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).The input impedance of analog VOM’s and the slow response of DMM’s makes the test shown above much more difficult to perform.You can check for leakage faster and with a visual indicator using a VTVM, and it is a worthwhile test.The plate supply is 100VDC, and we wish to measure the voltage at the plate itself, with a 500K (designated above as .5 Meg) plate ‘load’ resistor.Using a VOM with an input resistance of 100K, the voltage divider action yields a voltage measurement of 14.3VDC!
Hopefully, you can understand how much this accuracy was appreciated some sixty years ago.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.Some very old VTVM’s I have use a 6SN7, but these are few and far between.