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2359glenn | studio

Discussion in 'Headphone Amps (full-size)' started by silent one, Mar 9, 2012.
  1. gibosi
    Interestingly, "6J5 S1" stamped in the base corresponds to an RCA date code indicating January, 1957. And even more interesting, "DL" in the Sylvania two-letter system indicates November, 1957. So sometime in 1957, but we can't be any more precise. And I wonder... do your ears tell you this is an RCA? Or a Sylvania?
  2. whirlwind

    Great work Keenan :)
  3. whirlwind
    I have just discovered this fellow and thought that I would leave this hear for those interested in this sort of stuff.
    Listening with GOTL 6080 Bendix/ECC33/ZMF Atticus


  4. mordy
    Hi gibosi,
    Did not think of cross checking if this is a relabeled RCA tube for two reasons: Based on my readings, I was under the impression that the metal tubes were fazed out in the late 40's since the glass tubes were cheaper to manufacture.
    Secondly, the green oak leaf Sylvania logo appears on older tubes and it is my impression that they did not use it in the mid 50's but I may be wrong regarding that.
    In the same lot were two Tung Sol tubes with a date code of 5652 which would indicate December 1956. One of them has a code 3Y stamped in the base edge and the other Y3; the latter codes possibly indicating either month in a two month period.
    So here is my conspiracy theory: These tubes are older production, and labeled/relabeled by the date of sale. But maybe not....and mid 50's production? The stamped letters SI look different than the stamped letters 6J5 on the Syl/RCA tube.
  5. gibosi
    "Y3" and "3Y" stamped in the base corresponds to the RCA date code indicating 1956. And as you surmise, Y3 indicates May, 1956, and 3Y indicates June, 1956. And the date code 5652 indicates that Tung-Sol packaged this tube in December, 1956. Also, there is considerable evidence that RCA continued to manufacture metal tubes using this dating system at it's Harrison factory until at least 1968.
  6. GDuss
    The tube-identification knowledge of all of you experts is amazing. I have a question related to RCA 6J5s and their age. Does the design of the RCA logo have any meaning in terms of when it was manufactured? I am under the impression that the one on the right in this link is an older-style logo and the one on the left is a newer logo, but I could be totally wrong.

    chrisdrop likes this.
  7. gibosi
    I don't really have all that much "tube-identification knowledge". I just know where to look. :)

    Yes, the one on the right, with the "meatball" logo is the oldest. "K1E" indicates Jan/Feb, 1943. The one on the left, with the stylized silver letters "R C A", is later. "LM" indicates Sep, 1959.
  8. chrisdrop
    I just had a material reduction in my noise floor. Putting the amp on something squishy removed noise - presumably; transformer -> tubes. This has made my day. Some sort of isolation feet are in my future. I can't believe how stark the difference is...

    On a less Glenn amp related note, I am happy to report more positive "cable flogging"; As recommended to me by keen cable DIYers; Neotech UP-OCC copper headphone cable is proving a step up from stock cabling and also from my Forza Hybrid (copper/ silver). Solidly recognisable.

    And finally, a few more 6J5 pics. Not used yet. Still enjoying current setup too much!


  9. OctavianH
    Question related to the KT66/KT88: why on the GOTL compatibility table these are marked as "non compatible"?
    The voltages and currents seem in line and these are really kicking ass on the F.A. amps.
  10. GDuss
    Knowing where to look is important knowledge.

    And this is what I thought about the logos. On RCA tubes with the circle logo where I have been able to identify the date (or the seller has), they are commonly 40's. On tubes with the text RCA logo where I knew the dates, these have commonly been 50's. I've also seen the term "meatball" used and I assumed it was referring to this circle logo but that had never really been clear to me. Thanks for this information.
  11. GDuss
    I see one of these in your future

  12. chrisdrop
    Looks like a mag-lev butcher block :)

    My kitchen onion dicing will NEVER interfere with my listening again!
  13. L0rdGwyn
    Generally these tubes are not used in OTL amplifiers because they have high plate resistance, meaning they will have a high output impedance without an output transformer, and therefore, a very low damping factor = distortion. I don't know much about the FA OTL topologies, so no idea what the output impedance would be with these tubes in those amps, but it's probably pretty darn high. If they employ negative feedback, could drop the output impedance down to a usable range with high-impedance headphones maybe? Glenn might know more about those amps.

    The reason you see 6AS7G, 6080, etc. used so often in OTL amplifiers is because they have a relatively high plate current with low plate resistance, leading to a lower output impedance without the need for a transformer (with a sufficiently high resistance load).
    Last edited: Nov 26, 2019
    DecentLevi, leftside and OctavianH like this.
  14. GDuss
    Well multitasking right?

    It's a vibration-isolation table and what is not shown are the air tanks used to pressurize the tabletop so that it floats on an air cushion. You can usually get these for about 4-6K (US), so you'd have to decide between one of those or a few more Glenn amps :).
    chrisdrop likes this.
  15. 2359glenn
    The main reason they will not work is. In FA amps the two half's of the 6AS7 are tied together in parallel and wired as a cathode follower.
    In my amp the two half's of the 6AS7 are separate and not a cathode follower. Run as SEPP
    To use these tubes a special adapter would have to be made Each KT88 wired as a triode would be hooked to each side of the 6AS7 socket.
    And there internal impedance is to high to get good sound in a OTL without a output transformer. The bass would be bloated and not tight
    might sound like lots of bass but not good bass.

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