radiation to sustain the temperature constraint. This quickly led to the
realization that such valves would require enormous, impractical, glass bulbs.
‘Hard’ glasses such as ‘Pyrex’ were becoming available to replace ‘traditional’
lead glass, and these could withstand much higher temperatures without
gassing. However, this advantage was countered by the fact that hard glasses
were not so transparent to heat as lead glass.
Higher power also meant larger anodes, otherwise the excess (wasted) heat
would evaporate the (usually nickel) metal too rapidly causing blackening of
the bulb and raising the glass temperature above the safe limit. Later glass
valves employed molybdenum as anode which withstood higher temperatures
without undue evaporation and increased dissipation from *300 to (a still
modest) 600 W. Neither was the grid absolved from concern. Too great a rise
in temperature caused it to act as a filament giving rise itself to ‘primary’
electrons which once more destroy control of the valve.
Another important part of the valve was the filament, the intended source
of the ‘primary’ electrons and a component that influenced both the efficiency
and the lifetime of the valve. Filaments, transmitting or otherwise, were
usually made of tungsten, a metal with which Robert was familiar both from
his time at the ‘Auer’ and from his earliest days at the GEC. A heated
tungsten filament under vacuum has definite rates of electron emission (per
unit surface area) and of metal evaporation at any given temperature. If a
longer life was required for a given rate of emission, the total surface area of
the filament had to increase; this makes the filament larger taking it longer to
evaporate and at the same time increasing its emissivity. Equally, larger filaments consume more power and this had to balanced against the longer life
and reliability. Experience showed that filaments usually ‘burnt out’ when
evaporation has reduced its diameter by 10%. But as other factors are always
present to influence the life of the valve, it was pointless to increase the
filament life beyond a certain value. For the GEC, lifetimes between 3000
and 6000 h for glass transmitting valves were considered ‘economic’, but
these lifetimes were not particularly long, and if we were to parallel a large
number of such valves there was an unavoidable risk of failure and interruption of transmission service.
In terms of high-power radio frequency (RF) transmission therefore, the
decade quickly established an obvious fact for the GEC; an entirely different
type of valve had to be developed, one which would permit much higher
anode dissipation. To achieve this, a radical re-design was necessary, a design
in which the anode would have to be part of the glass envelope of the valve so
that it could be immersed in water, or in oil, or blown by air to facilitate a
very high surface heat dissipation. Thus, in 1923, the ‘cooled anode
15 The Post-war Years 1919–1930
351
realization that such valves would require enormous, impractical, glass bulbs.
‘Hard’ glasses such as ‘Pyrex’ were becoming available to replace ‘traditional’
lead glass, and these could withstand much higher temperatures without
gassing. However, this advantage was countered by the fact that hard glasses
were not so transparent to heat as lead glass.
Higher power also meant larger anodes, otherwise the excess (wasted) heat
would evaporate the (usually nickel) metal too rapidly causing blackening of
the bulb and raising the glass temperature above the safe limit. Later glass
valves employed molybdenum as anode which withstood higher temperatures
without undue evaporation and increased dissipation from *300 to (a still
modest) 600 W. Neither was the grid absolved from concern. Too great a rise
in temperature caused it to act as a filament giving rise itself to ‘primary’
electrons which once more destroy control of the valve.
Another important part of the valve was the filament, the intended source
of the ‘primary’ electrons and a component that influenced both the efficiency
and the lifetime of the valve. Filaments, transmitting or otherwise, were
usually made of tungsten, a metal with which Robert was familiar both from
his time at the ‘Auer’ and from his earliest days at the GEC. A heated
tungsten filament under vacuum has definite rates of electron emission (per
unit surface area) and of metal evaporation at any given temperature. If a
longer life was required for a given rate of emission, the total surface area of
the filament had to increase; this makes the filament larger taking it longer to
evaporate and at the same time increasing its emissivity. Equally, larger filaments consume more power and this had to balanced against the longer life
and reliability. Experience showed that filaments usually ‘burnt out’ when
evaporation has reduced its diameter by 10%. But as other factors are always
present to influence the life of the valve, it was pointless to increase the
filament life beyond a certain value. For the GEC, lifetimes between 3000
and 6000 h for glass transmitting valves were considered ‘economic’, but
these lifetimes were not particularly long, and if we were to parallel a large
number of such valves there was an unavoidable risk of failure and interruption of transmission service.
In terms of high-power radio frequency (RF) transmission therefore, the
decade quickly established an obvious fact for the GEC; an entirely different
type of valve had to be developed, one which would permit much higher
anode dissipation. To achieve this, a radical re-design was necessary, a design
in which the anode would have to be part of the glass envelope of the valve so
that it could be immersed in water, or in oil, or blown by air to facilitate a
very high surface heat dissipation. Thus, in 1923, the ‘cooled anode
15 The Post-war Years 1919–1930
351