A lot of transformerless tube radios were sold as AC/DC, and wouldn't
have worked if it was a Phil claims. You just had to make sure the
power plug was inserted the right way, or you got no B+ for the tubes.
Even if the dc was in pulses, the radio would work. When plugged in the
right way, the current would still flow through the rectifier tube and
be filtered by the same filter used for ac operation.
I had a portable radio with a hand crank generator. The generator is
nothing more than an ordinary Mabuchi DC motor as you'd find in
motorized toys. Spin the motor and you get something resembling DC out
which is sufficient to charge a small NiCd cell as well as power the
radio even if the cell is completely dead. A generator might not provide
absolutely clean pure DC but I'm pretty sure they could have made it
work. Cars used generators up until at least the 1960s, they made radios
work in those too.
This would all be easy enough to test. Take a mains voltage permanent
magnet motor out of something like a string trimmer (weed eater) and
spin it with another motor to get around 110V out of it, then connect
that to a transformerless radio, lots of those around still. Or heck,
just scope the output of a DC toy motor being spun by another motor.
Thjis is why in my previous post I mentioned that a modern generator
would have various bells and whistles that may not haver been present in
early Edison generators. A generator is likely to have a rotating brush
assembly so that the brushes could be placed where the arcing is
minimized. Interpoles will also help commutation. The brush material
itself is somewhat resistive to damp out noise a bit.
Remember that in the days of six volt car batteries charged by a dc
generator, radios worked just fine.
They have plenty to do with this discussion. The battery is low
impedance and does tend to "hide" pulsing just like the B+ filter cap in
the radio. As I mentioned, the hand crank radio would operate fine, even
when the battery was old and completely worn out to the point that it
would no longer take a charge. Anyone is welcome to try this, I see lots
of speculation but no hands on experiments here aside from my own from a
long time ago. Speaking of old car radios, most of them used a
mechanical vibrator to generate the B+ for the plates, talk about hash,
the crudest brushed generator would have a cleaner output than one of
those still, with shielding and noise filtering, they worked.
Additionally, if homes were fed by pulsed DC mains, they could be used
to maintain the charge on a bank of batteries similar to the old 32V
rural power systems in which batteries were charged by windmills and/or
Generators are nothing.
In the earlier days of car radios, they used miniature vacuum tubes.
Tubes didn't work so well with 6V on the plate, so the radios had
things called "vibrators" (no not THAT kind), something that rapidly
interrupted or reversed current flow to a step-up transformer, the output
of which was rectified to a B+ voltage that tubes could use. Standard
filtering of the leads allowed them to work.
I once had a police 2-way radio that used a bunch of 12AT7 class tubes.
If the brushes in Edison's DC generators didn't contact two segments
at a time at the current they produced, the arching would destroy the
commutator in a few minutes. Just like the similar design of a
universal motor, it is the biggest problem with commutated brush type
motors or generators.
To intentionally pulse the DC, the gaps between segments would have to
be wider than the brush so they would operate in a break before make
I worked on hundreds of them, in the '60s. Have you ever studied the
vibrator supply design? There are buffer capacitors across the primary
winding of the transformer to reduce the loss of contact material. The
vibrators ran at 115 CPS (Hz) to reduce the hum. It was a tradeoff
between the cost to filter the DC, the life of the vibrator, and the RF
interference caused by the vibrator. The vibrators were in thick steel
housings, with a thick molded foam insert to reduce the mechanical
noise. The socket had a spring loaded clamp that held the vibrator case
tightly to ground it to the chassis to reduce EMI/RFI noise. Even after
all that, the life of a vibrator could be as little as a month, if the
radio was in a taxi or someone who drove all day with the radio on. The
foam deteriorated from the heat and contaminated the contacts, so the
shelf life wasn't good, either.
The first solid state replacement vibrators came out in the '60s,
built with Germanium power transistors in T)-3 cases, inside a standard
vibrator can. If the buffer capacitor was bad, or failed, they blew
The DC had some ripple, but it wasn't pulsating. I have looked at the
output of a generator on more that one pre '63 car with a scope. One of
the first signs of a failing generator was noise in the radio.
