16 Mar
2013
16 Mar
'13
3:12 p.m.
Most all the the portable radios of this time used
thin directheated filame=
nts.Most table models used a secondary cathode because the filamentswere st=
rung together in series to run directly off the mains. Thiseliminated the n=
eed for a transformer.Most of the tube radios could also be run from the AC=
line.The filament voltage was created by a wasteful resistoror ballast tub=
e. Neither was optimal as it required thatthe filament be supplied with rec=
tified and filtered DC.The Selenium rectifiers of the day will be sure to f=
ail withuse. They need to be replaced with silicon diodes andin some cases =
a limiting resistor.In any case, the direct filament heats almost instantly=
.The indirect cathode requires more time to come up to temperature.The port=
able radio uses less output power as well usuallyusing a 3V4 output tube.Dw=
ight
Most AM portable sets obver here used 4 vavles. The signal valves
(frequency changer, IF amplfier, detector%1st audio) had 1.5V filamanet,s
originally at 50mA eachm, later 25mA. The otuput volve had 2 filamnet
sections that were connected. For batter-only sets all 5 filament
sections were connected in parellel and powered by a 1.5V battery (yes,
often several cells in parallel, so 'battery' is correct).
But we also had battery/mains sets. These normally put the 5 filament
sections in series and used a 75.V (!) LT battery. For mains, the HT
(90V) came form eitehr amins transofrmer and rectifier or a dropping
resistor and half-wave rectifier straight off the mains (no isolation).
The heater chain was then fed form thsi DC HT line viaa fat droppign
resisotr. Not exactly efficient, but...
One trap for the unwary is that when you put hte valve filaments in
series, the current throug them also includes the anode current.
Thereofre the volve at the earthy end of the chain is goig nto be carying
rather more filament current than it should do, since it will also have
to carry the anode current of all teh valves above it. The crue was to
fit shut reissotrs across the filaments to get the currents right again.
I seem to rememrb that if you ran a DL96 or 3V4 or... with the filimant
sections in seires you had to shunt the more -ve one (resisotr from the
filament tap to the -ve side fo the filament) due to the anode current of
this valve alone.
-tony