Resistive mains cable

[saddlestone-man]

Did resistive mains cable, fitted to lots of domestic radios to drop the mains voltage, come in standard reels of so many ohms per foot?

Were there different types, and who were the main manufacturers?

best regards .. Stef

[peter_sol]

I think the leads were of standard lengths almost "modular" most radios of that type would have been similar in their requirements.

[AC/HL]

I believe there were standard values in ohms/foot, could be in the region of 60 -100?

Edit: Chas Miller's book "Valve Radio & Audio Repair Handbook" quotes 60 ohms/foot for 0.3A cord.

[llama]

I once had a reel of RadioSpares line-cord cable. I seem to recall the resistance per foot was marked on the reel (but it was at least 50 years ago!)
Graham

[WaveyDipole]

I didn't know such a thing existed. How do you identify it from ordinary mains cable?

[llama]

The reel would've been labelled "line cord".
Stripping it back, there were the normal 2 low resistance wires in there and a third resistive one in heat-resistant (asbestos?) sleeve.

In use, the resistive and one low resistance wire would go to L in the mains plug. At the receiver, the resistive one would go to the series heater chain and the other 2 used at full potential.

Graham

[G6Tanuki]

The radios I've worked on which came from the factory with a 'line-cord' often had the ends of the resistive-element crimp-terminated a few inches back from the radio- and the plug-ends and a pigtail of normal copper-wire attached [often overbraided yellow to distinguish it from the white and black standard-US cord provision].

The nichrome resistance wire doesn't take solder at all nicely so a crimped connection here makes sense.

[henrymarpla]

Example some photos

[HamishBoxer]

Be careful with it re asbestos also I believe it can be a fire risk. Safer to use a 110V transformer. As normally this cable is used on American sets.

[Peter.N.]

If I remember correctly there were three different types, for 0.1, 0.15 and 0.3 amp heaters. I can remember us having several different reels in stock.

Peter

[Herald1360]

Originally Posted by llama

In use, the resistive and one low resistance wire would go to L in the mains plug. At the receiver, the resistive one would go to the series heater chain and the other 2 used at full potential.

That works for a set expecting 240V fed HT.

American conversions fed the whole set via the resistive line with both copper wires in parallel for the other side of the mains. I found this out rather noisily at the age of about 12

I believe there were other possibilities with tapped resistive bits as well.

[andycapp]

I have a Pilot little Maestro with an original line cord, and a male double round pin plug. I intend to power it from an adapter through a light socket, I am just wondering will it make any difference which way around the plug goes. Thanks Andy

[broadgage]

The set should work just the same with the plug either way round.
"Back in the day" it was very common to work sets from lamp holders or other reversible two pin connectors, and no one worried about polarity on AC mains.

It would however be safer to connect it such that the chassis is connected to neutral rather than to live.

[andycapp]

Thanks broadgage, not sure with my very limited knowledge how to find out if the chassis is connected to neutral.

[Radio_Dave]

Andy, That rubber insulated mains cable is possibly over 70 years old and almost certainly not safe any more.

Why do you want to plug it into a light socket as well? How do you intend to determine the polarity of that?

David

[emeritus]

I still have some of the original TRS rubber wiring (Pirelli) that must date from about 1937 that I removed when I rewired my house more than 30 years ago, and it is still in pristine condition. I use the 2 core 15A cable to run 12VDC from a wall-mounted battery charger in the garage to occasionally trickle charge the car battery in winter so that mains voltage stays indoors. The rubber cable stays flexible in frosty weather. On the other hand, in the 1970's, the wiring of my cousin's house dating from the same time was so brittle that some of the lighting cable in the loft had crumbled so much that bare conductors were exposed in places.


Here's some info on line cords from the 1956 edition of Odhams "Radio Television and Electrical Repairs".

[andycapp]

Thanks emeritus i'll have a read of that and David the person I purchased the radio from sold it as in working order. I have also spoken to him a few times and he told me it would be perfectly okay to power it on via a shaver switch but not to leave it on for any long period, so I thought a light socket would do a better job as its most likely that's how it was originally powered. I don't want to play the radio for any length of time I just want to see if it works and then leave it as original. The line cord does look in remarkable condition.

[Herald1360]

Powering it from a shaver socket is a sensible suggestion insofar as there is an isolating transformer in there, but I think the caveat about not leaving it on for long reflects the likelihood that the shaver socket transformer rating is probably too low to carry the radio load current.

Whilst if your radio cable, case, knobs and any other original insulation are in first class condition, powering it from a light socket won't be any more dangerous than it was when the set was new, perhaps the light socket itself should be fed from an RCD type plug. Probably the best (least worst?) way to do this would be to fit the RCD plug to a stable based table lamp and find a two way light socket adaptor to plug into that and feed both the radio and the lamp. Make sure the radio cable is in plain view and don't leave it on unattended.

[andycapp]

Thanks herald1360, I have a two way light socket adapter, I will have to sort out a decent lamp to attach it to. All of my sockets are protected with RCD.

[SteveCG]

As Chris says: Wouldn't the power consumption of the radio overload any isolating transformer in a shaver socket? I thought those transformers were only for a few Watts load.