I remember some "universal" battery-line
sets that used a 117Z3
rectifier to drop the line voltage and also to supply the filament
voltage for battery sets when run from the AC line. This was before
selenium rectifiers became common.
Yes, those directly-heated valves needed a DC fillament supply and before
metal (or semiconductor) rectifiers became common. about the only way to
get that from the mains was to use a dropping resistor from the HT (B+) line.
At least one set over here used a mains transformer and a full-wave valve
rectifier (I am thinking 'EZ40', which has a 6.3V heater and a B8A
'rimlock' base) to provide the HT (about 90V), with a series string of
filaments (7.5V totla) run from that via a dropping resistor in mains
mode. On batteries, of coursem it used separate 90V and 7.5V (!) dry
batteries.
Did it have haedphones as the audio output
devices? If so, it only
takes one little insulation breakdown to turn it into a fair version
of the electric chair...
Yes, but it used a resistor as a plate load and a capacitor coupled
to the headphones. An interstage AF transformer would have been
much safer--but again, cost was an object.
Well, as you well know by now, I do feel that many safety precautions are
way OTT< but I am not going to wear headphones driectly connected to a
hot chassis device like that. Yes, a transformer may be expensive, but I
think it's cheaper than a funeral!
If you were any kind of a scavenger, getting on the
air meant at most
finding an old AM radio or two and a TV set. You could go to a swap
Presumavly you wanted the line output valve ('sweep tube'?) from the
latter to use as the transmitter PA.
meet for transmitter crystals or beg one. A 6v 150 ma
pilot lamp
would serve as an adquate plate current indicator. Coils could be
easily hand-wound.
You mean it's all changed now? :-(
-tony