A bit of history
In the time that zener diodes didn’t exist, a gas filled voltage reference tube was used to get a stable voltage. Nowadays these are seldom used and that is why I dedicated this article to explain what different types there are and how they work.
Different types of the voltage reference tube
I will briefly describe the principle of a neon tube: In a small gas filled glass tube, 2 electrodes are placed opposite. Electrons and ions present in the tube get a certain speed on their way to the electrode. The speed is dependent on the applied tension and in case of the 85A2 the tension is 85V. As soon as the speed is reached, the electrons collide with other atoms and this results in more free electrons. The snowball effect will eventually result in a glowing light. The color of the light is dependent on the gas the tube is filled with. For neon, the color will be orange, for argon it will be blue.
In case of the 85A2 the tension upon the tube will remain constant. This is only applicable when the current remains between certain margins. Each tube has a certain kindling tension to start the snowball effect. When the amplifier is switched on, it will take 5 to 10 seconds (this can be type and brand dependent) before the tube flashes. Then it will shine a glowing light. This kindling tension is higher then the stabilized tension (125V for the 85A2).
For these kind of tubes, it is important that the current is held between the max. and min. working point with a resistor. These neon tubes are still easy obtainable and the lifetime is very long. A spin-off is the magnificent visual effect when the little tube is glowing on top of your amp.
Connecting a stabilization tube is simple. Even though it has 7 or more pens, internal it has only 2 connections: the anode and the cathode.