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.

Are you sure you aren't using the Home Theater bypass inputs?

But looking at the underside photo on the web site, it actually *is* possible that the KT150 worked.  There's a zener or diode wired from pin 8 to pin 7 on the rectifier sockets, and that's where the B+ is taken from. Maybe the the KT150 was acting as a half-wave rectifier, with the screen as the plate?  It wouldn't be elegant but it might work.

I will say, I have to hand it to PrimaLuna for designing a preamp that's fool-proof enough that dropping a totally inappropriate tube into the rectifier slot doesn't result in a catastrophy. ;-)  No knock on you, audiodidact.  It happens.  Much worse damage can occur when an octal tube's key is broken and the tube gets inserted in ways that can cause some serious damage.  It's possible the KT150s could have continued to operate that way, but it wouldn't be wise.  Sorry you lost the magic, but best to stick with a true rectifier tube.

But speaking of magic, you might want to try a pair of Mullard GZ32's.  Should not cause any problems, and that's a really magical rectifier, IMO.

If you want the best possible rectifier for your PrimaLuna, then spring for a pair of vintage NOS Mullard GZ34s. The price may seem off-putting, but they will literally last a lifetime in that preamp, and offer the best possible sound, IMO. For maybe $500 you will get a $2K upgrade in the sound of the preamp. No modern GZ34, and I mean NONE, even remotely compares to the quality of a NOS Mullard.

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. ;-)

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. ;-)

Ah, okay.  Is this the fellow that makes the add-on rectifier boxes?

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

Well, basically, you're fiddling with both the amount of HV voltage fed to the mother unit (which will change the operating points of the tubes in that unit), and with the impedance of the power supply itself, along with whatever colorations result from the choice of rectifier tubes.  

Like I said, it's a combination of factors.  One way or the other, different rectifier tubes will impart a different sound to the amp or preamp.  A lot of technically-minded folks will disagree with that, but I've always found it to be the case.