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Why plate degeneration in a cathode follower?

Discussion in 'Electronic Design' started by Robert Baer, Jan 9, 2013.

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  1. Robert Baer

    Robert Baer Guest

    What is the benefit of adding plate degeneration in a cathode follower?
    All i see is that it decreases the follower gain, say from 95% to 90%.
  2. Tim Williams

    Tim Williams Guest

    At least for a triode... should be almost zero effect for a pentode (if
    the screen doesn't also connect below the resistor).

    Same thing a collector resistor does on a BJT follower -- dampens the
    common-collector oscillation mode. Base/grid/gate stopper is more
    traditional though, better efficiency.

  3. Robert Baer

    Robert Baer Guest

    ? Common collector?
    The device i am fixing has triodes (a number of 6SN7s to be more
    specific) and every cathode follower has a 10K resistor in the plate, NO
    pickoff for phase splitter-type use.

    The plate resistor reduces the cathode follower gain, and i do not
    think that it could possibly reduce might even increase
    the distortion (K-A voltage varies more).

    ...Now if usage was as inverter and resistor was added in the cathode,
    again the gain would be reduced but the distortion also would be reduced
    (less G-K resultant drive).
  4. [...]

    1) Equalising load currents between several valves in parallel without a
    significant increase in output impedance?

    2) Peak current protection in the event of an inter-electrode short or
    other abnormal operating conditions?

    3) Reducing the difference in the output impedance between positive and
    negative half-cycles so as to avoid high frequency rectification effects
    due to charging and discharging the cathode-heater capacitance?
  5. Rocky

    Rocky Guest

    The plate resistor increases the Miller Effect which causes HF roll off.
  6. Robert Baer

    Robert Baer Guest

    ...Please wait; my access to my site is down temporarily.
  7. Robert Baer

    Robert Baer Guest

  8. Tim Williams

    Tim Williams Guest

    Come on Jim, ain't your rusty ol' brain remember this stuff?

    If Robert's referring to V2, they aren't doing anything helpful.

    Very old circuit. Mine has "Eico" painted on it, but uses 6K6s for the
    higher power bits. 6X4? Gah, a submini in an octal set... what an
    abomination :)

  9. Robert Baer

    Robert Baer Guest is an AN-TEEK!
  10. Tauno Voipio

    Tauno Voipio Guest

    He's preparing to grill the final tube pair, the cathode bias is
    destroyed by moving the grid resistor low ends, and the plate
    current protection resistors are shorted.

    The oscillator quality is compromised by moving the output
    tap from the buffer to the feedback line, and the buffer
    isolation is compromised by adding the cathode bypass.

    Hope he has spare tubes ...
  11. Robert Baer

    Robert Baer Guest

    The "X" indicates where i broke a connection, the arrows show a new
    connection (eg: move grid resistor from ground to cathode), and adding
    the jumpers put across the 10K plate degeneration resistors.

    Lastly, the arrows show that the wire from the sine/square/pulse
    switch was moved from the divider to the (other) plate.
    Maybe i should experiment trying V5A instead of V5B plate for even
    greater overdrive (level was 6Vpk-pk now 30Vpk-pk; cold be 65Vpk-pk if
    used V5A).
    PS: Sorry i forgot how the Abbott & Costello joke went..
  12. Robert Baer

    Robert Baer Guest

    Maybe, but having a signal generator with sine output FIVE TIMES LESS
    in amplitude WRT other waveform output selections just ain't kosher.
    The CFs do not run any harder than before, the V5A bypass gives more
    drive for a better square / "pulse" waveform and has little effect on
    the oscillator; V5A is nowhere near to grid or plate glow..
  13. josephkk

    josephkk Guest

    For anybody that is interested:

    First find of course.

  14. Robert Baer

    Robert Baer Guest

    Sorry, i do not live in the PLAINs area of the US; i live in WA.
    Itza signal generator; selectable output sine, square, pulse.
  15. Tauno Voipio

    Tauno Voipio Guest

    The OP could start by restoring the switch labels (and tube envelopes)
    in the schematic.
  16. Robert Baer

    Robert Baer Guest

    I give up; if you cannot read a schematic..

  17. Wasn't The Who on first and Jimi Hendrix on third?

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  18. Guest

    I think that was The Who. ...or CSI, I can never remember.
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