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Old 24th Feb 2007, 4:28 pm   #1
pmmunro
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Default Sets designed for DC only?

In another thread one contributor speculated on the size of main switch which was needed in a house with a DC mains supply - a good thought since pure DC is much more demanding on a switch than AC, which passes through zero volts every half cycle, provided of course that the power factor is kept reasonably high.

The question is, how pure was the DC waveform? Had it been true DC, would there have been any need for a rectifier in a radio set? (I assume that some filtering would have been necessary on any mains supply so the filter components would still have been needed).

Other than economies of scale in producing universal sets, the prospect of eliminating the rectifier valve would have been an attractive cost saving, especially given the royalty payable on each valve holder up until the 1950s.

For the same reason, it would have seemed that metal rectifiers would have been attractive to British set makers. My impression is that they were more popular with European set makers than in th UK.

I don't know if there were any DC-only mains receivers. If not, it would seem a little strange since almost every other conceivable technique was tried at some time.

Peter M. Munro
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Old 24th Feb 2007, 5:11 pm   #2
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Default Re: Sets designed for DC only?

I'm almost certain that, on one of my random trawls through the World's old radio sites, I spotted a DC-only console set dating from the very early '30s.

My memory really doesn't work properly, but I shall post here again if/when I find the set of which I'm thinking.

Hang about, Here's some information from this very site! Not about any specific set though but, interesting all the same.
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Old 24th Feb 2007, 6:05 pm   #3
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Default Re: Sets designed for DC only?

There were many DC-only radio receivers in the 1930s - and a few television receivers, too, even into the early 50s. My father used to recount how, when stationed in the "dry navy" on the Isle of Man, he was often called upon to repair "broken wirelesses" for the civilian population, and was amazed to find that they had ... no rectifier ! In most cases the valve had simply been removed and the rectifier anode and cathode "bridged" by an AC/DC plug on the mains voltage selector. (Presumably if the radio didn't work when first plugged in, the plug had to be removed and inserted "the other way round".

I seem to remember (from my days of childhood servicing) that there was a similar ploy with some of the old televisions. Because (AC-derived) HT rails tended to give "more than their RMS worth" it was recommended to select a mains input voltage lower than the correct value.

It's all a long, long time ago now, and I can't remember any specific instances, but I know such things happened. At least one television even had an "alternative fuse" for use on DC: it effectively shorted out the last section of mains dropper !
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Old 24th Feb 2007, 7:52 pm   #4
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Default Re: Sets designed for DC only?

I am afraid DC mains was not so clean as you would have imagined.There was mains hum,not the 50 cs type but 400c/s plus,this was the ripple from the dynamo,there also was some commutator noise evident,so you would need good RF filtering..You should remember DC mains was in the towns where you had a lot of machinery,lifts,neon signs etc.Rual areas only had AC,where the load was often only lighting.
I have only came across a few DC only sets,most were Universal,and some of these had a selector which shorted out the rectifier with a fuse,if you plugged the set in with the wrong polarity the fuse blew,most left the rectifier in circuit just in case of wrong connection.
I lived in a Town that had AC and DC mains,you never knew what supply you had,customers used to remind you,"you know we are on DC here"(or AC),I spent a long time at one house taking a TV set apart because it had no HT,checking around the rectifier with my meter(which I think must have been moving iron)revealed a fault with the rectifier,replaced the valve,and still the same,I was a bit perplexed,then customer said something like"oh we are on DC here".I reversed the plug in the wall socket all came to life,what an embarrassment!
Then there was the people who moved house from one street to another from AC to DC,the TV would not work and the mains transformer was on fire,also the Electricity board would swop your washing machine motor if your new supply was different.I always look back and remember the days of DC,the town and buildings were well lit with incandescent lighting that give a steady flicker free light.The substations had rotary converters,balancers/boosters,you would hear these as you turn the corner.
The areas with AC used to be 200V,220,sometimes 240,we often used RS autotransformers on the 200v lines to give 240v.
DC can be more dangerous due to arcing,modern switcthes would not work and quickly burn up.The advantages are that this IS electricity and is so versatile,even more so today with electronics,we can have commutator less motors,switch mode power supplies,CF lamps work on DC,if it does not operate by magnetic induction,then its generally ok for DC mains.We are beginning to recognise the advantages of HV long distance DC transmission,we may see DC return to public supplies,wouldn't that be fun!
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Old 24th Feb 2007, 8:03 pm   #5
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Default Re: Sets designed for DC only?

