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6SN7GT & 6F8G - An exploration of WWII-era octal tubes

  1. larcenasb
    I was encouraged to make a new thread for this, so here we go. These are just my impressions of some WWII-era 6SN7GT and 6F8G tubes—a handful of American treasures that I collect...well, collect and enjoy like fine wine. :) I've written this some months ago, but kept revisiting and adding to it as I listened to the tubes over and over. These observations span a few years overall listening with these tubes. I'd also love to discuss with others about the 6SN7GT, and especially the 6F8G, since there aren't too many threads about that older tube. Please post your own impressions as well!

    For context, my audio chain is: PC -> AudioQuest DragonFly 1.0 -> DIY Canare 3.5mm-to-RCAs IC -> fully-modded Bottlehead Crack + Speedball (shown below) -> AKG K240 Sextett MP. Also, the output tube in my Bottlehead Crack is an RCA 6AS7G (black-plates, clear-top, bottom-getter). Lastly, of course, YMMV...For example, if I change my DAC from the AQ DF to a Musical Fidelity V-DAC, the power in vocals through some tubes diminish. If I change out the RCA 6AS7G to a 6080, the soundstage closes in and highs seem recessed. So, of course synergy is huge here. Despite that, what I will describe below are the changes each of these input tubes make in the context of the rest of my system; only the input tube changed from impression to impression. I have explored options with my system for years and found a sound that really enthralls me...I hope you find yours as well.


    Like many, I discovered the world of these majestic octals after transitioning from the 12AU7 tube type using adapters (special thanks to Tomm11 for first suggesting this to me years ago). The 12AU7 generally has a more immediate, cleaner sound—which may or may not be a euphemism for a constricted sound, lacking in character. Personally, the best 12AU7 I’ve heard is the Mullard long-plate (date code: K61)—its tone and balance just seem so right, much like the Tung-Sol round-plate 6SN7GT. Except the Mullard is subject to the limitations of its tube type: it lacks a deep, expansive soundscape as well as weight in the mid-and-lower-mid range, both hallmarks of the octals (however it’s better at these than most 12AU7s). It’s these properties of the larger tubes that make the listening experience so transportive and emotionally fulfilling—even nourishing—for me. So, in case I haven’t been clear…I’m TEAM OCTALS. Plus, I’ve read that audio engineers and radio enthusiasts in the 50s complained that the 12AU7 was an obvious sonic compromise to the 6SN7GT of the 40s. But the 12AU7 was cheaper to make, so… yeah.


    Let’s begin (in alphabetical order):

    Ken-Rad [Owensboro, Kentucky]


    6SN7GT clear/black glass, staggered-plates

    Everyone talks about the bass performance with these,—and they rightfully should—but it’s more subtle to me with my vintage AKG K240 Sextetts. If I had modern cans with a more propulsive sound, I suspect the bass’ tightness and slam would be more apparent, but I definitely still notice the bass quality.

    More noticeable to me however is a trait that I have come to appreciate with high quality amps: presence. Voices simply sound like they come out of a fleshy water-bag with a soul who is trying to communicate with me, rather than from a person talking into a microphone…cable…capture device…post-processing…on and on…then through my headphones. The sense of presence and realism is truly magical with the Ken-Rad.

    Detail is very good from the bass to the upper-midrange. If a vocalist has a raspy voice, you hear all the starts-and-stops—there’s very little smoothing out or bloat added, yet there’s still a nice sense of weight. I don’t agree with some who say the highs are grainy (perhaps a determination of different gear), the highs in my experience are smooth but there is definite roll-off before reaching the highest, spine-tingling frequencies. Overall, a favorite for male vocalists and for modern, bass-focused music.


    6F8G round-plates

    I regard these as my pièces de résistance, even over the highly sought-after Tung-Sol 6SN7GT round-plates. Their sound signatures are similar however. “Glorious” and “high-class” are the first determinations my brain comes up with. They have some traits of the 6SN7GT staggered-plates,—especially bass performance and presence—but seems smoother and more refined overall like the Tung-Sol. And airy, which the Ken-Rad 6SN7GT isn’t really. By "airy" I mean I can sense the space between instruments really well; the soundscape isn't claustrophobic, it's wide and "air" seems to separate the instruments and vocals. The sound from the bass to the treble is very balanced as well, again similar to the Tung-Sol RP. So, even when considering others’ preferences, this tube probably allows my system to be at its objective best. Meaning it does the most things right over any of my other tubes (followed closely by the Tung-Sol).

