Gosh, it's great to learn there are other survivors who remember tubes!
I saw a sample of undersea cable and a repeater at Murray Hill in 1965.
I think the loss between repeaters was 60 dB. Is that correct?
Most filaments were 6.3volts. Even 12 Volt tubes like the 12AU7, 12AT7, etc.
had center taped filaments and were usually powerd by 6.3 Volts. Higher
voltages were popular for line operated radios where the filiments were all
in series adding up to 121 volts, no transformer needed. Five tube sets had
50, 35 and 3, 12 volt filaments in series. Other voltages were available for
battery operated and specialty equipment as somebody mentioned.
The five volt rectifier was a special case requiring five volts at high
current isolated from the other voltages, 6.3V etc. Isolation was required
because these were directly heated cathodes where the filament was often
several hundred volts above ground on the rectifier cathode. This could not
be connected to other filament circuits which were near ground potential.
They always had their own insulated transformer winding. The voltage was
low, five volts because the filaments were thick and had low resistance, but
the current was high. Smaller, receiving tube filaments used thin tungsten
wire in a metal sleeve that formed the cathode. The thin wires were higher
resistance and also lower power than the rectifiers. These usually ran at
near ground potential to keep the filament to cathode voltage a low as
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