Here is the complete schematic diagram of the Franklin English Reading Tube Checker. The 3000 - ohm variable resistance just below the line-voltage meter operates in conjunction with the knob and scale on the front panel of the instrument. There is a particular adjustment for each type of tube

Above: Schematic diagram of the Franklin H - 33 Tube Checker. Note that provisions are made for resistance and continuity testing. Below: Panel view of the Franklin D 33 Tube Checker

Complete schematic diagram of the Franklin Model 1 Volt - Ohmmeter