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I have been into tuning so much, and so long, my preferred envelope is probably relatively narrow, and perhaps somewhat subtler stuff sounds big to me???
That in mind, I like variety within this envelope, and have been testing a bunch of 5U4s the last several days, the first rectifier I got into more extensive exploring with quite a long time ago ... I started with a smallish bottle RCA 5U4GB I recall trying because Lon liked them back then in his room. The particular tube I got was notably weighted toward bass, and my room, with Bob's MG944s and the Torii III (both designed for big bass), this particular tube tended toward an overly thick/full bass, so a bit too thick/muddled, while darkening the mids, and the highs were recessed, not cutting through the bassy veils very well...
Bad combo for me, this was compared to the stock Ruby 5U4G-ST that came with my very early Torii III, and some reproduction JJ and Sovtek 5AR4s. The Ruby was pretty neutral and open to me, but a little thin and uninteresting, and the 5AR4s were more potent....fast/dynamic, with bold tight bass, articulate, and spacious, tubes I used mostly then.
Then I found a late 40s Raytheon 5U4GB with a two step base made to accommodate a fat/straight bottle, black plates, and top/side horseshoe getters on both sides. Seems most fat bottles, the glass narrows on the bottom to fit into a narrower bass, so this tube caught my attention as unusual, and early. Still weighted toward bass in the balance, but also nicely dynamic and really good at fine detail and space.... and having finally found 5AR4s a little in the face on dynamic hit in my system/room, these were interesting to me. The bass needed some adjustment with some leaner tubes elsewhere, but then, instead of dark, I would call this tube "warm" having good space and detail within its dark leaning sound, even slightly euphonic, the complex warmth having a sweetness.
Chasing more 5U4GBs, I ended up with several really good sounding ones (to me). And who knows, with the better ones, I might still be using them had I changed back then to more open OC3s, but I only had warm/smooth OC3Ws at the time, nice tubes, but with two baseline tubes tending darksish, I didn't get into these OC3 with the 5U4GBs. Though I never liked OA3s... too forceful in my setup, there was something about the slightly milder/more open, and generally smoother feel from OB3s I loved here, so I tended to stay with them mostly for years.
The other day, starting these tests, I had been running sophisticated and supple sounding mesh plate 4V, 4 pin RGN-1064s (a Tesla still in the CSP3), at the time with OB3s in the heavily modified Torii IV. Though my system and room are much more refined, balanced and resolving than during my first foray into 5U4GBs, for me, they were still tending too dark, thickening/slowing the bass too much (especially on recordings leaning that way), and in turn masking the mids a bit much. A really good sound, but I need open space and relative neutrality, with tight/fast bass, complex detail, and within a mild warmth. So I put in my current fav OC3s that I actually have been using most in the last year, and they did a lot to balance the 5U4GBs in my system.... more open, textured, spacious, less bass thickness....now most seem to work on most recordings here (an important tuning standard for me) though still it is comparatively a darkish, yet resolving tube here.
I ended up gravitating to fat bottles and tall/fattish bottles, most with laddered black plates. All sound different, most good in my setup if you like that "flavor." Overall, they are notably powerful sounding dynamically, and tending bassy, but with the right company, reasonably tight bass, open/textured mids, and pretty complete detail with most of them..... and most are slightly euphonic without being syrupy in my system/room.
In early quests, seeking something similar, but less weighted toward bass, I found some value NOS RCA 5U4G-STs with a top/side getter, and this tube was more neutral, less forward, with more fine detail complexity, and pretty smooth in general...sort of like a sophisticated, fleshed out Ruby. An interesting change, I started watching for them. A few I tried had a little bigger/heavier black plates and bottom getters, nice also if the system needed more bass, but a little full/dark/solid here. Whereas the slightly lighter build plates and top side getters are more neutral and open here...maybe later 50s and early 60s models??? But finally these tubes ended up a little predictable to me they were so dependably relatively neutral in most every balance.... not much sticking out, not necessarily a bad thing, but I prefer a tube now that sucks me in more with a little more characterful complexity.
Comparing the famous '55 Winged 5u3Cs today (5U4G-ST equivalent), the Russian tube is similarly warm as some of the nicer 5U4GBs, but in comparison, has less speed, hit, detail complexity, and spaciousness, so not as alive and engaging to me. And though bigger warmth is not my thing in general, I do like a taste of it that is not particularly obvious, as long as the darker hue is relieved by good detail complexity that is not too recessed. These particular 5U3Cs are definitely warm, and the understory relatively balanced within that. But they are slow enough to feel veiled and lacking in fine detail for me.... Maybe the appeal of this tube has to do with more sharp system/rooms... don't know but lots of folks love this tube, so guessing different needs.
I have not tried 5Z3s, but by reports these are in the range (and look) of the 5U4G-STs, but generally quite a lot cheaper, the oddity of needing adapters presumably helping to keep them that way so far.
So that is a little 5U4 view from my room today anyway... hope it is of some use. I am enjoying playing with 5U4GBs...
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