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Firstly, welcome to the forum and the Decware family!

Now, about your 274B. A USN Sylvania 274B is a rare and highly sought after tube so I'm wondering what the backstory is. If it is what it says it is, and not worn out, or a counterfeit, it will likely work fine and sound really good. But it should cost a lot more than a Type 80 rectifier.

Hey PointThatThing.

I have not looked at this in a long time, having ruled out looking at vintage 274Bs, so a little vague in mind. But I think what Steve is talking about with "some 274Bs," is whether they can handle the rectifier cap of an amp or not, and some recent 274B tubes are more tolerant of higher value caps, and better able to handle the 47 uF cap Steve often has used in the past. Whereas, some of the older 274Bs, as I recall (?), have a pretty low rating and tolerance, needing lower value caps (for longevity anyway). I could not find a data sheet on these Sylvanias, but guessing James has.

Type 80s, especially globes, according to forum posts here, can be vulnerable with these higher value rectifier caps too. I had no issues with mine when I was using them, but it seems I may have been luckier than many.

Also Sophia, 274Bs, even though rated for 47uF, can be touch and go here, many loving them, but apparently on the edge and vulnerable to failure with some Decware amps.

Good luck, and have fun with your new beauty maker!

the current rectifier is the factory Decware tube. None of my other amps use tube rectification except for my Woo Audio headphone amp, I have a couple of 5U4 tubes for that, nothing of any note (RCA and another that i can't recall right now) and sadly have generally found them to be noisy in comparison to the factory supplied tube that came in that amp. I presume that headphone listening puts a microscope on any noise though and for me is less than 5% of my listening anyway. I guess the 274b navy was just something that grabbed my interest, I have asked the seller for more info about use, but as a private seller they likely won't have the capacity to test. So maybe i'll play it safer and try some of the other options you suggest from a source that can test before i buy

Woo is the WA6-SE, i wasn't amazed with it at first and so rolled some tubes and probably did too much too soon, when i went back to the factory set and gave them a chance to burn in properly and for me to get settled with the sound i really liked them and it, maybe a lesson in that for me!

Hey James. I too have had a number of experiences with the cap rating not necessarily applying, but others where is might, implying there is more to it than just the cap value. And I agree, it seems pretty clear that inrush is what causes a rectifier to spark and damage the cathodes, or fail altogether, and sparking or blowouts tending to show with startup. Relative to that, also vague having been a while since I looked at this, so figured I should ask you rather than try to research it in a web so full of opinion or vagaries. But what comes to mind (not fact, but memory) is that the rectifier cap is after the rectifier, and at some level, the power needed to quickly charge a higher value cap can over stress the rectifier???

Very timely, just spotted Steve’s latest video, I’m thinking of rolling the dice and taking a punt on it

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Very cool, looking forward to your experience.

Hey Will,

After sleeping on it I'm going to speculate what is going on. Tubes work by cooking negatively charged electrons off the cathode, which flow to the positively charged anode, aka plate. But when these electrons hit the plate they kick out other, so called secondary electrons, from the plate. The secondary electrons interfere with the original electrons from the cathode and are a major limiting factor in tube performance. Tetrode and Pentode tubes are capable of much higher power than triodes because they have extra screen and/or suppressor grids specifically to deal with secondary electrons.

But rectifiers are diodes, they don't have any grids and the secondary electrons are free to run amok. So, when the rectifier needs to surge current to charge the filter cap, it is also surging secondary electrons. And I think too many secondary electrons fighting it out with the "primary" electrons is what causes the sparks and arcs. Note that while rectifiers don't have grids, some rectifiers, typically for use in high power radio transmitters and the like use gas like mercury or xenon to deal with the secondary electrons. And these can operate at much higher voltage and currents than vacuum rectifiers.

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Short answer: Yes. Longer answer: I'm confident recommending a $400-$500 rectifier with a 2.5A peak current rating for a UFO amp with 47 uF cap, even though the datasheet says 4 uF. To be clear, I'm NOT suggesting ignore the datasheet cap value. I'm saying look at it in the context of other specs, like the maximum current and voltage and compare that to how it will be used in your amp.

