We found in an old 1968 document an interesting megohmmeter (insulation meter) that makes use of two 12AU7 tubes and a voltage regulating tube OB2 to 106 V.
The circuit 'can measure resistances up to near 100 M ohm using as an indicator an analog milliammeter of 1 mA full scale.
In this circuit the resistors of the input divider network R1 to R4 must be precision resistors. The circuit has the indicated voltages and critical components, plus the valve is the power transformer and the filter shock.
For the first there are alternatives such as the sources we described in our article Sources for Valves and for the shock can be used the primary of a small transformer of 110 V / 6 V x 250 mA or 300 mA.
We observed that, because it is a measurement circuit, the insulation of the network is important and solutions that use a self-transformer must be avoided.
The source diode can be 1N4004 or 1N4007. In figure 1 we have the complete diagram of the apparatus.
