most of it. Most of them come in under 1% accuracy anymore, don't pay extra for more. You won't really notice much difference between a 0.05% and a 0.08% meter, honest, and especially when the readings you take are by sticking your probe tips down into some ancient gobs of solder. If the extra accuracy comes with the model, no problem, just don't let THAT be the reason to spend and extra $100.

It is nice to have accurate low resistance readings, though most of the time when I am measuring fractional ohms it is just to see if things are open or not. Measuring a 0.33 ohm resistor when the probe tip contact resistance is twice that can be difficult enough. Not a deal breaker for a "first good meter," to me anyway.

Current. Most all of them handle that, but when I bought mine, the main difference between mine and the next lower model was the extra 10A scale. Oh how many times have I been glad I went for it. Meters that go only up to about 2A are nowhere near as versatile. 6VAC heaters can draw a ton more current that 2A. 10A 15A 20A, whatever the high end is these days, get plenty. I think most have a low end scale too, like 300ma on mine. I want that for shunt readings of output tube current. Trying to read 38ma on a scale that can't decide if that is 30 or 40 is no help. Make sure it has a low current scale in it. Of course, if nothing else, your own 1 ohm resistor makes the voltage scales into curren tmeters, so don't spend any extra for that. So there is some self-contradictory advice right there.

AC volts. Man i think most of them come "true RMS" these days. I guess if I were making power measurements of complex waveforms, I'd be more concerned. But keep in mind that your sine wave will be the same either way.

They love to add features. SO be it, just don't pay extra for them. I have a freq meter in my shop. In all my years fo bench service in pro audio, the ONLY think I ever use it for is this. When servicing a tape deck, I mount a test tape, connect the line out to my freq meter, then adjust capstan speed so the freq counter says it is the exact freq of the test tape. ABout the only other time I care about frequency is 60 vs 120 in ripple, and I can tell that by ear just listening.

Capacitance? WOn't tell you if they are leaky, but if the readings are way off, the cap is funny. Not a sale point for me. Might heelp identifying oddly marked or unmarked caps you find. Or to verify that you deciphered the value code properly - does that "820" mean 820pf or 82pf? Or measure the capacitance of a hunk of shielded wire.

Temperature? I already know tubes are hot. I don't really care what temp they are exactly. DOn't pay extra for it.

I think all of them have diode test these days, and you do want that. As to the transistor tester, no value to me, and I work on tons of solid state.

You can find a perfectly useful FLuke or other quality brand for under $200 still.

I have other fancier meters, even an HP on the bench, but my old Fluke 75 is what I reach for just about every time. SInce this is a first good meter purchase, I'd say stick to the basics. COmes a time in your life when you REALLY DO need a $400 meter, then you will be in a position to buy one, I'd bet. And that $189 FLuke you had been using up til then would still be perfectly useful every day.

[ricach]

Well like most purchases, I spend too much time researching and hemhawing around and as to date I have not purchased a meter yet. I did, however, get a Craftsman for Christmas from my wife. Silly her, doesn't she know it takes much more effort and research for such an important purchase? lol. So, I look at the specs anyway, and check out a couple reviews, and now I'm wondering why this $29 autoranging DMM won't do the trick for me? It's not True RMS, but when would I really need such a feature? Anybody see anything else in the specs that might be a problem for me down the line? Keep in mine I have an O-scope for anything I need true accuracy on.

(Second number is the resolution.)