I guess if there is one question to which I'd like to know the answer, it is how did tube companies determine the datasheet cap value? Is it because operating in equipment different than a tube audio amplifier is more stressful? Are they being really conservative? There are other power supply designs, that use caps with inductors, aka chokes, instead of resistors (like most tube amps I know) that are more stressful. This is because of reactance and phase angle and stuff I knew once for a class 40+ years ago. Because to me, a 4 uF cap value for the Sylvania 274B in our application seems pretty harsh.

Of course there is always a disclaimer, which I don't think is news to anyone who has much experience with tube amps. I'm going to quote from a post I made a couple days ago:

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Thanks. I found it digging around. I have always preferred NOS rectifiers, and have tried lots of the 5V tubes made.

But the ones I am using now are also out of spec for Decware, 4 volt/4 pin RGN1064, UU5, and MU14, the The RGN1064 pretty low power and sounding sort of like slightly mild Euro GZ32s, but to me more musical and sophisticated than nice GZ32s. Other than a pretty high reservoir cap rating of 60uF, ratings were a little suspect to me, including being rated for 4 volts, and maybe just good luck with the tubes I bought, but I have used them for years losing only one Tesla mesh plate RGN1064 from the 50s as I recall. The UU5 and MU14s are higher power and sound a little more like GZ34s to me, more dense and forceful, which can also be nice with the right company here, and though I use them less, no failures from them so far. All need adapters.

Here anyway, in the end, I always prefer the milder RGN1064/AZ1/AZ11, these three having the same spec but different adapters. Those I have in now are Telefunken mesh plate globes. I have come to realize that my moving gradually toward less powerful tubes than stock was in part my Torii voicing being a little strong for me, and as time passed, where now everything here, from power, to PCs, to ICs, through to speakers, is modified for musical resolution, speeds and transparency, more comes through with more ease, so less powerful tubes sound plenty powerful to me.

And BTW, I have had pairs of RGN1064 go longer than a year, maybe lucky, EDIT: and for years I was using 40uF power caps in the place of 47uF stock, but I agree with Steve's saying to not necessarily count on it, and have spares around... This is not a problem for me having accumulated a few storage boxes full of nice rectifiers. I have had affairs (explorations seeking the big beauty of each type) with many types for extended periods over the years...so have a lot of tubes, and as I said, this includes my having had good luck with Type 80 globes where many did not in their Decware.

On the other hand, I had a rough time with some Telefunken labelled GZ32s, I think likely early models, the main difference I could discern in construction being narrow cathode tubes compared to other Euro GZ32s I had used for years. The seller resent several pairs before we figured out they just could not stand up to my Torii or CSP3. GZ32s were my last rectifier affair before the Euro 4 pin/4 volt types, those GZ32s with wider cathode tubes and presumably having more tolerance electronically. Interestingly, it was the seller of the Telefunken GZ32s that did not work here who finally sent me my first Euro 4volt tubes to try, pairs of UU5, and MU14.

Some other failures over time did not seem type specific, but I have had troubles with modern, supposed to be more tolerant in our amps than the venerated old 274Bs, the Valve Art 274Bs that came with my amp, one failed during burnin not too long in. Then I tried a Preferred Shuguang 274B, looking like the same tube, but "preferred" sounding good conceptually, to hear it and finish burnin with, and one failed further in, but..... And as I have said, many here have had problems with Sophia Aqua 274Bs, also a modern 274B with uprated power handling specs compared to those like you are getting.

So my cautions are not about absolutist spec hounding, but about potentially losing out from pushing limits too far, especially with high-dollar tubes, and NOS clearly does not necessarily mean durable, specs, handling, etc playing into it. Also this discussion has interested me for trying to further understand how specs relate to all else relative to rectifiers, for which I feel a little further along, but still a bit baffled.

Related, the last modification I did a few years ago in my Torii MKIV was actually to reduce the base power supply cap value associated with the rectifier. With a lot of cap experiments over years, I got to thinking that, all else similar, with bigger film caps dimensionally, lower values may filter as well or better than higher value electrolytics... not sure if this is true, but changing from a bypassed 40uF Jenson electrolytic to a bypassed 15uF Clarity CSA film (that might be 4-5 times the physical volume of the Jenson electro), I could hear only sonic refinements. And I do feel relatively confident that it has to be safer on my rectifiers, making less tolerant tubes more usable for longer, so all good.

Anyway, wishing you good luck with your new rectifier and I look forward to impressions.