Basic tube pinout/construction

Discussion in 'Amp Tech Center' started by W.L.Weller, Sep 20, 2019.

  1. W.L.Weller

    W.L.Weller Tele-Holic

    May 20, 2014
    I'm thinking about building a single-ended tube amp that'll use a 7591A power tube (because I already have a half-dozen, and an amp using a pair of them configured push-pull) (and I have a different single-ended 6V6 amp already).

    My question is about the difference between the pinout of the 7591A and the 6CA7(EL34).

    On a 7591A (and for that matter a 6L6), the G3 and K internal connections share a pin.

    On a 6CA7, the G3 connection and K connection are separate pins.

    Looking at the ur-text of SE amp circuits, (not actually, since the ur-text would use octal preamp tubes etc), the 5F1, I only see (non-heater) connections to the anode, two (2) grids, and the cathode. The 6V6 and 6L6 have an identical pinout, and while the 7591A does not match 1:1, it shares the internal connection between G3 and the cathode in common with those tubes.

    The 6CA7, alternatively, uses pin 1 as the connection to the third grid, and pin 8 as the connection to the cathode.

    Obviously, you can not plug a 6L6, 7591, or 6CA7 into a socket wired for one of those tubes specifically.

    My question is whether changing a single-ended circuit designed for a 6CA7 to use a 7591A involves accounting for the G3, K difference specifically, outside of the pinout differences.
  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    One can plug a 6L6/6V6/7591 tube into a socket that is prepared for an EL34. One cannot plug a 6CA7/EL34 type of tube into a socket prepared for a 6L6 type of pinout IF that socket is wired to use pin 1 as a contact point for another purpose as with a Fender that places the 1500ohm control grid stopper resistor on pin 1 to feed the control grid at pin 5. When one wants to run an EL34 in a Fender amp, one has to reroute that bias voltage feed in order to clear that pin 1 for the connection from pin 1 to pin 8.
    The connection to G3 is thereby made good for the 6CA7/EL34, and pin 1 is empty in the 6L6 so that jumper from pin 1 to pin 8 is of no effect with a Beam tetrode type of tube.
    Last edited: Sep 20, 2019
    W.L.Weller likes this.
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