Crucial TUBE question

@gs5556 Has it right, they are not pin compatible so it's very problematic to do this swap. You had luck with you that day.

OK I pulled up the pinout for both tubes.  Nothing shorts out (obviously based on your experience).  The heater will not have current in a KT150 so I don't see how it works well if at all.   Pin 4 is a grid in one tube and a plate in the other.  Other pins aren't used in both.  

Makes me think there is another rectifier in the PL.  Perhaps the tube is just for show.  

Jerry

Another thought.  Perhaps the tube you used was not a KT150 but a large globe 5U4G?  double check that.

If the Prima Luna employs two tube rectifiers for a split power supply, as their ad copy says and a photo of the underside seems to confirm, and one rectfier isn’t working, then only one channel would work. So I don’t know what you put in there but there should have been no sound from one channel if it was a KT150.

Yes, you can use tubes other than actual rectifiers as rectifiers, though a pentode or beam power tube would likely not work as well. Some Western Electric 300B amplifiers used 300Bs as rectifiers, and there were broadcast amps that used 211s as rectifiers. A rectifier is a just a diode, so if you take a triode and short the grid to take it out of the picture, you’ve got a diode.

Regardless, whatever you did in this case I would recommend you not do again.  Use the recommended tube to avoid harming your preamp.

I thought OP replaced both rectifiers with a matched pair of KT50 - maybe he can confirm.

Well, pin 2 on both 5AR4 and KT150 (same as KT88 pinout) hits the heater, but the other side of filament is pin 8 (also ties to cathode) on the rectifier and pin 7 on the KT. I don’t know enough to say how that works out, but if it does fire up - the ratings of the 2 filaments are close enough for the KT tube to be ok.

On the other side, the KT tube has its screen grid hooked up to what should be 1 of 2 plates in the rectifier (each plate handling a "half wave"). There appears to be nothing hooked up on the KT to serve as the other plate. So at best it’s acting as a (sort of) half wave instead of full wave rectifier, and the PSU isn't outputting as flat DC as it was designed to. KT150 is generally extremely robust on many parameters, but (total guess here) the danger is vastly exceeding the screen grid’s power capabilities and melting it at some point. You might get away with it...for a bit.

Hopefully someone with actual tube circuit knowledge can clarify.