> ... ... there are NO reasons to buy an analogue test meter.
I persist in insisting that a tube-amp hacker "should" have a VTVM (or good FET-VM).
Terminology:
> VTVMs or VOMs simply used a valve to increase the input impedance of an analogue meter movement
Term-useage changes. But in my day, a "VOM" was a passive meter, a "VTVM" had a tube and wanted you to know it.
> and iron out the non-linearities inherent in these.
Not sure what you mean here. The Volt, Amps, and Ohms ranges of a classic VTVM did nothing about linearity. Meter movements are pretty linear. Not exactly; but when H-P wanted to improve accuracy beyond what their meter-movement would give, they implemented a system to custom-engrave the meter scale for the movement it was mounted on. The only "linearity" issue the VTVM addresses is low-level AC voltage: vacuum diodes are a little smoother than copper-oxide, and even so the lowest-ACV range used a custom scale.
> voltage readings on older circuit schematics were most likely taken with a meter which had an input impedance of 20,000 Ohms-per-volt.
It varies. Older lower-tech gear sometimes was marked for 20K/V passive meters (VTVMs are not handy on a service call). Most Fenders are marked for 5Meg or 10Meg meters. Note though that a 20K/V passive meter on the 500V range is 20K*500= 10,000K= 10Meg, same as a VTVM.
> their ability to indicate slowly fluctuating readings,
As you say, this can be super-useful around tremolo circuits, or motorboating, which completely baffle most DVMs.
They also twitch on sudden change, which is sometimes good to know if a point is supposed to be steady.
> Modern digital test meters, even cheap ones, typically have input impedances of 10Mohms or greater
They emulate the old 10-Meg VTVM custom.
> far greater accuracy
So? Vacuum-tube audio amps are not precision systems.
An audio voltage-amplifier plate typically sits at 50% to 75% of the stage supply voltage. If it is ANYwhere in that zone, it is probably not your problem. With a VTVM, you tap the B+ and the plate, look at the relative needle-angles. A fast VTVM will take both readings close-enough quicker than a DVM will auto-range one point, and then you have to turn two numbers into a ratio to know if it is roughly-right.
It can be too easy to obsess about "197.3V" versus "207.6V" differences on a DMM. A needle reminds you that everything is relative and approximate.
> virually all DMM's nowadays have diode junction testing capability
Almost any -analog- ohm meter can rough-check diodes. Yes, a good DMM is better for this... I'm not arguing against owning a basic DMM, I own(ed) 5 or 6.
And between chip technology and modern safety concerns, many DMMs have shockingly low "maximum voltage", often as low as 600V. Marginal for big tube work. Yes, the "600V max" may be more CYA legal paperwork than actual limit. OTOH, digi-meters die "for no reason", and over-voltage into a teensy junction may be the reason. Most VTVMs have a 1,500V range. Sometimes you wonder if the jack and first resistor are really safe at this level. And I have blown-out the ACV coupling-cap a few times. But they were meant to be used around tube voltages.
There absolutely are good reasons to own DMMs. It FLOATS, no ground reference. You can't bang the needle (a good VTVM won't melt a meter, but a VOM sure can). If you need to know 178.9V within 0.1%, a VTVM will only say 180V +/-5V (2% at best; 0.5% on some H-P). Some diode-checks will read junctions in the presence of other parts without human interpretation (which may be good or bad). They are now cheap enough (I remember when they cost a week's pay) to own several and monitor many points or to cross-check against each other.
Topic: Analog Voltmeter? (Read 14037 times)