Quote:
Originally Posted by Dave Moll
I have a Murphy BU183 that has this arrangement - using an AD38 for the 7½V supply (in my case a replica containing five "AA" cells. |
The normal reason for that was that, of course, these little directly-heated valves need a DC filament supply. In battery/mains sets this was often obtained by a dropping resistor from the HT+ (90V or so) line in mains mode. Having the filaments in series reduced both the voltage dropped across the resistor and the current through it, thus saving power, and more importantly heat production.
When making replicas, what do people do about (a) the casing (folded cardboard, or something stronger) and (b) the socket? I am assuming I don't have an old battery to re-stuff.