DC mains

[Graham G3ZVT]

This is prompted by a comment "emeritus" made in the DAC90 thread currency running in the radio section.
It was about DC supplies typically having an audible generator whine, and the suggestion that this is the real reason for the infamous capacitor across the mains in AC/DC chassis.

It got me thinking that few of us will have any experience of DC mains, I certainly haven't, and would like to know more.

What about the role of the rectifier?
If the set had a 2 pin plug you might plug it in and patiently wait for the valves to warm up only to find there was no HT, so you would have to repeat with the plug reversed. So the rectifier was only there to protect the electrolytics from reverse polarity?
Has anyone set up a DC "mains" supply in their workshop?
Many years ago I had a dynamotor that I used to supply HT to a valve transmitter when operating mobile, and it occurs to me that would simulate a DC mains supply to test out how our sets perform on DC?

What thoughts do others have on the subject?

[Radio Wrangler]

I've designed equipment for DC mains as part of the day-job. It was for the US navy who have 275v DC power on their aircraft carriers!

Anti-RFI measures are a little more difficult. Finding DC-rated mains switches is a right pain.

David

[electrogram]

If the set has a rectifier it will work on AC too. It had not occured to me that this would protect the electolytics in a reversed pilarity DC supply. There are some early 30s DC only sets with no rectifier, those made by HMV/Marconi use unpolarised paper block filter capacitors, but then so did most of their AC only sets at that time. So did all DC only sets use unpolarised filter or smoother caps to prevent capacitor explosions ?

[Radio Wrangler]

AC/DC sets running a half-wave rectifier on AC export the DC component of their current consumption back to the substation. This isn't too popular with power distribution people as it can use up some of the saturation allowances in their transformers. It was a huge problem in the days of AC/DC tellies and radios everywhere. When tellies went over to SMPS with bridge rectifiers, it wasn't just the sets' reservoir capacitors that got an easier life.

David

[winston_1]

I've come across DC mains 3 times.
When I was very young we had DC in Barnes SW London. My parents had a Pilot Little Maestro radio with a 2 pin plug on it. I remember if it lit up but didn't work the fix was to unplug it and reverse the plug.

Next DC was on a Hook van Holland to Harwich ferry in the 60's. The shaver sockets in the bathroom were labelled 220v DC. I had a Phillishave that would work on DC so I smugly had a shave whereas others wondered why their shavers did not work.

Finally in 1970 I was in a hostel in Calcutta and I plugged in a cup water heater to make coffee (successfully) and someone said that socket did not work for his cassette player. I found out why when I withdrew the plug for my water heater and drew a long arc. I wondered if the other guy has burnt out the transformer primary in his cassette player!

[Cobaltblue]

I was told by my good friend Barrie (Ex SWEB) that the last area in Devon with DC mains was Okehampton I think he said that was the case up till the early 60's.
Apparently one of the main advantages was the generators were turned off at night and the consumers supplies ran on batteries.

Anyone know if that date is correct?

Cheers

Mike T

[G6Tanuki]

There was 200V DC mains in parts of the Nottinghamshire coalfields up until the mid Sixties. Provided (free) by the pit.

It was a split system, a pair of houses shared a common return path but one house was fed +200V and the other -200V. The supply cable was a big steel armoured one that ran up the side of one house to a large rusty cast iron box from which the feeds then dropped down to the kitchen of each house, where there was a large wooden cased fusebox with ceramic fuses inside.

The coming of the National Grid and its AC was expected, so people started buying modern equipment. My relatives bought a modern AC-Only radiogram and my father built them a DC-to-AC converter that used four large valves that looked to be of KT88 sort of size.

When the Grid arrived, there was some local disharmony because people had to pay for their electricity rather than the previous free power arrangement.

[ex 2 Base]

When I worked for BR in Lincoln we had 3 wire DC, POs, Neg and Neutral. Lighting, one floor, 230. POS & Neutral, Next floor 230 NEG & Neutral etc Motors 460 volt. Ceramic fusing and glass fronted control gear cabinets. A pleasure to look at and see it working, real craftsmen who originally carried out the work. Eventually the power station cut the DC supply and we fitted a pair of big Mercury Arc Rectifiers and transformers to go with them. The building was a listed one in detail.

[emeritus]

My original comment was prompted by a recollection that guy I used to work with at Plessey who had had experience with wiring on military vehicles, mentioned that the DC supply cables were twisted pair to mitigate the radiated interference.

Cardiff still had both public AC and DC mains supplies in the 1960's. The docks had used the DC supply for operating things like cranes and winches, but by 1969 the only remaining customers for the DC mains were the municipal trolleybus system and the University engineering department's laboratories. I remember doing an experiment where a dynamo driven by a 600V DC mains-supplied motor, drove a 3-phase alternator that had to be synched to the public AC mains supply to feed power back into it. The electronics lab benches had 13A sockets for both 240V AC and DC, the DC ones being the "Walsall" gauge ones where the pins were rotated 90° relative to the normal 13A ones. The Engineering block had only been opened circa 1964, so their DC wiring installation was possibly the last one to be installed for operation from a public supply.

The Public DC supply probably ended when the trolleybus system closed at the end of 1969, and the engineering department had to make its own DC mains supply. I had left Cardiff by then. The power lab did have mercury arc rectifier experiments, but I think they may have been going to use thyristors for their DIY DC mains. I had opted for electronics in my final year, so didn't get involved in the heavy current stuff then.