    I never leave this tube in 24/7, though I’ll use it when I’m aching to listen to a favorite album all the way through, like Joni Mitchell’s Blue or Nirvana Unplugged in NY—to enter those atmospheres in a more real way than the other tubes allow, and to spend some rejuvenating time with the songwriters. In that sense, listening to the Ken-Rad round-plates is very much akin to having a glass of one’s special reserve label.

    Side-note: As you can see in my photos, one of my Ken-Rads (marked “WARDS SUPER AIRLINE”) has copper tabs holding the plates…I notice zero difference in the sound compared to its non-copper counterpart, but it certainly is a nice trophy piece.


    National Union [Orange, New Jersey]


    6SN7GT black glass, flat-plates

    Just looking at these, I love the quality of their construction: the copper and those support rods. Soundwise though, they are soft and timid. I mean they’re not at all aggressive or dynamic…just very easy to listen to. Good airiness around notes. Good presence but lacking the tiniest bit of weight in vocals. Good for jazzy female singers and piano; not good for Daft Punk or film soundtracks. I tend not to listen to this tube too often, but I imagine a day when all my favorite tubes die out and, in that instance, I’d still be more than thrilled to plug this American treasure in and let the music flow. :)

    Raytheon [Cambridge, Massachusetts]


    6F8G clear glass, T-plates

    Airy and pristine treble are these tubes main strengths. As well as good width and depth to the soundscape. It’s somewhat similar to the National Union but with dynamics and more authority. There’s a lack of weight in the lower-mids though that I always notice with T-plate tubes—and, being T-plates, these unsurprisingly sound similar to the Sylvanias.

    6F8G smoked, flat-plates

    Ah, I love these. Incredible power and emotional engagement. But what do I mean by “emotional engagement”? Well, it’s remarkable how different tubes can actually elicit varying degrees of physiological responses (heart pounding, goosebumps, catches in the breath, jitteriness).
    Here’s a peculiar scale… the degree to which Adele’s voice reaches me and stirs me up during the chorus of “Hello,” specifically when she first wails out “Hello from the other siiiiiii-IIIIIIIDEEEE!” (from 1-10, 10 being the most powerful and gut-wrenching):

    Mullard long-plate 12AU7 = Beautiful tone but doesn’t reach me, treble seems recessed. 6/10

    Sylvania/Raytheon t-plates = Controlled and clean, but that control makes it seem a bit restrained. 6.5/10

    Ken-Rad staggered-plates = Realistic, but the highs don’t quite reach me. 6.5/10

    National Union = Liquid smoothness to her voice...but not quite there...there seems to be a ceiling on the treble reach. 7/10

    Ken-Rad RP/Tung-Sol RP = The reach stirs me up a little bit and gets me smiling. Like the ceiling now has a nice big skylight. 8/10

    RCA = Whoa, treble isn’t often praised with this tube, but in my set-up, it's uninhibited and far-reaching. I’m forced to hold my breath a bit and act a little jittery. There is no ceiling! 9.5/10

    Raytheon smoked, flat-plates = WOW. *EYES BULGE* *HEART POUNDS* *THE JITTERS* Like the RCA, but cranked up a significant notch. Incredible viscerality. Her voice really grabs me and winds me up! I’m grinning and breathing harder after the chorus. Her voice literally blasts off at each crescendo! 10/10


    As for other aspects of the smoked Raytheon’s sound, it has a little bloat to the mid-and-lower-mid range like the RCA. Cash and Sinatra on vinyl sound very lifelike with this tube; great palpable weight to their voices…and their pain…with exquisite, natural tone. Just listen to Sinatra's "Watertown [Vinyl]"… “Ollllllllld Watertowwwwn! Nothing much happeninnn’ dowwn on Main, ‘cept a little raiiiinn…” …each word comes through with great plumpness and weight, the little inflections in his voice are detailed and wonderfully rounded.

