Crucial TUBE question

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.

@facten 

Transistor (or triode tube) can be equivalently represented by 2 diodes nearly same way. In fact, one triode or transistor can be used as full wave rectifier.

As to applied quiescent voltages, I cannot guarantee that they will match to the original rectifier tube, but I can assume that the pins might be matching

@czarivey -  Are you stating the use of any tube in a generic way , meaning if you were doing say a dyi build that you could design the circuit to accommodate any tube as a rectifier? Otherwise, I remain hard to convince that you can arbitrarily swap any tube as a rectifier into a manufacturers circuit that was designed with specific rectifiers  in mind. In the Op's case apparently the folks at Upscale, whose owner has a long standing hands on relationship with Prima Luna ,didn't give the all's okay to swap in KT150s.  I'm not being argumentative I'm just trying to better understand your position.

I did indeed use 2 KTs in the 2 available rectifier slots. The pins actually did fit in the available holes. I had robust, deep bass and sparkling highs for the brief time I had them in. No exaggeration, the soundstage doubled, the sound was tactile. But after all of this great feedback (very positive I might add, and I thank you), I realize I just had impossible dumb luck in not blowing up my preamp. I truly appreciate all your educated ideas to help me understand the “why” of it all. I now have new 5r4gy’s in. Sounds good, but not any way near as good as what those power tubes produced. Thanks all!