Tube direct box cicuit

Discussion in 'rec.audio.pro' started by paul tumolo, Sep 1, 2003.

  1. paul tumolo

    paul tumolo Guest

    i am looking for a circuit diagram for a tube direct box. something with an
    input impedance of at least 10Mohm. thanks.
  2. Scott Dorsey <kludge@panix.com> wrote:

    > Go to the RCA Radiotron Handbook and look up "cathode follower." Put a
    > transformer on the output to balance it. The input Z is entirely due to
    > the leak resistor, so if you use a 10M leak resistor, you have a 10M input.
    >
    > Very clean and transparent if it's biased right. The transformer will
    > be pretty much all of the coloration and it'll be damn near impossible to
    > overload.


    How do you calculate the output impedance of a cathode follower? I
    think I know how to measure it, but I'd rather do the math. It can't
    be higher than the cathode resistor, right?

    Also, I know any cathode follower will have a gain less than unity, but
    what can you typically expect from a 12AX7 or 12AY7? I know a 12AY7
    has lower gain than a 12AX7 in a typical plate-loaded circuit, but is
    there a substantial difference in the signal level you'll get out of
    them in a cathode follower circuit? Seem like a good idea to use both
    halves of a 12AY7 in parallel and thereby use a lower-ratio output
    transformer.

    Sorry, I dunno where Johnny put the Raditron. So I ask and I ask.

    ulysses
  3. Scott Dorsey

    Scott Dorsey Guest

    Justin Ulysses Morse <ulysses@rollmusic.com> wrote:
    >Scott Dorsey <kludge@panix.com> wrote:
    >
    >> Go to the RCA Radiotron Handbook and look up "cathode follower." Put a
    >> transformer on the output to balance it. The input Z is entirely due to
    >> the leak resistor, so if you use a 10M leak resistor, you have a 10M input.
    >>
    >> Very clean and transparent if it's biased right. The transformer will
    >> be pretty much all of the coloration and it'll be damn near impossible to
    >> overload.

    >
    >How do you calculate the output impedance of a cathode follower? I
    >think I know how to measure it, but I'd rather do the math. It can't
    >be higher than the cathode resistor, right?


    The math is in the Radiotron Handbook. For a triode with a single
    cathode resistor, Rk, the output Z is Rk/1+gmRk. For a pentode you
    fudge the gm for whatever the operating point you have the screen set
    at. I don't remember the other configurations but they are all in
    the book.

    >Also, I know any cathode follower will have a gain less than unity, but
    >what can you typically expect from a 12AX7 or 12AY7? I know a 12AY7
    >has lower gain than a 12AX7 in a typical plate-loaded circuit, but is
    >there a substantial difference in the signal level you'll get out of
    >them in a cathode follower circuit? Seem like a good idea to use both
    >halves of a 12AY7 in parallel and thereby use a lower-ratio output
    >transformer.


    You can get as low an output impedance as you want, as long as you don't
    want much voltage swing. From a 12AT7 you should be able to drive a
    50-ohm load up to -10 dB without any linearity issues. Look at the output
    stage of the Harman Kardon Citation II for a nice example of a well
    designed follower output stage.

    >Sorry, I dunno where Johnny put the Raditron. So I ask and I ask.


    You need it! It has everything you need to design tube or fet circuits.
    --scott
    --
    "C'est un Nagra. C'est suisse, et tres, tres precis."
  4. On Tue, 02 Sep 2003 18:21:01 -0500, Justin Ulysses Morse
    <ulysses@rollmusic.com> wrote:

    >How do you calculate the output impedance of a cathode follower? I
    >think I know how to measure it, but I'd rather do the math. It can't
    >be higher than the cathode resistor, right?


    The active device's output impedance is the reciprocal of its
    transconductance. Circuit resistances all contribute as you'd
    expect 'em to.

    >Also, I know any cathode follower will have a gain less than unity, but
    >what can you typically expect from a 12AX7 or 12AY7? I know a 12AY7
    >has lower gain than a 12AX7 in a typical plate-loaded circuit, but is
    >there a substantial difference in the signal level you'll get out of
    >them in a cathode follower circuit? Seem like a good idea to use both
    >halves of a 12AY7 in parallel and thereby use a lower-ratio output
    >transformer.


    Lightly loaded high-mu devices have gains of about mu/mu+1, IOW,
    pretty darned close to unity.

    A good way to think about loading is that loading should not be
    heavier for a cathode follower than for common cathode use. The
    device operates on the same load-line, same plate curves, etc.


    Chris Hornbeck

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