DC Voltage (V DC)
400mV 0.1mV ±(0.5% reading + 2 digits)
4V 1mV ±(1.0% reading + 2 digits)
40V 10mV ±(1.0% reading + 2 digits)
400V 100mV ±(1.0% reading + 2 digits)
600V 1V ±(1.5% reading + 2 digits)

AC Voltage(V AC)
(40 - 400Hz)
400mV 0.1mV ±(2.0% reading + 30 digits)
4V 1mV ±(1.5% reading + 30 digits)
40V 10mV ±(1.5% reading + 30 digits)
400V 100mV ±(1.5% reading + 3 digits
600V 1V ±(2.0% reading + 4 digits)

DC Current (A DC)
400μA 0.1μA ±(1.5% reading DC Current + 3 digits)
4000μA 1μA ±(1.5% reading DC Current + 3 digits)
40mA 10μA ±(1.5% reading DC Current + 3 digits)
400mA 100μA ±(1.5% reading DC Current + 3 digits)
10A 10mA ±(2.5% reading + 5 digits)

AC Current (A AC)
(40 - 400Hz)
400μA 0.1μA ±(1.8% reading + 5 digits)
4000μA 1μA ±(1.8% reading + 5 digits)
40mA 10μA ±(1.8% reading + 5 digits)
400mA 100μA ±(1.8% reading + 5 digits)
10A 10mA ±(3.0% reading + 7 digits)

Resistance
400! 0.1! ±(1.2% reading + 4 digits)
4k! 1!±(1.2% reading + 2 digits)
40k! 10! ±(1.2% reading + 2 digits)
400k! 100! ±(1.2% reading + 2 digits)
4M! 1k! ±(1.2% reading + 2 digits)
40M! 10k! ±(2.0% reading + 3 digits)

Capacitance
4nF 1pF ±(5.0% reading + 10 digits)
40nF 10pF ±(5.0% reading + 7 digits)
400nF 0.1nF ±(3.5% reading + 5 digits)
4μF 1nF ±(3.5% reading + 5 digits)
40μF 10nF ±(3.5% reading + 5 digits)
200μF 0.1μF ±(5.0% reading + 5 digits)

NOTE: Accuracy specifications consist of two elements:
• (% reading) – This is the accuracy of the measurement circuit.
• (+ digits) – This is the accuracy of the analog to digital converter.
NOTE: Accuracy is stated at 65oF to 83oF (18oC to 28oC) and less than
70% RH.

Thanks.

[Alex R]

Well bless her generous heart. Probably it will be fine. Cheaper meters tend to have less good overload protection - be careful you have the right ranges engaged. The current ranges in particular may well just blow out the meter if you accidentally ground a high voltage through them - astonishingly easy to do inside a valve amp.

Have fun.

[Enzo]

It will be fine. As Alex said, maybe less protection from overloads, and maybe not as forgiving about being dropped onto the floor - a regular occurence for my Fluke.

You will now learn how to use a meter, and come time to buy a new one, you will have a much better idea of what you use, what you don't, and what you wish it included.

If you use sine waves, your non true RMS will work just fine on AC volts.

And I use my scope for rough voltage readings and viewing waveforms. If I want to know the output at the speaker, I will reach for my meter first, not the scope. 23.6VAC is not so easy to read off a scope screen.

[Ampclutz]

Harbor Freight sells meters on sale for less than $3. I would not want to have to use one of these for my only meter. But, I haven't killed one yet. I keep one in the car and one in the tool box. They will do a good job of checking continuity, wall voltage, diodes, battery voltage and a bunch of other stuff. And you don't risk having your good meter lost, stolen, or blown up because you won't really care and nobody will steal the cheap piece of crap. They even have back lit screen models for less than $4.

I recommend at least one IN ADDITION TO your good meter.

AC

[Enzo]

I am always eyeing those things. When I don't care about accuracy so much, it would be handy to have a couple secondary cheap meters.

For example, when I care that a voltage is present or not moreso than what its actual value is.

[Ampclutz]

I feel honored to be able to make a recommendation to Enzo! Next time you get near those get yourself some. They seem to be better than any analog meter I have had. And as a benefit the batteries are included...

AC

I am always eyeing those things. When I don't care about accuracy so much, it would be handy to have a couple secondary cheap meters.

For example, when I care that a voltage is present or not moreso than what its actual value is.

[Enzo]

I will, we have a HF here in town.

One reason I want to try them is so I can answer all the people who ask about them from time to time. You have no idea how many emails I get...