    RCA [New York City, New York]


    6SN7GT smoked

    This was the first octal I’ve heard and, so, I’m sentimental about it. Especially also because it is still my go-to choice after all my auditioning. And I just want to say: it is a little difficult for newcomers to notice subtle differences between the tubes…therefore I think it’s important to first note the affordability of the RCA—which makes it a mandatory consideration. If you know the date codes to look for, so you don’t have to just search “VT-231” and end up with inflated prices, then you can find some identical pre-1946 tubes for much lower… I mean the Tung-Sol routinely sells for over $100 today…for one tube! While the RCA can be had for around $20-30 if you know where to look. And both are miles ahead of any 12AU7. But compared to each other? No, the Tung-Sol isn’t five times better.
    Tip: Date codes with three digits are post-1945. Pre-1946 tubes have a letter (year), a number from 1-6 (months), and sometimes “E” (OEM sales). Here are the codes for the year:

    1936 Z, 1937 U, 1938 T, 1939 X, 1940 R, 1941 Y, 1942 S, 1943 K, 1944 H, 1945 V

    So, “K1E” means the tube was shipped in 1943, between Jan-Feb, and is OEM. And “Y6” means the tube was shipped in 1941, between Nov-Dec.

    Another tip: Some tubes post-1945 are virtually identical to the wartime versions. The things to look for are the top rectangular micas that also scratch lines into the grey coating of the glass. If you find a tube with these micas but has a three-digit date code for a good price, I say go for it. It’s the same construction as the wartime version. I can’t tell a difference in their sound myself; the construction is what’s important. Without the micas though, I can definitely notice less refinement and magic.


    Now to the sound: Reach, emotional involvement, lower-mid weight, and bit of lovely tube bloat! Listen to Ray Charles’ version of “Ev’ry Time We Say Goodbye [SACD],” and at the start when he says, “OHHHHHHHHHHHHH!!” you can really hear the guttural density in his voice—with a t-plate tube, the sound is clearly leaner. The RCA is very similar to the Raytheon flat-plates. There is however some sparkle around edges of notes, as many like to say, that prompts them to describe these as “romantic.” Maybe another euphemism, this time for grain…a quintessential aspect of vintage tube sound—and that nostalgia perhaps explains the use of the word “romantic". Bass is full but not the tightest… basslines, particularly in complex passages, are more clearly separated from the rest of the mix with this tube. Soundstage is big, and here’s the most impressive pairings of traits to me: a big soundstage yet still doesn’t allow the singers to be distant or uninvolving, as I’ve said before…the reach with this tube (meaning the closeness and intensity felt from vocalists) is among the best.

    All-in-all, it’s stupendously incredible to me how overlooked this tube is if it’s not marked “VT-231”.


    6F8G smoked

    Set up a double-blind test and I doubt I could differentiate the 6F8G from its little brother, the 6SN7GT. Though without the strictures of the scientific method, my bias influences me to think the 6F8G is a little better overall, sounding bigger and more refined than the 6SN7GT…but maybe not.

    Sylvania [Emporium, Pennsylvania]


    6SN7GT T-plates

    I was pretty disappointed with this tube. Just…it’s antithetical to the big, weighty sound of the RCA that I prefer I guess. But I definitely can appreciate what this tube offers: airiness, no grit or grain, a pretty balanced sound, and pristine highs. Zero etchiness in the sound. And if you have headphones that are darker or weightier unlike mine, then this tube could be a great match for you.

    6F8G smoked, T-plates

    Same as difference from the RCA 6SN7GT to the 6F8G. That is to say… subtle or maybe none at all.

    Tung-Sol [Newark, New Jersey]


    6SN7GT round-plates, oval-mica

    Despite my list being alphabetized, there’s a sense that we saved the best for last, isn’t there? But I did already say I favor the Ken-Rad round-plates above all the others (remember that not many today have even had a chance to listen to the rare Ken-Rad). Regardless of my opinion, the Tung-Sol’s level of fame is completely mental. This tube is legendary. And even with the hype, it is worthy.

