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Many of today’s tube ‘experts’ do not seem to own a VTVM, and I am always puzzled by this.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!In the scenario above, we see where a VTVM performs a function with a visual panache that no DMM can even dream of doing.If you want to be serious about servicing vintage tube amplifiers, you should make a routine habit of checking any coupling capacitors for leakage.Some very old VTVM’s I have use a 6SN7, but these are few and far between.The biggest benefit here is that the meter movement cannot ‘load down’ the circuit you are trying to measure.

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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.Now the voltage divider action results in a voltage reading of 49VDC, which is pretty close to the ‘expected’ 50VDC.Hopefully, you can understand how much this accuracy was appreciated some sixty years ago.Next, use your isolation probe, and set your VTVM DC voltage scale to ‘150’.

Connect the isolation probe to the ‘open’ end of the coupling capacitor, and the ground clip to the chassis.

Today’s DMM can achieve comparable accuracy, yet there are still instances where a VTVM is the only meter to have!