Hi Paul. Wouldn't it be the same thing if you set-up a reference
voltage that is -1.07V or whatever the drop is and then pushed the
relative button?
If all you are trying to do is improve upon the input Z of the DMM, , my
opinion is that I have never seen a measurement in troubleshooting any
tube set that could not be done with a modern DMM. At 10M inputZ, it
isn't going to do much in the way of load.
Using one for an RF probe is slightly more of an issue but with a
properly built RF probe, it still is no real issue.
Perhaps I have misunderstood what and why you are trying to acheive.
FWIW
Curt
KU8L
On 11/17/2012 12:29 PM, Paul Christensen wrote:
I finally got around to trying a good 'ole fashion VTVM probe with a
Fluke DMM. Recall that most VTVM probes have a switch that allows for
either DC Volts in one position, then AC/Ohms/mA in the other
position. The DC position typically has a 1-meg isolation resistor,
highly useful for VT grid measurements. Without value compensation,
a modern DMM cannot use such a probe and is otherwise useless for
serious tube receiver work.
I took a Simpson VTVM probe with a BNC connector and connected it to a
Pomona BNC-to-Banana adapter with standard 3/4" centers. With the
adapter, the probe easily connects to a DMM. My first measurement was
a precise +12V source. When using the VTVM probe in the DC position
(series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is
being dropped across the 1-meg resistor. Assuming the resistor is
close to 1-meg in value, the input Z of my Fluke 8060A calculates to
10.215 meg. Essentially, a 10:1 voltage divider is being created
between the 1-meg iso-resistor, and the internal Z of the DMM. The
drop is creating the value discrepancy. VTVMs are compensated in
design and manufacture for this.
I have several Fluke DDMs, including 8060A (my favorite DMM), and an
advanced model 189. However, peering through the manuals, I see no
setup routine to create a user-defined DC offset. What I want is the
ability to measure a precise DC voltage, then enter a menu that allows
me to assign a new display value to compensate for the voltage drop
across the iso-resistor. I have a lab-grade Keithley bench-type DMM
that does allow for such an offset, but hauling it around is a pain.
So, does anyone know of a DMM that allows for DC voltage offset? This
is different than the "Relative" button seen on many DMMs. Relative
is used to "zero the display" for any input value. I want the same
thing but instead of zero, assign a new value of my choice.
Paul, W9AC
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