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Old 25th May 2023, 6:11 pm   #1
Courtney Louise
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Default Mains cable with resistance wire thread

Hi all,

I am trying to restore something that uses series heaters designed originally for 110v, a 1948 Wireway Wire Recorder/Record Player

They also produced the model I have which was intended for 220v 50hz countries and has the motors wound for 220v @ 50Hz from factory and the original very crispy mains cable had 450 ohms of resistance wire threaded through and attached to the neutral lead at the plug end. At the other end it was fed off from the mains coming in and to feed the whole amp section at 110v for European use.

Is there anything I can use to replace this wire? It's a fairly heavy current use I would think with 5 valves

Courtney
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Old 25th May 2023, 6:35 pm   #2
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Default Re: Mains cable with resistance wire thread

PS, I was thinking of an auto transformer with a centre tap just to power the amp section but I have no idea on power requirements of just the amp section and don't have a plug in wattmeter/ammeter to find out.

It's going to take me a while to recap the little beauty but I know mechanically it's fine after a decent amount of de-greasing and re-greasing.
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Old 25th May 2023, 6:51 pm   #3
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Default Re: Mains cable with resistance wire thread

OK so,

4 x valves heaters in series

35+35+35+12=127v

UK mains is 240v

I need 113v to be lost in a transformer.

so based on the fact the original resistance wire was 390r and i was dropping 113v I make that just 26 watts.

So a 50va autotransformer should suffice?
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Old 25th May 2023, 8:22 pm   #4
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Default Re: Mains cable with resistance wire thread

The schematic shows a 5 valve heater chain ( 3 x 35V and 2 x 12V) which is under run at 117V AC.

The half wave rectified current for the HT will pass through the autotransformer and I assume this is not ideal for a standard autotransformer.
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Old 25th May 2023, 9:52 pm   #5
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Default Re: Mains cable with resistance wire thread

Alternatively, could you use a capacitive dropper as per about half way down https://www.vintage-radio.com/repair...per-calcs.html?
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Old 25th May 2023, 10:55 pm   #6
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Default Re: Mains cable with resistance wire thread

Read this for some ideas Courtney.

https://www.cool386.com/dropper/dropper.html
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Old 25th May 2023, 11:38 pm   #7
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Default Re: Mains cable with resistance wire thread

Hi guys and thank you for your help so far.

The only problem I have is I can only account for the heater current and not the rest of the HT in the circuit as the 117v is split, 1 for the heaters and one for the HT.

All the heaters are 150mA in series so I am guessing the calculation has to be at least 750mA? What should I assume the HT to need?

I have used these figures for now but that doesn't allow for the circuit current draw.

Values

Supply voltage 240 Volts

Supply frequency 50 Hertz

Heater voltage 125 Volts

Heater current 0.75 Amps

Surge limiter drop 10 Volts



Results


Dropper resistor 154 Ohms
at 86.6 Watts

Dropper resistor with diode 60 Ohms
at 33.9 Watts

Dropper capacitor 12.01 uF
with surge limiter resistor of 13 Ohms
at 7.5 Watts
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Old 25th May 2023, 11:52 pm   #8
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Default Re: Mains cable with resistance wire thread

The heater current is 0.15 amps not 0.75. They are in series.
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Old 26th May 2023, 1:42 am   #9
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Default Re: Mains cable with resistance wire thread

I've had brief dealings with one of these in the past and from what I remember it should have had an isolated step down transformer fitted for use on UK mains. Unfortunately, someone had pinched this transformer and fitted a standard 13amp plug directly to the removed wiring, so when the chap plugged it in there was some smoke after a couple of minutes of operation. You could see where the transformer had originally been fitted inside the case. 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. He had the chassis side connected to neutral and he was using it like this - I told him that it was potentially dangerous the way it was and to be very careful with it!

I took a few pictures of it when I went to his house to look at it (as seen below), guessing this is the same as yours. Also be aware that the unit incorporates a low power medium wave (pantry) transmitter as standard - he could pick his up on medium wave on a portable radio from inside his house down to the bottom of his back garden with no trouble at all.
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Last edited by Techman; 26th May 2023 at 1:49 am.
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Old 26th May 2023, 6:57 am   #10
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Default Re: Mains cable with resistance wire thread

As Winston says redo your calculations based on 150mA.

Using your calculations you will blow the heaters!
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Old 26th May 2023, 9:11 am   #11
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Default Re: Mains cable with resistance wire thread

@techman

This one never had a transformer fitted from factory. The factory solution was a resistance wire in the line cord. Thatís on the diagram and also what I have. The motor chassis is pretty well isolated from any power shorts risk but the amp section Iím not so sure. But this is how it was originally designed and was on a 2 pin plug so no obvious polarity when new.

It sure is a strange beast for sure.

@all I will redo calculations on 150mA but what do I allow for the HT side or do I just base it on the heaters?
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Old 26th May 2023, 9:26 am   #12
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Default Re: Mains cable with resistance wire thread

Check the data sheets for the valves and add together all the anode and screen currents.
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Old 26th May 2023, 9:47 am   #13
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Default Re: Mains cable with resistance wire thread

I work that out to be approx 160mA which I guess is in parallel to the heater circuit so makes 310mA total?

Quote:
Originally Posted by Station X View Post
Check the data sheets for the valves and add together all the anode and screen currents.
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Old 26th May 2023, 9:49 am   #14
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Default Re: Mains cable with resistance wire thread

Values

Supply voltage 240 Volts

Supply frequency 50 Hertz

Heater voltage 110 Volts

Heater current 0.31 Amps

Surge limiter drop 10 Volts


Results

Dropper resistor 419 Ohms
at 40.3 Watts

Dropper resistor with diode 193 Ohms
at 18.5 Watts

Dropper capacitor 4.75 uF
with surge limiter resistor of 32 Ohms
at 3.1 Watts
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Old 26th May 2023, 10:05 am   #15
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Default Re: Mains cable with resistance wire thread

This is the text from the parts list about the line cord dropper.

Like I said the mains cable is split, one for the motors which is direct and the resistance wire split goes to the amp to feed 110v ish
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Old 26th May 2023, 3:47 pm   #16
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Default Re: Mains cable with resistance wire thread

Beware of that original line cord, it might have asbestos heat insulation.
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Old 26th May 2023, 5:09 pm   #17
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Default Re: Mains cable with resistance wire thread

Be careful.
If you try to do a dropper for the whole device then the valve heaters will be grossly overrun until the emission starts to load the ht: if they survive at all.

It is much safer to just do separate dropping of the heater chain supply ie., (150mA), and then deal with the HT as a separate issue. I'm assuming though that the resistive mains cord section means that the rest of the electronics was meant to run on 240Volts, and doesn't need further voltage dropping for the HT is that correct?

To put it another way, does the resistive section of the cord only run the valve heaters or the whole device electronics?

Last edited by G.Castle; 26th May 2023 at 5:12 pm. Reason: Added word
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Old 26th May 2023, 6:13 pm   #18
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Default Re: Mains cable with resistance wire thread

The resistive dropper only does the heaters and the amplifier, the motors were 220v as standard.
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Old 26th May 2023, 6:15 pm   #19
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Default Re: Mains cable with resistance wire thread

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.

Last edited by Maarten; 26th May 2023 at 6:20 pm.
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Old 26th May 2023, 6:49 pm   #20
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Default Re: Mains cable with resistance wire thread

BTW the correct value for a capacitor dropper for just the 150mA heater chain is 2.4uF.
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