| Sorry everybody. I was thinking 5Y4, not 5Z4. Not sure how I did that. The 5Z4 is interchangable with the 5Y3 like Billy said. The 5V4 is also an interesting tube that I have sometimes used as a swap for the 5Y3. If you are one of those who prefers the radios with the center tapped rectifier filament winding, both the 5Z4 and the 5V4 give you the next best thing. The problem with using a filament as a cathode is that filament voltage is added and subtracted from the cathode voltage unless the cathode is tapped from some central point. Since the cathode in the 5Z4 and 5V4 tubes is not part of the filament circuit other than being connected to one side (it handles no filament current), the cathode is pretty much neutral with respect to the filament voltage--filament current has no effect on cathode current as long as cathode current is drawn directly off of the cathode and not through the cathode heater. As most of us know, though, with proper filtering, center tapping the filament winding of a filamentary rectifier is not necessary. Sorry for the mistake before. Thomas |
Will Hodges 
TonyC 