The picture displays the tube tester with its mesurement indicator, a tester. In the tester scale there is a sector marked as "GOOD" "?" "NO GOOD" wich is useful to verify the tube under test.

Main control panel of the instrument.

Closeup picture of the scale where is possible to check the tube status (go/?/nogo)

Detailed view of the inductor wich has been added to the circuitry to avoid auto-oscillations. This tube tester was build by myself about in the '70, it was sold by "Scuola Radio Elettra".

More details HERE , with the schematic diagram that explains inside a red squares the modifications required.

Image of the voltmeter to be used with this tube tester.

Internal view of the voltmeter PC board.

View of the tube tester during an heather glow check. With this instruments is possible to check some important parameters of the tubes.