Vacuum Tube VoltMeter 1/1965
Vacuum tube voltmeters (VTVM) were once what digital multimeters (DMM) are now. Very versatile multi tools for electronic professionals and hobbyist alike. Of course, time and technology have moved on and tools, uses and requirements of today are very different from what they were back in 1960's. In everyday use modern digital multimeter will do everything VTVM can do and much more, but there are still some very specialized uses where old tube meters are preferred. If you would like to align an old AM-radio, this is still a great tool for that.
Vacuum tube voltmeters have been very common and popular do it yourself projects and kits in the past. Heathkit released several different models over the years, and you can still find them on ebay. Just remember: those were kits! Build quality may vary. Some factory build units like AVO-meters on the other hand are very good quality, desirable and very expensive. Most of us don't need anything like that, so it's still a good idea to build your own. This project describes the most basic meter with basic functions with two different probes. Yes, probes are important part of the meter!
While this project is very simple, it requires some uncommon components. Meter is a analog panel meter with 50uA full scale deflection (FSD). These are available online, but the quality may vary. I used an old unknown type Russian meter and I adjusted other component values to suit. It was not my best decision. You will also need a 22.5V battery. You can make this yourself with fifteen 1.5V LR44 batteries and heat shrink tubing. Heater voltage is 1.25V and don't exceed it! Use one Ni-Mh cell. And then there is the all-important tube! This meter uses a subminiature tube type CK6088. This is a tiny pentode tube still available online. I bet other tubes will work too. I should say almost any subminiature types will work, pentodes or triodes. Experiment!
This is a very simple build and nothing is really critical. Also, we have a benefit of modern tools and for example calibration is super simple with lab power supply. But like I said before, probes are must with this unit. You can build probes into an old sharpies or other plastic tubes. Use Google for inspiration! This is another fun and an unusual project. If you have an old tube radio you can align it perfectly with this meter and some simple RF generator. Or use this as a DC-voltmeter for that authentic vintage feel. We are very privileged to have modern DMMs!