electrons

The electron tube is a vacuum element using electron emission to operate, Its main electrodes are the cathode, heated by an electric current, and the anode. The cathodes are of two kinds, directly heated and indirectly heated. The directly heated cathode consists of a filament on which is deposited an emissive layer; usually a mixture of barium and strontium oxides (Figure 25).

An indirectly fired cathode has an emissive layer deposited on a nickel tube, which is annealed by a tungsten filament insulated with alumina (Figure 26).

The anode surrounds the cathode and is usually cylindrical in shape; it is made of nickel or iron plate, or nickel mesh. The cathode is connected to the negative pole of the source, the anode usually through a suitable impedance to the positive pole of the source.

The electrodes are in a glass (or also metal or ceramic bulb) which is either depleted or filled with a dilute gas (e.g. neon) or metal vapour (mercury or celsius). In order to achieve the best possible vacuum, a tube (steel, nickel) with a getter is placed inside the flask, which contains usually barium with an addition of aluminium or magnesium or both at the same time. After depletion, the getter tube, forming a short thread, is heated by an induced high-frequency current to red This vaporizes the getter, absorbs the gas residue and condenses on the wall of the flask to form a metallic coating. The cathodes of most vacuum tubes are heated by an electric current, the magnitude or size of which of the glow voltage is indicated by the first of a group of characters (letter or number) indicating the type of tube.

In the middle and on the left is the electron tube meter (on the right is the meter I used when making the boxes)

A few collection electronics to show you


Tube amplifier

Vintage Philips tube radio.