> In message <001101bf926f$27c65e00$df36fea9@roland>, Roland Andrag > writes > >I just wanted to thank everyone who too the trouble to reply to my question! > >Looks like I will have to do a bit of maths to figure out how the cap in > >series with rectifier works.. I will be very glad to get rid of 10 W > >resistors though! > I would be interested to see how your maths works in practice, many > years ago a particular range of Ferguson B/W TV's used a capacitor to > feed the heater chain (rather than the usual huge ballast resistor). > Being an inquisitive type I worked out the reactance of the capacitor at > the 50Hz mains frequency - it was nowhere near the required value to set > the current at the required 300mA for the valve (tube for our USA > readers!) heaters. I've always presumed there was more to it than simply > the reactance of the capacitor, but have never really understood why?. > -- I'm digging back a long way in the memory vault here so I could be wrong, but ISTR that many old TVs had a line output valve that used 600mA heaters. In this case the other valves were arranged in a series-parallel config to make the whole heater chain 600mA. Dunno if your Ferguson was designed like this though. Also the capacitor is almost pure reactance so there's a j component you need to consider when calculating the drop. .