Curt,
Yes, that works but it will not save to memory. So, each time the DMM is
powered up, I would need to set the Fluke 8060A against the 1.07V reference.
And probably, the 189 may need a slightly different reference voltage for
best accuracy. It's an inconvenience, but it may be worth building a
precision voltage regulator in a small project box, powered from a 9V
battery. I have not found a precision low-voltage regulator that will
output below +1.24 V. I could use a pass transistor after the regulator,
fed from a pot as a voltage divider to get it adjustable down below +1.24V.
To invert the sign to achieve -1.07V, the leads would be reversed just for
calibration.
I've not had a good experience troubleshooting the grids of receiving tubes
when using a DMM. There's often enough distributed lead C that creates
measurement problems. I rarely run into this when using a VTVM and a
switchable 1-meg probe. Nothing wrong with using a VTVM or an FET VOM with
an isolated probe, but like my Keithley DMM, they're not as portable as I'd
like.
Paul, W9AC
----- Original Message -----
From: "Curt Nixon" <[email protected]>
To: <[email protected]>
Sent: Saturday, November 17, 2012 2:22 PM
Subject: Re: [Drakelist] VTVM Probe with DMM
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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