Crucial TUBE question


I  placed a KT 150 power tube into a Prima Luna Evo 300 preamp rectifier slot. I know. But what happened was surprising. The sound was, well, magnificent glorious. Nothing burned out. No smoke. No arc lights. I asked Upscale Audio to advise me as to what damage I may have done or could have done to the PL. They only said they would not advise placing anything but the original tubes in the slot. They would not commit to whether or not that power tube could be used in place of a rectifier to tube. So, is it an absolute or not? If so, why?  I’d appreciate feedback (please, no slams) from anyone having any experience with this. Thanks!

128x128audiodidact

I wish I were smart enough to know how the filament voltage made it through the diode to power the filaments, but I'm not. ;-)  It worked somehow.

"I don't know what it is that makes power tubes in the rectifier position have such a big bold sound."

Well, you're only getting a half-wave supply (one diode) as opposed to a full-wave supply (two diodes in the 5AR4).  It would be noisier, but if there's anough filtering it would work, I supposed.  Also reduced voltage at the output, which would change the sound.  Mainly, though, I'm having trouble imagining why a manufacturer would suggest, or even allow, replacing a twin-diode rectifier with a single-plate power tube, unless it were a 6080 or similar twin-plate regulator tube.  Another possibility is that the unit uses SS diodes and the tube rectifier is just a buffer to provide a slower turn-on.  In that case I guess a power tube could work.  But most power tubes aren't designed the handle the amount current that rectifiers can.  So it seems like a risky thing to me. ;-)

Also, it's likely that a schematic wouldn't indicate that pin 7 is used as a tie point, so you'd still be scratching your head. ;-)

@dogearedaudio the unit I'm talking about has 2 half-wave rectifier tubes, but the small manufacturer does state that if using a power tubes as rectifiers, you should only use for preamp, dac or low powered set amp. I'm sure there isn't going to be much voltage drop in a preamp that uses one rectifier for each channel.

Yes, called super rectifier, they do make a huge difference for some reasons I understand and some I don't.