This is still significantly higher than the typical 20,000 ohms-per-volt of analog VOM’s of the day.
What is further done usually is to use an isolation probe, where a 1Meg resistor is soldered into the probe casing.
In practice, these two triodes are a single 12AU7, which is the most common tube to be seen in every VTVM.
Some very old VTVM’s I have use a 6SN7, but these are few and far between.
To be sure, there are many instances where a good digital multi-meter is actually preferred, but a simple, old-fashioned VTVM can help maintain your tube amplifier in ways any Fluke simply doesn’t measure up to (pun intended).
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To give you a basic understanding of what a VTVM is, and why it is so valuable, let’s start with a crash course. At the end of the ‘Lesson’ I will impart with a book list that can help you become a better technician in no time flat!
Just as the name implies, VTVM is the acronym for Vacuum Tube Volt Meter.
This isolates the low-resistance meter movement from the circuit being measured.
The meter movement is located in the cathode of the bridge circuit, and the differential voltage is displayed.