15 Mar
2013
15 Mar
'13
10:02 p.m.
On 2013 Mar 15, at 9:28 PM, Josh Dersch wrote:
On the subject of tube radios, I picked up an RCA Victor BP-10
awhile back; this is a portable from about 1940 and it's actually
pretty darn tiny for a tube-based unit. (And it runs off a 63.5V B
battery (#467) and one D-sized 1.5V battery. Amazingly you can
still buy the 63.5V batteries new...). Anyway, I bring it up
because aside from its small form factor it has one other attribute
I've never seen in a tube radio -- it starts playing almost
immediately (within a second) of turning it on and the tubes never
seem to get warm at all. All the other tube units I've ever seen
take 10-20 seconds to warm up first. Makes me wonder a) what's
special about this unit and b) why other units didn't do the same
-- seems like a pretty useful feature.
There was a range of tubes built for 1.5V (and some 3V) filaments for
use in battery-operated receivers, both 'farm' radios (rural areas
that were late to get electricity) and portables, the tubes specially
designed for low power requirements, the 1.5V of course so the
filament can be operated off a single cell.
Partly they don't have to heat up as much (in the dark you can just
see the filament glowing a dull red, like that in VFD displays), but
I think the main factor in the quick on time is they were directly-
heated (the cathode is the filament), there was no separate cathode
to heat, and no waiting for thermal transfer from the filament to the
cathode.
Actually, to make this sort of vaguely on-topic, there may have been
some carry-over from the low-emission/low-power design lessons
learned from those tubes to the design of VFDs.