Dennis,
Many thanks. I'll give that a try once I find a set of 100x probes. Darn, I
didn't know 100x probes even existed for scopes as I've only worked with 10x
types for the past 30 years!
Paul, W9AC
----- Original Message -----
From: "Dennis Monticelli" <dennis.montice...@gmail.com>
To: "Paul Christensen" <w...@arrl.net>
Cc: <drakelist@zerobeat.net>
Sent: Wednesday, November 21, 2012 3:38 AM
Subject: Re: [Drakelist] VTVM Probe with DMM
Paul,
The best probe I have found for the purpose are the old 100X scope
probes. They have a very low capacitance on the order of 2.5pF in
parallel with 10Meg so circuit loading is minimal. Most have extra
voltage capability which is handy for working around transmitters.
While designed for AC & DC measurements via scopes, you can use them
accurately with DMM's because the 100X divider swamps out the loading
effect of the DMM. The numerical accuracy of the DMM compensates for
the high voltage division ratio.
Dennis AE6C
On 11/17/12, Paul Christensen <w...@arrl.net> 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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