[Richard_FM]

I heard some American cities had DC mains, Boston being one from what I remember.

This led to some newly arrived students at Harvard wondering why the appliances they brought with them went with a bang when plugged in!

[broadgage]

Originally Posted by Richard_FM

I heard some American cities had DC mains, Boston being one from what I remember.

This led to some newly arrived students at Harvard wondering why the appliances they brought with them went with a bang when plugged in!

Parts of New York still had DC mains until 2007 ! I think Chicago may still have DC service in parts.

DC radio sets invariably had a rectifier so as to permit of use from AC as well.
DC had to be of the correct polarity, if the heaters lit but no sound was produced then supply polarity had to be reversed, easy with non polarised 2 pin plugs.

When determining the polarity of the supply was important two methods were used. "pole testing paper" could be used, this was a white blotting paper type material, supplied in books of small pieces. To use, a strip was wetted and DC of unknown polarity applied via test prods or bared wires. A bright red colour appeared around the negative pole, for a "high voltage" supply of more than about 125 volts, connect a small lamp in series.
The other method used a battery and a small lamp, connected in series to the unknown polarity DC mains. By observing which connection of the battery gave the brighter light, polarity was readily determined.

[FIXITNOW]

Got one off those Farnell E350 PSU's, so have used it to power up AC/DC sets with dropper resistors, had a very hummy set so went for powering it up via DC.

[Station X]

I'm sure I read somewhere that the polarity of a DC supply could be checked by pushing the leads into a potato and observing the colour change. I wouldn't like to try it on mains voltage though.

[Cobaltblue]

I had a polarity indicator I am sure I did a thread on it:

A quick search an I have located it:

https://www.vintage-radio.net/forum/...ad.php?t=64330

Grahams Potato trick gets a mention as well

Cheers

Mike T

[Vintage Engr]

When I was at school in the 1950's we had DC mains in the school.
However, not everyone was aware of the fact... After all we were only children.

On an open day, one lad brought in a 9.5mm projector, with a transformer, which was duly plugged in to the nearest 15A round-pin socket. There was a 'bang' & a flash from the expiring transformer, with the teacher unable to explain!

My first introduction to DC mains.

David.

[duncanlowe]

I never experienced DC mains, only experienced knock on effects.

Earliest, the local cinema where I got to do a tour of the projection facilities. Changes in voltages, and change from DC to AC had resulted in the addition of a resistor room, an AC transformer, and a mercury arc rectifier over different periods. The carbon arc projectors were original.

In the same town, my first house was built when there was DC mains (or possibly added just after). Wiring was for lighting only and the original wiring was still providing lighting. Ceramic circular wired fuseholders in both circuits, the main switch (on the same board as the fuses) was two separate brass switches with a wooden link handle. That house also had lead pipes for the gaslamps.

[Dave Moll]

All this talk of DC mains reminds me of my early visits to a Christian conference centre near Lynton in North Devon back in the 1960s. Its supply wasn't exactly DC mains as it wasn't connected to the mains supply at the time, so it had its own diesel generator pumping out a DC supply around the complex. I recall various inverters dotted around to power electronic equipment such as radios, record players and the like.

[TonyDuell]

Of course the 'voltage' of DC mains was just that, whereas the voltage of AC mains was the RMS value.

As a result, if you had 200V AC mains you could expect to get rather more than 200V on the HT+ line after a rectifier, even allowing for the voltage drop in said rectifier, but with 200V DC mains you'd get rather less than 200V. That's when bypassing the rectifier could be a good idea.

I am sure I read in an old book on televsion servicing that there was a TV set with a fuseholder wired in parallel with the rectifier. To use the set on 200V DC mains you set it up as best you could then put a fuse in said holder to bypass the rectifier and push the HT up a bit. If the set was then accidentally connected to the mains with reverse polarity, hopefully the fuse would blow before too much damage was done

[Tractorfan]

Hi,
I've not experienced DC mains myself, but a works colleage, who was ex-merchant navy, gave me an old DC to AC inverter that he used on board to power his record player in his cabin.
It has six PL81s in parallel push-pull and an ECC81 oscillator. A few larg-ish dropper resistances and an output transformer.
There's nothing on it to indicate a maker, nor output. I imagine it to be horribly inefficient, but I doubt that'd be an issue on board ship.
Cheers, Pete.

[emeritus]

Originally Posted by TonyDuell

Of course the 'voltage' of DC mains was just that, whereas the voltage of AC mains was the RMS value.

As a result, if you had 200V AC mains you could expect to get rather more than 200V on the HT+ line after a rectifier, even allowing for the voltage drop in said rectifier, but with 200V DC mains you'd get rather less than 200V. That's when bypassing the rectifier could be a good idea.

I am sure I read in an old book on televsion servicing that there was a TV set with a fuseholder wired in parallel with the rectifier. To use the set on 200V DC mains you set it up as best you could then put a fuse in said holder to bypass the rectifier and push the HT up a bit. If the set was then accidentally connected to the mains with reverse polarity, hopefully the fuse would blow before too much damage was done

That could explain why some Bush TVs in a 1953 leaflet are specified for use with 200-250VAC but only 230-250VDC.