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Thread: Question about 7591 tubes

  1. #1
    Senior Member
    Join Date
    Sep 2010

    Question about 7591 tubes

    This drawing looks like I make the 348v grid connection at both pins 4 & 8. Can anyone shed some wisdom on me about what's going on there? Tube data shows they are both G2, why not be able to connect to only one of the pins?

    ~Semi-No0b Hobbyist~

  2. #2
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Wernersville, PA
    The screen pins should be tied together for "efficient operation of the screen". (from the data sheet/ last page)

    Justin Thomas and mort like this.

  3. #3
    Old Timer
    Join Date
    May 2006
    Chill-Ville, VA
    And, Ampeg did exactly that. They're the most frequent guitar-amp user of those tubes. Of course, for "inefficient" screen operation... doesn't say anything will explode now, does it?

    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  4. #4
    Senior Member mhuss's Avatar
    Join Date
    Oct 2006
    West of Philly
    I wouldn't be surprised if the two pins were connected to each end of the screen grid (in fact, that's how it's drawn in the RCA manual page). In that case, connecting both == minimum impedance, best result, but either end would still work.

  5. #5
    Supporting Member loudthud's Avatar
    Join Date
    May 2006
    Near Dallas Texas
    Often times the internal structure of a tube dictates the pin connections. In this case note that pins 4 and 8 are located directly opposite each other. The screen grid consists of a pair of vertical rods that go through the bottom mica disc, straight through the glass and into the base without any bends. A spiral wire between the vertical rods forms the screen grid. This may be the only support for the internal structure of the tube while it is being assembled.
    sgelectric likes this.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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