Hickok tube testers have used lamps in their tube testers since the late 1920's. It is very important to use the correct lamp number/s in your tube tester to insure the intended circuit protection (fuses). Not only that but for the correct test readings. These tube testers are not laboratory instruments and offer no voltage regulation. Using incorrect lamps and the Solid State replacement rectifiers will alter the Mutual Conductance readings.

Here's why; (1) -read why the solid state rectifier in your Hickok is not a good idea. (2) the lamp used for the main circuit protection fuse is in the power transformer primary. If the lamp draws too little or too much current, the Mutual Conductance reading will vary accordingly. Not only that but if the lamp's current draw is too high, little power transformer protection is less. using a higher current lamp is like using a larger fuse. (3) the #49 lamp is used in the bias circuit to protect the bias adjustment potentiometer. It's most important not to use any other number lamp as it only takes a moment to fry the bias pot due to a shorted tube.