Philips 2514

Below is the set of valves used in this model (all Mullard types)

 

S4V (tetrode)

 

PM24 (output triode)

 

154V (triode)

 

DW2 (rectifier)

 All these Mullard valve types were obsolete by 1932, such was the progress in development of new and more efficient types of valve. Considering the age of this example I'd have thought that newer types may have been fitted, however the set may have developed faults early in its life which rendered it not worth repairing and ended up in someone's loft for 60 years.

The valve on the left is a screened grid type, which in those days, constituted a really major breakthrough in valve design. It offered very high and stable gain compared with the old triode which had been the mainstay of receiver design for many years. The tetrode was almost as much a breakthrough as adding a control electrode to the diode had been years before, and was proclaimed as such in the names of sets of that era (see the "Valve line-ups for 1933 sets" in the "Really Useful Information" section where you will see references to "SG" or "screened" in some names).

Of course once the principle had been established grids were added willy nilly. Within a short space of time there was even a nonode valve with seven grids.

The shiny or "mirror" finish on a valve is a by-product of its production. Inside a valve was fitted a small amount of magnesium known as a "getter" and after a decent vacuum had been obtained from a pump, and the glass envelope had been sealed, the magnesium was vaporised, using RF energy, and adhered to the inside of the glass. This process used up any remaining non-inert gas in the envelope, producing a better vacuum, and a much longer lasting valve. If a valve is broken open, or loses its vacuum integrity, the metallic magnesium converts within seconds to magnesium oxide and the glass envelope clouds over with a white powder.

Another interesting effect can be seen with some valves. The speed of light in air is less than the speed of light in a vacuum and if a working high power valve is viewed in darkness a strange blue glow can be seen, where electrons missing the anode strike the glass. The glow is a side effect of the change of speed of the light. Old transmitter valves such as the 807 exhibit this effect quite well.

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