Wow. Props to Upscale for being so calm in their response here lol. Those tubes are not pin-compatible; they’re not even function compatible. The 5AR4 is a full-wave rectifier and you maybe sort of got this KT150 scheme working as a half-wave rectifier? - I don’t even know.
Anyways, what this tells you is that you like the sound of LOTS of distortion (the euphonic kind, not the harsh kind). I know how that feels - I once inadvertently subbed 5814 (12AU7) into the 12AX7 phase inverter slots of my Rogue Apollo amps, and thought it sounded awesome! Then discovered the mistake a couple days later, and felt silly.
But yeah, this is much worse than that 12AU7 / 12AX7 swap - please respect the gear and don’t keep doing it. At the least, it’s truly a testament to how resilient tube circuits are to most mistakes, versus Solid State.
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Solid states have a lot more "room" for swapping. Any tube or transistor can work as rectifier
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@mulveling thanks for the kind kick in the pants. I should’ve known better. and @czarivey, thanks for that info too. I’ve since pulled the KT150s (cryo and matched for crying out loud!), and I will have to sell them or save up for a powerful enough intergraded tube amp I can use them with that will push 1.7i Maggie’s.
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Did the tube glow at all? The pin configuration of a rectifier puts no voltage to the KT150 plate and does not complete the heater circuit. In other words a tube with no voltage to glow the base or the top. No glow means no power and therefore a dead channel. Unless the preamp has a solid state backup in case a tube rectifier fails which would cause problems if both channels are fed from one transformer.
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@OP It sounds as if you like the sound of mono if you left the 5AR4 rectifier for the other channel in situ.
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This is in part because of the power supply voltage design and transformer design .
in some amplifiers it allows several choices for they built it much more rebudt.
in a preamplifier look up the Ma for the kt150 voltage draw, you didnot specify your rectifier tube ,let’s say a 5ar4 it may have a different Ma voltage parameter.
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@czarivey "Any tube ... can work as rectifier"
I don't know one way or the other if that is factually correct, but I haven't come across any tube amps, DAC,s or preamps that use anything but a tube that is in fact specified as a rectifier tube as a rectifier. And, the manufacturers specify which rectifier is appropriate for the circuit and if there are options. For instance, my Modwright preamp specifies "5AR4, may also use 5U4GB and 5V4GB".
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Accident is sometimes the mother of invention. that said, I doubt this is a great discovery but you never know.
Upscale audio's response is predictable legal CYA when they have no idea what the technical answer is.
Hope someone who actually knows what happens when a power tube goes into a rectifier socket replies.
Jerry
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@gs5556 Has it right, they are not pin compatible so it's very problematic to do this swap. You had luck with you that day.
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OK I pulled up the pinout for both tubes. Nothing shorts out (obviously based on your experience). The heater will not have current in a KT150 so I don't see how it works well if at all. Pin 4 is a grid in one tube and a plate in the other. Other pins aren't used in both.
Makes me think there is another rectifier in the PL. Perhaps the tube is just for show.
Jerry
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Another thought. Perhaps the tube you used was not a KT150 but a large globe 5U4G? double check that.
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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.
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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.
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@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
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@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.
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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!
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@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
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@carlsbad2
I know! But everything checks out: the PL is in the signal path alright, volume control for all sources, turn off the PL = no sound and vice-versa. The KT 150s lit up as well (mostly at their tops, because KT105s don’t glow much because of how they are constructed). I’ll say this, not as an excuse, because i should still have checked further: but Upscale promoted the KT 150 as being compatible with “all” of their Prima Lunas. There is no where on there tube info dictating to NEVER place a KT150 into a PL preamp. But, in fairness to them, they probably assume no one’s that stupid. What made me stop to ask them when everything was going great as far as I knew? I couldn’t find any posts, reviews, reports, ANYWHERE about using the KTs the way I did, only with integrateds and amps. It spooked me, so I wrote Upscale about it. Glad I did,and glad I posted about here too. All of you are being super helpful. But, still don’t know how come it worked.
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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.
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One more thing…
What got me started down this dark path was the simple quest to acquire the best (rectifier?)sounding tube I could get for the PL. My PL feeds 2 amplification outputs (Luxman switches between them): 1. Parasound Halo A1+ 2. Two Quicksilver M60s monos (with Zeros, into GR Research outboard crossover) into 1.7i Maggies. (I use the QS for quieter jazz/acoustic, or just for a different flavor, if not great power). So, really I wanted to have the PL kick a bit more juice to the monos. Maybe I should’ve just started here with asking you all what tubes you’d use to get something like that accomplished, if any tube other than the standard PL 5r4gy might do it.
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@dogearedaudio,
Just saw your post after my last upload. That’s sounds promising. I’ll definitely look into it. Thanks.
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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.
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@dogearedaudio,
Just checked-out what’s available for NOS GZs. You’re right, they fetch a hefty price. I’l l just put it out there that since I can’t use them, I’m willing to do a straight swap of the essentially brand new Tung-Sol KT-150s (cryo/matched), for a pair of used good matched and balanced pair of the NOS GZ34s.
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I don't know what it is that makes power tubes in the rectifier position have such a big bold sound. I have a unit that can use power tubes in the rectifier position, it just plugs into a 5ar4 rectifier socket.
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Sounds like @dogearedaudio found the connection between pins 7 and 8 that allowed it to work.
I also agree with his recommendation of the Mullard GZ34.
Jerry
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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. ;-)
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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. ;-)
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@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.
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Ah, okay. Is this the fellow that makes the add-on rectifier boxes?
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Yes, called super rectifier, they do make a huge difference for some reasons I understand and some I don't.
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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).
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"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.
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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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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.
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Lots to digest here. I’m impressed by the depth of knowledge you all bring to this area of audiophilia, and I’m learning a ton. I will try to tread more carefully and when in doubt—-I won’t without running it by folks with experience. Thanks to all for helping me better understand how to avoid stepping on my own tail.
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