03-01-2012, 10:51 AM
They are to balance the ends of the filament to an equipotential point with reference to ht and ground. I have seen this a lot with directly heated rectifiers in top end equipment. Also they act as part of the smoothing circuit plus as a bleeder network to discharge ht in the event of a fault and power off.
Little cheat I once used on an amp with an AZ3 rectifier when the centre tap of the heater tx was open circuit. 2x250 ohm resistors in series across the filament with the ht+ to the newly created centre point. Worked well and also slowed down the heating somewhat to allow everything else to come up to heat before the ht got fully going. They were big 40W wirewounds. Values chosen at random from the junk box (what I had in the bag that was hefty enough.. 4x6v6 running all bridged class A in that 1950's amp, about 25 watts output) after working out the filament resistance at working temperature and getting completely stumped... told you I learned some really dodgy tricks on the road. So theres' a useful "get you home" trick for anybody with an open circuit centre tap on a rectifier heater winding.
Little cheat I once used on an amp with an AZ3 rectifier when the centre tap of the heater tx was open circuit. 2x250 ohm resistors in series across the filament with the ht+ to the newly created centre point. Worked well and also slowed down the heating somewhat to allow everything else to come up to heat before the ht got fully going. They were big 40W wirewounds. Values chosen at random from the junk box (what I had in the bag that was hefty enough.. 4x6v6 running all bridged class A in that 1950's amp, about 25 watts output) after working out the filament resistance at working temperature and getting completely stumped... told you I learned some really dodgy tricks on the road. So theres' a useful "get you home" trick for anybody with an open circuit centre tap on a rectifier heater winding.
