2 Feb
2011
2 Feb
'11
10:38 a.m.
There was a brief time when AC/DC/battery radios were
offered that
could be switched between line and battery operation. A 117Z3 or
similar rectifier was used to power both the filaments and as a plate
supply for the low-voltage tubes in the receiver section. Later
versions used a selenium rectifier instead of the 117Z3.
Yes, we had those over here too...
The battery-only radios would use 4 valves. The later series (25mA
filaments) were DK96 (pentagrid frequency changer), DF96 (IF amplifier
pentode), DAF96 (detecotr diopde and audio pentode) and DL96 (output
pentode). The last had a pair of filaments that could be wired in series
or parallel. Theser were directly heated valves, 1.4V per filament.
In battery-only sets. all 5 filament secions were wiredf in parallel
normally. The LT (A) supply was a 1.5V battery with 2 or 4 cells in
parallel. The HT (B+) supply was a 90V (noemally) layer battery.
The filaments had to be run off DC of course. So the mains/DC models put
all the filaments in series for a total of 7.5V, and put baalsat
resistor around bits fo the chain to compensate for the fact that in a
directly heated valve the anode currnet passes through the filament (and
with such low filament currents it did matter). For abttery operation,
the LT supply was a 7.5V dry battery. For mains operation, the filament
string was run throug ha dropping resistor from the HT+ line, and the
latter was nomrally supplied from the mains via a dropping resistor and
selenium rectifier.
There were exceptions. I have an old Vidor battey/mains portable that
does much the smae thing with the filaments (in series, run off the HT+
line in mains mode), but the HT comes from a trnasofemr and valve (EZ40
IIRC) rectifier. Tht at leat means it's not live-chassis.
-tony