On 2009-08-22, Bruce L Bergman wrote:
On Fri, 21 Aug 2009 14:48:23 -0400, jeff_wisnia
wrote:
Jon Elson wrote:
[ ... ]
My God, what COULD you run off DC back then?
Light bulbs fer sure. G
Anything that was a straight resistive load works on DC - toasters,
hot-plates, coffeepots.
Now if the hotplate has a thermostat and that switch wasn't rated to
break DC, you could have some excitement...
Well ... hotplates were purely resistive loads, so you would
have the excitement that breaking an inductive load could produce. The
thermostats typically had fast click action, so the arc should be rather
quickly self quenching.
[ ... ]
Now they could use a Self-Winding style clock that has a universal
motor to wind a spring for a mechanical escapement movement, but that
would tend to be inaccurate. And those movements made noise when they
rewound every so often, which would be as loud as heck in a silent
room at 3 AM.
I remember a car which had that -- a loud KLUNK ever ten minutes
or so.
I guess smaller TVs and table radios would
run on DC, but that might be about it!
Many small radios back then were referred to by the cognizanti as "AC/DC
Sets". They had no power transformers in them and they used tubes with
various filament voltages like 12, 35 and 50. The typical 5 tube set
would run all the tube filaments in series from 120 volts (AC or DC, it
didn't matter which.) I can't recall whether the line cord was polarity
sensitive and had to be plugged in the "right way" to get electrons to
flow through the rectifier tube (usually a 35Z5 ?). Perhaps some antique
radio buff will check in here on that.
That's the classic "5-Tube AC/DC Superhet (-rodyne) Radio" design.
I owned several through the years. You can Google up more
information using that term than you can handle - they even had AGC on
the incoming signal.
An amazing amount done with very few tubes.
The trick to putting all the filaments in series is they all needed a
common filament /current/ so they all worked in series.
I've seen some with ballast resistors so the total filament
voltage need not reach the full 110 V. (120 V was fairly uncommon then.
Over the years it has marched from 110 to 115, 117, and now 120.
Squeeze a bit more power through the wires at a given current.
And the other
trick was not needing more than 120V B+ plate voltage, so they didn't
need a high voltage section that requires a vibrator or dynamotor for
DC. They developed a set of tubes specifically for the usage.
Yes -- a voltage doubler isn't going to work without AC -- let
alone a transformer. :-)
But, I don't recall ever seeing a TV which would work off DC line voltage.
Not without a vibrator or a dyanmotor (or a power-transistor driven
Inverter or DC-DC Converter in new gear) you wouldn't - you need a few
thousand volts of B+ to get the electron stram to go from the gun
through the picture tube to light the phosphors...
But that was produced by an inverter of sorts anyway. The
flyback transformer (which produced the sawtooth wave for the yoke to
scan the beam across the CRT) also was a very efficient high voltage
step-up transformer. Yes, it took yet another tube to rectify the high
voltage. It's filament was run from a single turn around the flyback
transformer at the high voltage end of the winding.
And AFAIK all the old sets have used the line frerquency as a
reference to get the Vertical and Horizontal Hold cues from. They
added the Colorburst crystal to get the timing for color TV, but the
utility AC frequency still gave a nice stable sync reference.
Yes -- that helped. But I have used B&W monitors powered from
12V DC -- (transistors of course) so tube based 120 VDC should not be
that difficult to achieve even with tubes.
And for the extra tube count -- after you hit the limit of 6V
filaments in series (20 tubes) you could always set up a second parallel
filament string of half the tubes.
The main thing back then was that transformers were cheaper than
trying to design a tube system which would run from 110 VDC and be
stable through the gain drift as the tubes aged. The design was simpler
with transformers. :-)
Enjoy,
DoN.
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