For reference, Ekco produced a DC only version of the RS2, a 3-valve TRF with filaments in series and a 'resistance lamp' and chokes to soak up mains fluctuations. There was a rather complicated series of dropping resistors, too. Other makers must also have produced the odd DC only receiver, surely.
-Tony
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Old 24th Feb 2007, 9:16 pm   #6
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Default Re: Sets designed for DC only?

Marconi had a dc only range of sets similar to the early 30s 263 and 438 types..............................ianj
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Old 24th Feb 2007, 11:20 pm   #7
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Default Re: Sets designed for DC only?

Quote:
Originally Posted by Aerodyne View Post
For reference, Ekco produced a DC only version of the RS2,
-Tony
I actually have one, or at least the remnants of the chassis. There is another example of a DC only receiver on Paul Stenning's website written by Nigel Hughes, under recent repairs. Its a 1933 Pye superhet. The article suggests that it may have been for use on board a ship, most of which used to be 220v DC.

There was considerable filtering for the power input since the mains, although DC, must have had an enormous amount of crud superimposed on it.

I also have some vague recollections of a range of DC mains valves, 20v 0.25A springs to mind as a possible heater rating.

Joe
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Old 25th Feb 2007, 12:03 am   #8
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Default Re: Sets designed for DC only?

I once had an Ekco DC74 console set. The DC version of the AC74. In fact there were 2 versions of the DC74. The ordinary one and a special one with extra HT filtering for areas where the DC supply was from mercury arc rectifiers and had more HF rubbish on it.
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Old 25th Feb 2007, 2:38 pm   #9
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Default Re: Sets designed for DC only?

Hi
I have the remains of a DC record player, I have never seen a dc motor (other than the small low voltage types) in a record player before, this is a big beast! mechanically governed , its fitted to a Garrard autochanger, it looks as if hum may have been a problem judging by the way the power cables were wrapped around each other plus the power cables are screened.
I dont know how it was originally wired someone had chopped the deck wiring nicked various parts inc the valves etc before I got it
I might be tempted to run the motor up and see how well it runs!
Rich.
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Old 25th Feb 2007, 3:05 pm   #10
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Default Re: Sets designed for DC only?

I have come accros these, DC mains record players,many had universal motors with a brake governor.There was a record player that used a shaded pole induction motor,the valves were series connected with an extra pair of UL84,these were only switched in circuit on DC,they produced a 50C/S to feed the motor,on AC mains these were not used.
Most of the DC mains sets I have worked on were 100-120,/200-250V,the 110v versions were mostly for marine use,since most small ships are 110V,while larger ships were 220V,although many of the smaller vessels are now 220V,its a lot easier to run a TV then!
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Old 25th Feb 2007, 5:26 pm   #11
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Default Re: Sets designed for DC only?

I grew up in a North Staffordshire textile town. In the 1940's, the mills were full of variable speed DC motors so our power supply was 240 V DC. The local power company supplied this from two huge diesel powered generators buffered by a battery of two volt lead-acid accumulators to accomodate peak loads.
These acted as capacitors to filter the commutator ripple of the generators.

The family radio was a Lotus "All Electric Bandpass Three, AC Model". The factory, or a local radio shop, had converted it to DC by removing the Westinghouse metal rectifier and replacing the valves with a type having heaters of 0.17 amps wired in series. The dropper resistor was a domestic 40 watt bulb mounted vertically on the chassis. This also served as the dial lamp.
John.
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