Crucial TUBE question

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!

@audiodidact 

@mulveling has confirmed my read that the heater would not be hooked up.  (needs both pins to have a circuit).  So KT150 would not light up and would not produce any current. 

At this point I don't think we need someone with tube knowlegde, we need someone with a primaluna schematic to figure this out.  

We all agree you should not have done any damage.  We don't understand why you got music.  

One other thing to check, follow your cables and make sure the preamp is actually in the signal path.  I assume it is as you must be using it for volume control if nothing else, but you could be using your DAC or Roon.

Jerry