2. amps-preamps
  3. 191147-148010

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
dogearedaudio
85 posts
 

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

invalid
1,147 posts
 

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

mulveling's avatar
mulveling
1,851 posts
 

Just to reiterate, the problem with wiring up a the KT screen grid like a "plate" here is going to be power dissipation - the fine wire obviously can’t handle power like a solid plate. We know that plates glow orange or red when they’re over-driven - imagine what happens to grid wire!

It’s amazing the tube can take it like this, even for a short session - but I admit I’ve made some serious mistakes and quite often the tubes (somehow) weather the storm.

Another anecdote: My VAC Master preamp has a switch to allow running 12DJ8 tubes in place of 6DJ8. The 12DJ8 uses fully twice the heater voltage. Once I had the switch on 12V for those tubes, and rolled in some nice NOS 6DJ8, but forgot to change the switch. Ran that for like 20 hours - it sounded great! Next time I looked in there, the little 6DJ8 were GLOWING LIKE THE SUN. Haha. Corrected my mistake, and tested the tubes. They’d lost ~ 20% of their transconductance since before the incident, but still tested "good", and still matched! They were NOS Siemens E88CC (nice tubes). Yes, I think they probably sounded a bit sweeter driven to molten hot temp lol. The preamp was perfectly fine too (this was years ago).

dogearedaudio
85 posts
 

"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.  

invalid
1,147 posts
 

If it was just amount of HV, then why does the sound change even if go from 2 866a tubes to 2 3B28 tubes, same voltage drop. I don’t think voltage drop is a problem in a preamp or Dac, the drop in voltage of an 5ar4 is 17 volts, but that’s not going to happen until higher current.

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