Carbon dating calibration
The meter movement is located in the cathode of the bridge circuit, and the differential voltage is displayed.
In practice, these two triodes are a single 12AU7, which is the most common tube to be seen in every VTVM.
Asking many old-time tube technicians what one piece of test equipment they simply cannot be without, the unanimous answer is the VTVM.
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).
Older paper coupling capacitors always get leaky over time, affecting the tone to the point that your Internet ‘guru’ espouses using only paper capacitors for ‘that sound’. Assuming your amplifier is not sounding as it should, you measure for any DC voltage at pin #2 and pin #7 of every 12AX7. This is definitely wrong, and we can confirm the problem thusly.
He has no idea what the heck is going on, and gives the ‘credit’ for this ‘sound’ that modern amplifiers cannot achieve (with ‘good’ capacitors) to the leaky capacitor. First, disconnect the end of the capacitor that was connected to the ‘positive’ grid.
In this example, the sensitivity is 10Meg on the 1-volt range, and 100K on the 100-volt range.
The biggest benefit here is that the meter movement cannot ‘load down’ the circuit you are trying to measure.If this high input impedance were not enough of a benefit, note that the voltage being measured is amplified by the two triode tubes.This isolates the low-resistance meter movement from the circuit being measured.On the outside, the meter looks like a typical old-fashioned analog meter. Despite the differences in outward appearance, probably 99% of VTVM’s have the identical circuit inside the cabinet.
The heart of every VTVM is the balanced bridge circuit. ), and the execution of this basic principle does not seem to have changed. You should note that the input resistance is constant though out the range switch.Just as the name implies, VTVM is the acronym for Vacuum Tube Volt Meter.Therefore, as expected, there are honest-to-God vacuum tubes inside your VTVM.In the scenario above, we see where a VTVM performs a function with a visual panache that no DMM can even dream of doing.