Dropping Voltage at PT

[olddawg]

I've used zenering to great success on 3 amps to drop the B+. 5W 9V-15V zeners are all you need (if you are going to mount them on tag strips away from the chassis). Just run more than one in series off the CT of the HT winding to get a range of drops. All you are doing is dropping the peak current x the voltage of the zener - i.e.: (say for a 5F6A) 200mA* x 12V = 2.4W - so 5W rating is ample

(* the assumed max current draw rating of the HT winding)

However if you are going to use chassis bolt-on mount zeners, you will want a higher power handling, because they soak up the natural heat from the chassis anyhow

When I have done it I put the zeners in the B+ line right after the standby switch. 2 or 3 of them in series on a tag strip mounted where they can breath works nicely. We run into this problem all of the time when using recycled PTs. I wouldnt wory about the filment voltage.