Round-pin sockets (2023 Update)

[Scott]

Did polarised plugs exist in DC when AIUI there was no earth (just red and black)?

[nib]

AC/DC universal supply.

nib

[nib]

Was there not usually a capacitor across the primary of the loudspeaker transformer to reduce any high voltages from the transformer acting as an inductor, such as when not loaded?

nib

[Joe]

I believe it did happen regularly with fairly high power amplifiers (i.e. more than about 10 watts) and the rule of thumb was that it was safe to short-circuit a valve amplifier or open-circuit a transistor amplifier, but not the other way around.

[Bob Eager]

Pretty sure they were meant to be. The (surface) box was 6-7in wide, 4.5in high and probably 2-3in deep. The button (push on, push again for off) was probably about 0.75in in diameter, angled forward on the top front edge.

I can't find a picture...!

[The Natural Philosopher]

I believe it could happen with valve radio TRANSMITTERS

But not audio amps.

[The Natural Philosopher]

No. I have never seen that.

With a class B push pull the primary is never unloaded. Only with a class A, overdriven into full off, might you get a few KV on the output valve anode.

But if it has feedback it should stop that. The feedback would turn the valve back on.

A valve RF transmitter, without feed back, is, however, a different animal.

[Scott]

I seem to remember as a child being told not to use my foot to turn the socket on and off, which suggests this was the practice at one time. I think they were flush mounted into the skirting board, so probably a different design. I remember being very interested in a socket on the floor until it gave me a shock.

[Scott]

On Tue, 18 Jul 2023 21:07:31 +0100, Scott [e-mail]> wrote: [snip]

I have asked my electrical engineer friend and he does not know :-(

My expectation is that when DC was still in use, all the plugs (including AC) were two pin. I remember them from my Granny's house. I assume there would be nothing to prevent creating an electrical earth for any appliance by bonding it to the cold water pipe. My Dad said you should connect the 'Earth' socket on the radio to the central heating.

[Max Demian]

That would be earthing for radio reception, not safety.

[Scott]

Yes it was. However my suggestion was that the plumbing also acted as an electrical earth until arrival of plastic pipes. I still don't know whether DC was a two conductor, two pin system or whether an earth could also be used. Put a different way, could three pin plugs be used with DC and, if so, what path was used for the earth?

[Andrew]

But Volks railway had enormous rectifiers to provide DC for traction current, surely ?. Why bother if the mains was DC ?

[SteveW]

Or just to protect DC devices from reverse polarity.

[Animal]

No. Sets with one wire to ground could be consistently neutral, but the popular practice of wire via switch to chassis ensured either, depending on plug polarity: when off live, when on neutral or when off neutral, when on live.

[Animal]

it saved tuppence. The danger was minimal enough to be considered accptable. 'Only a total idiot' etc.

no

[Animal]

double no.

[Animal]

There was some very good stuff, but mass market goods cut costs & didn't care too much.

[Animal]

yes, to stop rf feeding back & to control tone

doesn't have enough effect on that.

[Animal]

I've seen a Philips audio amp with spark gaps fitted for this reason. But hardly any had that.

[Scott]

"some parts of the town"