Portable Tube Tester for Directly Heated Tubes
(Source JC Jardine)
Indispensable device for testing both filament continuity
and the existence of an anode current in European tubes. Indeed,
the simple Ωhmic test of the filament on our old triodes or double-grid tubes
(1920 ~ 1930) is not sufficient for the tube to be usable. This
pocket tube tester allows me, during my bargain hunts, to test a tube
before purchase. The range of the milliammeter must be at least 500µA.
Calibration will be done either by comparative tests of a batch of tubes, or
by mounting a tube on a measuring bench and comparing.
The principle diagram
The LM317 voltage regulator is very easy to find. It is designed to provide a heating voltage of 4 V. It is advisable to power old European tubes with a voltage lower than 4V. Here the chosen voltage is 2.7 V.
Instructions for use
The heating voltage is regulated by the LM 317 circuit. It is 2.75 V for the resistor pair 680 / 820 Ω. This allows testing tubes heated under 3 to 4 V. The adjustable 27 KΩ resistor in position 1, to have the maximum deflection when the two F terminals are connected.Position 2 allows testing the cathode emission of the filament. When this is zero or weak: the tube is "pumped" and is no longer good for service. Reference tubes (pumped and in good condition) must be available and operate by comparison.
In position 1, by connecting two cords to the F terminals, the device transforms into an ohmmeter to test the coils of headphones or AF transformers.
Pinouts
To test "double-grid" tubes, a socket must be added (S5 in the photo) with G, G' and P connected.During my treasure hunts (flea markets and garage sales), I always have this small tool with me.
Europe 4 and 5 pin sockets