    Like the Ken-Rad RP, there’s a balance and finesse at work here that’s effortless and smooooooth. Nothing really jumps out, except that everything seems to sound “right” and “natural.” I’m entranced by my music through the Tung-Sol, and this kind of lulls me from being an analytical observer. That really is an amazing effect this tube has. It’s like you journey through the tube world, learning to accept bad traits of tubes with the good, keeping notes while seeing the different makes as different flavors for different music types—then you hear the Tung-Sol, drop your notepad and just bathe in the sounds. After a while, I went to taking notes to try and transcribe what I was hearing. The most noticeable difference to me between the Ken-Rad RP and the Tung-Sol is that the Tung-Sol has a little more smoothness and a little looser bass. Also, where the air in the soundscape of the Ken-Rad RP seems empty or alpine-like, the Tung-Sol's seems denser somehow. It's a sort of foggy or humid density yet nothing is obscured...that makes zero sense lol, I just mean to say there's great weight, roundness and analogue-ness to the entire presentation but it's still clear and nicely detailed. However, this tube isn't too exciting to me, it doesn't rile me up when listening to rock. But for more mellow music, singer-songwriter stuff, this is excellent. When I listen with this tube, I think of a quote from Herb Reichert of Stereophile, he was talking about how he went about designing his 300B amp, but it rings true here. He said: "What I don't want is distant, thin, or mechanical sound. [I want] lots of flesh and blood..." And the Tung-Sol is faithful to this human desire. That isn't to say none of the other 6F8G or 6SN7GT tubes sound true-to-life, but the Tung-Sol does it in a way like none of the other tubes do. I did my best to give you an idea, but it really must be heard...and you'll immediately understand what all the fuss is about.



    In the end, I can be objective and say the Ken-Rad and Tung-Sol (both round-plates) are the best, but subjectively I still prefer the weighty and more powerful presentation of RCA and Raytheon (both flat-plates). The round-plates generally get me thinking more that this is very accurate to what this music would sound like in real life in a real space, the flat plates generally still sound lifelike but has some nice vintage grain along with great weight, immediacy, and emotional power. And I just like it. So, here's my general preference: flat-plates > round-plates > t-plates. Remember though about synergy; I'm not a mindless cult leader saying "Stay away from the forbidden T-plates!" They just aren't my preference with my gear. I can imagine having the Sennheiser HD 650 that everyone says is quite warm...in that case, the RCA or Raytheon flat-plates might overdo the warmth, and the Sylvania might be the better equalizer. So, just take my impressions lightly, as a baseline detailing the subtle differences I hear with these tubes, and try to figure out how they'd factor in to the sound of your system. I'm hoping it'll be useful for you to at least have these ghosts of ideas, rather than shelling out tons of money without any idea at all about the tubes' sound signatures. Cheers.

    These tubes are nearing 80 years old—what a history! And they aren’t being made anymore, nor will they likely ever be made again. For there to still be quite a few NOS and good used tubes sitting on shelves is a great gift for audiophiles. Why not take advantage of this fleeting opportunity while we can? To me, it’s the same as asking, “Why not take advantage of the scarcely few breaths we have at all in this world?” Remember, “audiophile” means you care about how the music sounds, not just care about the music itself. I think how it sounds really matters to most audio enthusiasts even if they don't like the label “audiophile.” Caring how it sounds means you're showing respect to the music. I mean if it doesn’t matter how it sounds, then cheap earbuds is all you need. But, to me, it really matters. I could enjoy music fine with cheap earbuds, but if I am able to listen with more depth, realism and excitement…I will. And discovering these vintage tubes take me there has made me very happy. For those that say "tubes aren't accurate" or "they color the sound," I'd have to respond by saying, "Well, accurate to what? To the way the electronics captured the sound? What about the sound before entering the microphone? I'd rather aim to be true to the real sound than the captured sound." Anyway, Jimi Hendrix swore by tubes almost as if anything else was flat-out soulless, and legendary designer Bascom King acknowledged that the best, cost-no-object amplifier he could design required it have an input tube...These people know what they're talking about. And these tubes are still here, let’s enjoy this while we can…
    Last edited: Oct 6, 2018
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  2. alvin sawdust
    A rca smoked glass round plate 6f8g has taken up permanent residence in my Teton. Can't quite put my finger on it but it has a kind of ethereal quality to it.
    larcenasb likes this.
  3. alvin sawdust
    Great write up and pics by the way.
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  4. larcenasb
    Hi! RCA "round plate"?? I've never seen that, do you have photos!? :D

