I have a Heathkit Model IM-13 Vacuum Tube Voltmeter (VTVM) and the Model 336 high voltage probe. These instruments were sold by Heathkit in the mid-1960's. The VTVM is the "service bench" version that has a large 6-inch meter. The VTVM measures dc and ac volts and ohms. It has a very high input impedance of 11 meg-ohms (1 meg-ohm in the probe). The circuit used in the meter to make dc measurements is a balanced bridge (push-pull) using a twin triode. For ac measurements, the meter uses a half-wave voltage doubler using a twin diode. I have the probe for this meter. The probe is special to the Heathkit VTVMs in that it has a switch for ac/ohms and dc measurements. The switch removes the 1-megohm resistor in the probe from the circuit when measuring ac or ohms. I calibrated the meter using my Fluke 87 multimeter and the Heathkit meter reads very close to the Fluke multimeter.
A description and specifications for the meter from the 1966 Heathkit catalog is shown below.
I also have the Model 336 high voltage dc probe for the Heathkit VTVMs. This probe is the large red probe in the picture above. The probe can measure up to 30,000 Volts. A description of this probe and its specifications from the 1966 Heathkit catalog is shown below.
