In the mid-1930 in the USA, there were some radios designed with 3-wire power cords -- two copper conductors and a third resistive conductor. This was because the heater string voltage added up to 69 volts (at 300 mA) for a typical 5-tube radio. These cords soon acquired the nickname 'curtain burners'. Other radios had a "ballast tube" (a glorified light bulb) to drop the excess voltage (and add more heat inside the radio). Eventually UL forced the situation, and the tube line-up was redesigned so that the heater voltages added up to 120 V (at 150 mA).
If any of those power cords survive, the insulation is brittle by now, and so a standard power cord is fitted and a modification done to the radio. The simple modification to the radio is to add a diode in the heater string. I have pointed out to these hobbyists (I collect old radios as well) that this yields 85 Vrms for the heater string, not 69 Vrms, and so the tubes are being overstressed. I then gave ways to fix the problem (also add a resistor, or use a properly sized capacitor to reduce voltage). Donald Borowski EMC Compliance Engineer Schweitzer Engineering Labs Pullman, WA, USA From: John Woodgate <j...@jmwa.demon.co.uk> To: EMC-PSTC@LISTSERV.IEEE.ORG Date: 01/27/2012 10:45 AM Subject: Re: [PSES] Is this common knowledge - Electrical Ratings Sent by: emc-p...@ieee.org In message <4f22e536.60...@ieee.org>, dated Fri, 27 Jan 2012, Mick Maytum <m.j.may...@ieee.org> writes: >Dans experience reminded me of a guy who bought a 120 V coffee maker as >a present for someone in (old) England. Having some knowledge of AC >supplies he bought a 240 V to 120 V travel adaptor so the coffee maker >could operate on UK 240 V mains. In my youth (in Britain) I used to hang around the local radio dealer. People of my age will know what a 'line cord' is - a resistive mains lead used to drop the heater voltage for AC/DC radios, having a series string of 0.2 A or 0.3 A heaters. A guy arrived one day asking for a line cord for a 120 V 2 kW heater. He had even worked out the required resistance correctly with Ohm's Law. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK Some people who are peeling the finch of the financial crisis are thinking of biting a rook. - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
