Quote:
Originally Posted by Tubejunke
I take it that the diode has a similar forward resistance as the old wire-wound resistor had in order to maintain the proper voltage. I am sort of new to the whole concept of replacing one component with a totally different, more modern component. I have read up a bit on eliminating 5U4s or other tube diodes used in a rectifier configuration, and replacing them with silicon diodes and a resistor to drop the same voltage as the tube filament. I have seen a few sets with the selenium rectifier disconnected and some older (germanium?) solid state diode(s) wired in its place. I thought that selenium rectifiers tended to run a bit hot, hence the cooling fins. I KNOW that they smell terrible when the finally do burn up, but the few that I have seen burn up were caused by bad electrolytics.
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The silicon diodes have a lower resistance than the seleniums. That's why I have to increase the 10 ohm resistor to 22 ohms. I actually used a 32 and 75 in parallel each rated at something like 25 watts because that's all I had on hand. Really I think a 5-10 W would be fine.
As for using a diode in the filament string, that's a new concept to me as well. I first heard about it a while back as a potential replacement for the nasty VT71 ballast. I didn't actually try it until a few weeks ago in a Philco 37-611 radio. So cool that one little bitty device can replace a big 20W resistor and generate virtually no heat.
There is nothing wrong with the huge, original resistor and I could have left it alone. However, this little set uses around 130 watts and I figure any heat savings are a good idea.