    EDIT: And thank you for the kind words. :)
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  5. alvin sawdust
    6f8g 1.jpg 6f8g 2.jpg

    Apologies for the low quality pics.
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  6. larcenasb
    Ah, the meatball logo and military markings! Beautiful tube! I've only seen this as a flat plate though. But yeah, compared to the tops of my RCA 6F8Gs, yours does look different (1st photo). Are you able to see the round plates under the right lighting conditions? If so, does it look like the round plates of the Ken-Rad, Tung-Sol, National Union, or something else entirely (2nd photo)? Thanks for exploring this with me! :) IMG-0890.JPG
    rp constructions.jpg
    Last edited: Oct 4, 2018
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  7. alvin sawdust
    6f8g 3.jpg 6f8g 4.jpg

    Here is another smoked rca round plate that i picked up for 20 quid. Not as smoked as the other one so you can see the construction a little better.
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  8. larcenasb
    Ah, okay, much clearer! I can see these have the two parallel vertical flat plates like all of my RCAs have. Yeah, I've never heard of RCA making a round plate 6F8G. But there's nothing wrong with flat plates of course! Like I say in my original post...flat plates are my preference! Round plates generally have wonderful qualities that I adore, but flat plates generally have the weight and drive (and price) that makes them my go-to. Cheers!
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  9. alvin sawdust
    You're right, they aren't round but flat with a ladder detail on them, doh! Slightly staggered, just like me.
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  10. larcenasb
    Haha :) They are pretty much permanent in my set-up as well. That ethereal quality you speak of...to me it's the sparkly highs, pleasant grain, wide stage, and plump & meaty tones. Hopefully one day I'll get a GEC 6AS7G like you have there to go along with the RCA, I can imagine that must sound superb!
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  11. alvin sawdust
    Yes you are right, and ever so slightly forward, but that might in part be down to the Teton.
    To be honest I think the gec is great but way over priced and a little over rated. Us talking has gotten me trying other combinations and one of my favourites is the underrated Mullard 6080 and a pair of Mullard cv6 in place of the 6f8g. Also have a Mullard 5u4g in so all Mullard.
    Sorry for going OT.
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  12. larcenasb
    No, no, no! This is what it's all about! Exploring what these tubes can do, and especially how they synergize with other tubes. Thanks for your honesty about the GEC, my aching urge to get one has subsided a bit now haha. I'll just jump on it if I find a good price one day. The RCA 6AS7G really synergizes well and allows the RCA 6F8G to really show its strengths. For example, I have a Sylvania Gold Brand 6080 that is warm and pleasant but seems to mask the differences between the input tubes a bit...The RCA is much more revealing.
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  13. larcenasb
    "All Mullard" brought to mind this old ad I saved..."The Master Valve." :) I would like to try some British octals, and I see an adapter on eBay for dual-CV6-to-12AU7, but maybe the Brimar 6SN7GTY/CV1988 is what I'll try to get next. Others say it's similar to the RCA smoked-glass but more plump and refined sounding.

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  14. larcenasb
    Here are some other very cool vintage ads from wartime America for anyone interested in this exploration. I have the RCA one in a clear sheet protector next to my amp. :)
    ken-rad coming ad.jpg ken-rad prince tube ad.jpg ken-rad wwii ad.jpg raytheon-ad-feb-1943-qst.jpg RCA WWII ad.jpg
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  15. alvin sawdust
    Those are great old ads. Tubes do tend to conjure up feelings of nostalgia, especially as you pop in a new one that has been hiding in a cupboard for decades and watch it slowly light up. Part of the magic.
    The 6sn7 and it's variants are well documented on this forum. Ones I have are 6sn7, 5692, l63/6j5 x 2, cv6/det20 x2, 6FQ7/6CG7, 1578, 7193 x 2. Have even successfully used a 6dj8.
    I also have a rca 6as7g, looks like it was made in the 70s. Not sure how good it is but might give a whirl this evening.

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