VTVM and TVM's

[Dawn]

With the recent threads on multimeters and voms, you just know that there had to be an equivalent on electronic analog voltmeters. Back in my early days during the mid 60's and early 70's when I first started in this hobby, a VTVM kit was among the 2nd or 3rd pieces of test equipment that would have been sought for a basic bench after initially acquiring a hobby grade VOM. The 70's brought in FET and transistor based VOM's at the kit/hobbly level. Many of them nightmares in the short run and unable to deliver the reliability of an 20 y/o second hand VTVM. There were many things a VOM just couldn't do and nor could a latter DMM with respect to a VTVM. To this day, a VTVM does have uses that are not duplicated in a DMM. It probably would be fair to say that they are irrelevant nowadays with the current crop of electronics and certainly no place in a development laboratory. Yet in service, there are still applications they are used for occasionally and not easily substituted and merit being kept on a shelf if needed. Some applications that they excelled in were:

Most all could bias the movement for a zero center.
High frequency response up to several meg AC
Capable of reading very low ohm values right down to emitter resistors that regular DMM's cannot.
Higher end volt ranges often to several thousand volts directly.
Isolation resistor in the probe and tube amplifier gave a very high impedance that didn't disrupt circuit operation or detune it.
Like 20k/ohm voltmeters, were standardized instruments that procedures and reference voltages were documented in older service literature.

There are others, but suffice to say that if you're working in a service vocation, inevitably a VTVM would make your life easier at times.

Regarding the TVM's that never seemed to succeed the VTVM's. One of their worst malady's was the presence of RF. Sometime even probing around and oscillator would drive the meter full scale. That forcible "thwack" any time a transmitter was energized or around powerful oscillator circuits made many of them undesirable for service. While there were many TVM's that were designed to be RF proof, most were service instruments designed by and unique to the trade. Alignment was touchy and could vary with battery or supply voltage and short time ageing. Thankfully, DMM's at hobby and service prices began to make their debut shortly afterwards.