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6SN7 Tube Addicts

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  1. gibosi
    It is important to remember that there are significant differences in amps. Some amps are less susceptible to noise than others. And as leftside stated earlier, he has thrown out a number of noisy 68FG and kept the quiet ones. Quiet ones are just that: quiet.

    The top cap provides more isolation between the grids of the two triodes in an attempt to reduce RF-induced noise, and the use of a cap also freed up pin 1 to be connected to a metal shield, again to reduce RF-induced noise. But when you install a 68FG into an adapter allowing it to be used in a 6SN7 socket, these noise reducing measures are eliminated. The grids are no longer isolated in the socket and and there is no way to use a metal shield.

    So again, it is best to accept the fact that this tube is more susceptible to noise than a 6SN7 and it will often be necessary to roll through several in order to get a quiet one.
    L0rdGwyn likes this.
  2. L0rdGwyn
    Thank you for the lesson in 6F8G anatomy. I have since rolled through all eight of the 6F8G's I have on hand (three Tung Sol, two Sylvania, one Raytheon, National Union, RCA), they are all equally noisy and exhibit the 60Hz buzz.

    This would lead me to believe that it is a product of the amp circuitry and noise itself, since all tubes are equally affected.

    Would you mind sharing your thoughts on why the noise goes away when I touch a finger to the top of my amp? Is my body acting as a shield for the tube, and is there a way to possibly emulate this via modification? I appreciate the help.
  3. gibosi
    Well, I am not an electrical engineer, far from it, but like you I have noticed that touching the amp, and sometimes just placing my hand next to a tube reduces, and sometimes eliminates the hum. And like you, I suspect that my body is somehow acting like some kind of RF shield.

    I have thought that maybe the metal tube shields that were commonly used in old radios might be worth a try. But since they are in fact made of metal, I worry that I might accidentally short something out and I have never followed through on this idea...
    L0rdGwyn likes this.
  4. Pharmaboy
    This thread is pretty big & I have yet to read all of it (though I will). My concern is indirectly related to a recommendation for the best sounding 6SN7 for my application (Woo WA22), but it's a bit more involved than that:
    • I purchased this amp w/a complete set of NOS tubes installed (Woo's stock tubes have never been taken out of their boxes)
    • The 6SN7 pair is taken by a matched pair RCA JAN 6C8G (6SL7 equivalent), which are now in 6SN7 location
    • I'm already on the hunt for a legendary Mullard rectifier (not part of this thread, I know).
    This amp already sounds quite good as it is--still burning in. But I sense this is a real opportunity for add'l tube rolling to kick it up 1-2 sonic levels...

    With that background, can anyone here advise me as to the following issues:
    1. Do I have any sonic benefits ahead of me for reverting that pair of tubes to 6SL7 (the best-sounding ones available)?
    2. If yes, what kind of sonic benefits can I expect?
    Thanks in advance for any guidance I receive (I'm pretty inexperienced w/tube rolling, in case it isn't glaringly obvious!).
  5. leftside
    Pharmaboy likes this.
  6. L0rdGwyn
    I've taken a look, 6F8G shields are hard to come buy, and probably not worth the risk anyhow, I'm sure I can manage. Thanks again for your input.
  7. L0rdGwyn
    I was scouring new ebay listings the other day and scored a lot of eight 6SN7's for $70. Six are random GTB's (Tung Sol, Zenith, GE, Sylvania, RCA), really only grabbed it because it included the two tubes below.

    I am very impressed by the RCA VT-231. I know it is considered to be on the warm/euphonic side, but I paired it with what I consider to be a more bright and forward output tube (Tung Sol 7802) on my Crackatwoa, and the combination is a very balanced, clear presentation.


    I know this is old news for you 6SN7 vets, I'm just catching up :o2smile:
    Wes S likes this.
  8. Wes S
    I love the RCA grey glass! One of my favorite tubes! I also have a grey glass National Union, but have not heard the black glass version. The National Union black glass, is on my list to try.
    L0rdGwyn likes this.
  9. attmci
    I have no problem with most of my 6F8Gs. The noise level is non-existent on my amp.Yes, I am extremely sensitive to tube hum.

    I always let the tubes warm up at least 20-30 min.
    Last edited: Mar 8, 2019
  10. attmci
    Have you tried the smoke glass RCA 6F8G? Very nice tube.
  11. L0rdGwyn
    I had sort of a breakthrough on my 6F8G noise problem. I recently moved into an older home, some of the outlet grounding is questionable methinks. I used an extension cord to plug my amp into a remote outlet. Lo and behold, the noise level was significantly decreased! Only audible hum and RF noise was at listening levels that would make your ears bleed.

    So final diagnosis: the outlet I have been using is poorly or un-grounded, resulting in a ground loop on top of the intrinsic tube noise. Either the extension cord is going to become a permanent part of my setup or I'll be calling an electrician :)
    leftside likes this.
  12. attmci
    You need this:

  13. L0rdGwyn
  14. attmci
    Much cheaper than the $122 million F-35B.
  15. leftside
    6SN7 variations. GEC L63 with 6J5 to 6SN7 adapters. Some of the best sounding tubes I have.

    GEC L63-1008x756.JPG
    Monsterzero and L0rdGwyn like this.
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