BTDT, 40 years ago.
That wasn't my point. I was just saying that *if* for some reason AC had
not succeeded and we still had DC generators powering our homes, radios
could still be made to work directly from the generator, even generators
build with mid 1800s technology.
I didn't mean to imply that batteries need a pulsed voltage to charge,
but simply that hypothetically if design constraints of the generator
caused the output to be pulsed, it could still be used to charge
batteries and power devices.
In both cases I was simply countering arguments from someone else that
this wouldn't work.
Funny thing is with modern technology, DC power distribution would
actually have some advantages, though in a residential situation the
disadvantages of dealing with high voltage service and large DC-DC or AC
converters would far outweigh the advantages, but it could be done. They
use it in some long distance transmission lines afterall.
It is actually becoming the preference.
We could make a system for residential DC service with the batteries out
at streetside. That is a chemical and environmental mess though.
Until battery technology takes a few steps forward or until solar cell
technology gets so good that we care less about battery bulk, we will be
in an AC fed, AC consuming world. Make for easy conversion, but a
magnetically noisy environment.
DC makes for instant 'kill-n-cook' situations though. You really do
Edison's machines were true DC machines. Some earlier machines were
"Gramme Ring" machines which could have given you this impression
(falsely). As far as I can see from pictures, his machines were drum
armature 2 pole machines with a conventional commutator (which is a
synchronous switch). Note that a commutator, properly used, switches the
current only in the individual coils under the brushes, shorting the
coils (2 in his case) on the neutral axis-at a time when the individual
coil voltage changes polarity. During the time that the brush contact
moves from one side of the coil to the other, shorting the coil in the
interim, the voltage in that coil would be nearly 0. The total voltage
would have little ripple as the rest of the coils would be
producing normal voltage so the total voltage certainly did not go to 0
or near 0. Brush width and material was a problem until it was
discovered (by Brush or Thompson- I can't recall which) that carbon was
ideal. Modern machines are designed to compensate for armature reaction
which shifts the neutral axis and causes arcing. In Edison's day this
compensation was done manually. In operation, except for considerable
refinements in design, Edison's machines were essentially the same as
modern DC machines.
By the time the "mains powered" radios came along, the supply was AC. In
addition, while an arcing commutator produces hash, it produces more
important problems in large DC machines and the design of the
commutation system as well as proper maintainance of the commutator
surface eliminates the arcing.
Small motors such as used in drills do arc because a) nobody maintains
the commutator until the arcing becomes a problem, and b) compensation
for armature reaction is not provided.
Sure DC transmission has its advantages- at high voltages for long
distances or for underwater or underground cables or for asynchronous
connections between systems. All point to point rather than grid systems
where specific technical requirements are needed or the distance is long
enough that the savings in transmission costs exceed the costs of the
end equipment. For mid level and distribution, it has serious
disadvantages(no current zeros).- AC offers great advantages in
switching and flexibility of networks and the optimization of voltage
levels for the task at hand with relatively cheap and efficient
Some areas still had DC power well into the '60s and even early
'70s. A friend of mine in the Army told me what his dad had to go
through to get AC for the new elevator in his building near Chicago,
around 1970. The elevator company refused to repair the old DC model,
and only installed new AC powered elevators. People in those areas used
the transformerless radios on the DC power lines, and wouldn't notice
the switch to AC, as long as the usual two electrolytics following the
rectifier were still good.
Add to that, the crappy speed controls in variable speed power tools,
and you get even more noise, because they are too cheap to add a small
inductor to filter the noise before it enters the power line and is
radiated to nearby electronics.
I read somewhere recently that the last DC service to a building in New
York I believe it was, was shut off sometime in the 1990s. I had no idea
it lasted so long.
Somewhere there is a website with pictures of some of the gigantic
rotary converters used to convert 25Hz AC to DC to drive subway trains.
Some of these were still operational at least up to a few years ago.
I've seen relatively recent pictures of polyphase mercury arc rectifiers
still in operation doing the same job.