In article ,
"Asimov" wrote:

" bravely wrote to "All" (08 Apr 05 04:50:09)
--- on the heady topic of "VTVM versus new DVM"

bo From:
bo Xref: aeinews sci.electronics.repair:45265

bo Hi,
bo I am doing a tune up on an old turntable I have and according to the
bo service manual I need to make measurements with a VTVM (vacuum-tube
bo voltmeter). I own a Fluke 111 DVM, is there any reason why this can
bo not work in place of the VTVM.


JR,

The requirement to use a VTVM was given because its input is 10 Meg
ohms or more and so it wouldn't affect the circuit voltages. A DMM
typically meets a 10 Meg ohm requirement and is equivalent to a VTVM.
Rare VTVM's used a 100 Meg ohm input but the standard was 10 Meg.

Most "shop quality" VTVMs had 11 Meg DC and 1 Meg AC input impedance,
but there's another issue that could be much more of a problem.

When used for AC, all meter circuits *measure* some attribute of the
signal, and *indicate* an attribute which may or may not be the same one.

A D'Arsonval meter (Simpson 260, say) is average measuring, RMS
indicating.

A VTVM is most likely peak measuring, RMS indicating (a good VTVM will
have a "peak" or "peak-to-peak" scale, too).

A DVM can be anything, including RMS measuring, RMS indicating (and that
was *very* rare prior to DVMs.) Check the user's manual.

As long as the waveform you're measuring is pretty close to a sine wave,
the indicated voltage won't be too far wrong, but if the signal is very
"spiky", the different meters will give very different readings --
perhaps *none* of them very significant. Use a 'scope if you don't know
what to expect.

Isaac