Caps for Resistance Line Cords

For some time now I've been reading about how you can replace resistance line cords or ballast tubes with capacitors. I am at a loss to figure out just what type of cap to use, with or without a diode or extra resistor and if the cap should be film, oil-filled, a motor starter type, etc.

Currently I am using a small box at the end of a very short, heavy-duty extension cord that contains a few power resistors, an on/off switch, an "in use" light, a recepticle and a dial-type switch for different resistances needed for various curtain burners. My grandfather designed and built it some time ago and I'm sorry to say I never got the chance to ask about it before he passed away.

It sits just far enough away from the wall to not heat it up too much and it has about six settings for different sets.

Although I love the box I'd like to be able to make the sets independent and more like the originals that could be plugged in without any attachments. I also need to replace one of the resistors. They've been in there for quite some time now.

i have done this a few times .you need an unpolarized cap usually 7 or 8 mfd will work fine.there is a complicated formula to figure out exactly what you need but in these type of caps there isn't a great deal of choice anyway.i used solen caps i got from aes they were 250 v. and i think 7.2 uf if i remember right.they worked great but the sets did warm up a hair slower but not enough that you would really notice.i also picked up some caps in this range used on dishwasher motors but haven't tried any yet.butch

Where did I put that... wrote: I am at a loss to figure out just what type of cap to use, with or without a diode or extra resistor and if the cap should be film, oil-filled, a motor starter type, etc.

Hi,
This site has lots of explanation information and most important is the spreadsheet link.
http://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html

Basically, a capacitor will drop the 'apparent' voltage only on AC so you would be unable to use it PLUS a diode.

The caps are pretty easy to find. Digikey has a good selection (Panasonic line) of AC-rated caps but its a bit difficult to locate them on their website. Like Rich says, most situations fall in the 6 or 7uf range. If you need to mop up some excess you can use a resistor. That spreadsheet mentioned earlier lets you fiddle around with the numbers.

If you have dial lights to contend with often you need to add zener(s) across the lamp if it is in series with the tube filaments since you don't have the ballast resistance to slow down the in-rush. And forget using a regular voltmeter with the capacitive dropper! It will not read correctly in that scenario. Depending on the meter it may also not read correctly with the diode dropper since that is half-wave pulsating DC.

GL

www.goldmine-elec.com has them at good prices and a lot of other parts you want to get the capacitors for A.C. SMITHY

A person on ARF has come up with a way of making your own, safe to use NEW resistance line cords. It looks pretty easy. I have purchased some of the heater wire off ePay and will post photos here of how it is done. This is definitely the way to go in restoring these sets, the diode resistor combo, or capacitor idea are ok, but when an idea comes along that maintains the originality of these radios and is cheap and easy to do, I am all for it.

Hello,

Thanks for the kind words Larry. I was that guy that made the repro resistive line cord. I post here because ARF is still down. This started out as an experiment but ended up working out very well. The test subject is a Silvertone 6175,

http://www.nostalgiaair.org/PagesByModel/421/M0017421.pdf

a VERY small octal AA5. I really didn't like the cap, diode solutions and had no room for a power resistor nor did I want to change the tube line up. I think these resistive line cord radios should be preserved as the resistive line cord was a technological solution to a problem of the time. Plus this allows the radio to be as original as possible. Here a pic of the original wire.



I started out with new resistive wire. I could not find a supplier that would sell a small quality at a reasonable price. On a whim I thought I'd check ebay and found someone selling this stuff in a resistance that I could use. His stuff looks like the wire from here but this place was only interested in selling 1000's of feet.

http://www.springfield-wire.com/category.cfm?Category=21

The guy on ebay was willing to sell me a 17 foot section at a reasonable price. He still is selling this stuff.

http://cgi.ebay.com/HEATER-WIRE-25-5-OHMS-per-ft-FIBERGLASS-BRAID-30-ft_W0QQitemZ250235952321QQcmdZViewItemQQptZBI_Connectors_Switches_Wire?_trksid=p3286.m20.l1116

At 25.5 ohms per foot, and I needed 165 ohms, that came out to about 6.5 feet of cord which is what I thought I would like. The original cord was around 4 ft. Plus the longer wire makes for a cooler cord. Here's the wire.



I then purchased some cloth covered cordage from RadioDaze and removed the conductors from the cloth cover. I used the cover for the new cord. I tried their different sized cords but could only fish the new wire through their 8-conductor cloth covered cord.

My particular application required a tap of 22.5 ohms for the dial lamp. So I carefully spliced the resistance wire about 10.5 inches down from one end for this tap. Care must be taken when stripping this wire. I discovered the nichrome will break very easily if nicked with a knife. I had to carefully pick away at the insulation after slicing the outer fiberglass cover. Here's the finished product.



The cord only gets slightly warm and the radio originality is preserved. I'm looking forward to the warm weather so that I can finish the cabinet.

I know it's been awhile but...

Excellent information, ErikD! That will work well with some of my ballast tube sets, too.

I do have one problem, though. I have an RCA T4-10 that has a line cord resistance of 315 ohms. The cord you mentioned would have to be over 12 feet long.

BUT, would I be able to bend it back on itself to make it a double 6-foot length without breaking it as long as it wasn't nicked in the process?

How is it for flexibility? Can you shape or coil the cord to keep it short when not in use?

you could spiral wrap it around, that should solve your length issue.