Mains cable with resistance wire thread

[Courtney Louise]

Hi Maarten

Yes. The line cord/mains cable had a resistance wire third core that dropped the 220/240 mains to 110v just for the heaters and anode voltages with the mechanical items being fed direct from the 220/240v line/mains cable.

Originally Posted by Maarten

I would assume separate droppers (maybe none for the anode voltage) indeed. A design that overruns valve heaters for more than 10% for any amount of time, would be considered quite bad.

On the other hand, the Sams photofact schematic clearly shows a 390 ohm resistor in series with the whole lot. Back in the day they might have gotten away with it, but I would now use a capacitive dropper for the heater voltage and a resistive dropper for the anode voltage.

[Maarten]

By the way, an important reason for not using a capacitive dropper in series with everything or a separate capacitive dropper for the anode supply, is that it won't work as intended with a half wave rectifier was that would introduce a DC component to the dropper.

However, if you want to eliminate the resistive dropper for the anode supply, it may be possible to obtain the anode supply _from_ the capacitive dropper, as seen here: https://webspace.science.uu.nl/~tel0...es/Sanshin.htm

[Techman]

Originally Posted by Techman

He bought a cheap auto wound step down transformer to run it, which although it worked, made it a genuine 'live chassis' in that all the exposed metalwork is directly connected to one side of the mains.

That's your answer, problem solved.

When I first went to look at this chaps recorder it was when he phoned me and told me about the smoke and I said that it must be because it's intended to run on 110 volts American mains supply, regardless of the fact that someone had fitted a standard 3 pin 13 amp mains plug to it. I went down armed with a Variac and a meter and found that was indeed the case. I said it really needed an isolated step down transformer to run it safely and properly, but the next thing I heard was that he'd gone on ebay and ordered a cheap auto transformer, which arrived within a couple or three days and it was powering the whole unit perfectly. I've just looked on ebay and from what I remember it was one of these, or similar:-
https://www.ebay.co.uk/itm/155538347...Bk9SR86epqWLYg

I seem to think he also bought an American style plug to go with it. This solved the problem, it was just the live chassis and safety aspect that I wasn't happy about and I warned him that he had to make sure that the chassis was always connected to the neutral side of the mains via that transformer and what the risk would be if the neutral ever became accidentally disconnected or if both poles of the on off switch didn't break at exactly the same time that there was the possibility of him getting a belt off the metalwork as he switched it off. None of it was isolated and I quizzed him as to whether he'd received any shocks from it and he said he hadn't. I also asked him if it had ever tripped the house RCD (he said the consumer unit had recently been replaced with the latest type) and apparently there had been no tripping while the recorder was in use.

The bottom line is that the whole lot really needed running via an isolation transformer for complete safety.

[G8UWM-MildMartin]

Forum rules prohibit discussion of that particular item so, however desperate I am to, I won't!

[Techman]

What "particular item" do you mean, Martin? I don't understand what you're saying.

Surely when it's someone's potential safety that's at risk, we need to get it out in the open, otherwise we'll all be guilty!

[Techman]

OK Martin, I've twigged and I've guessed what you're referring to and a recent new forum rule. We are all only guests in someone else's house, so have to abide by the house rules, I understand that. However, I've made no secret that I'm 'personally' against that particular rule for very obvious reasons and have made it known in the past, but let's say no more about it.

[Maarten]

It should not be necessary to talk in riddles, that seems counterproductive to having rules to avoid unneccesary discussion. As a forum administrator I know very well that there's a fine line between being a good guest and being a good host, but I have found that on one of my forums the guests felt more at ease when the moderation team eased off a bit.

If this is about the ebay rule, I seem to remember that as long as that particular sale is not discussed it is permitted to talk about technical properties of a set that is for sale.

[Techman]

Hi Maarten. No, it's not the ebay rule, the link I posted is allowed, as it's for a 'component' part and the seller is not being discussed. I think what Martin is referring to is the 'house wiring' rule and it possibly being related to the consumer unit and RCDs - link to recent new rule below:-
https://www.vintage-radio.net/forum/...d.php?t=198565

[Courtney Louise]

Well to answer @techmans great reply and all very relevant in my opinion as this is why I was asking the question in the first place. Neither side of the chassis is connected to the mains and none of the common chassis tag strip mounts are used. It’s not exactly insulated but it’s not been designed to have a live chassis as if this were the original 110v unit it may not have been polarised.

[Station X]

The forum rule relates to "fixed" mains wiring which is not what is being discussed here.

Back on topic please.

[RichardGM]

In my young days, reading American publications, I used to be bemused by the phrase "line cord". I managed to work out that it referred to what I would have called a mains lead (and might now call a mains cable) but there seemed to be a presumption that it necessarily included a resistive wire.

I don't think (unless I've missed it) anyone has pointed out one reason for having some sort of current limiter for the heaters rather than just a fixed resistance. The heaters may not all warm up at the same speed, and in that case one of them may temporarily dissipate more than it should.

[Maarten]

Series string heaters are supposed to be rated for this situation, in that it should not shorten their life to below what was specified. An inrush current limiter will probably help them live a bit longer, though.

If the voltage drop is large enough, a fixed resistor will act like a current limiter. I should do the math and see if a capacitive dropper will work equally well or even better.

[G6Tanuki]

Yes you shouldn't need separate inrush protection resistors with series heater valves, many US radios were designed using valve lineups where the total heater voltage was close enough to 117V that the whole string went straight across the AC supply.

Same as some UK TV sets, series heater chain with no additional dropper. Yes the different valves warmed up at different rates, you often had a poor little EB91 glowing like a light bulb for a few seconds after switch on because of the low thermal inertia of its heater compared to that of the other heaters in the chain.

Didn't seem to harm them!!

[usradcoll1]

Originally Posted by RichardGM

In my young days, reading American publications, I used to be bemused by the phrase "line cord". I managed to work out that it referred to what I would have called a mains lead (and might now call a mains cable) but there seemed to be a presumption that it necessarily included a resistive wire.

I don't think (unless I've missed it) anyone has pointed out one reason for having some sort of current limiter for the heaters rather than just a fixed resistance. The heaters may not all warm up at the same speed, and in that case one of them may temporarily dissipate more than it should.

Regarding the term "line cord", old habits die hard. I'm pushing 79yo and that's the only term I'm familiar with! I guess it's American slang. My test equipment has "test leads", not cords.
Dave, USradcoll1,