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What does a cathode follower do?

Discussion in 'Amp Tech Center' started by Ed Storer, Sep 11, 2017.

  1. Ed Storer

    Ed Storer Tele-Holic Ad Free Member

    830
    May 27, 2010
    Seattle WA
    Title says it all, but in my ignorance of amplifier design - and the fact that only one of my three guitar amplifiers is cathode biased, I'm curious.
     

  2. Wyatt

    Wyatt Tele-Holic

    806
    Nov 3, 2004
    Cathode Follower is irrelative to Cathode Biasing.

    In guitar amps a cathode follower is most often used in place of a gain stage, it gets its name because the signal is outputed through the cathode instead of the traditional plate output. CF's are buffers, pure and simply buffers, with high impedance input and low impedance output; their super low output impedance allows them to maintain the signal integrity while driving the next stage.

    Most traditional Marshall circuits (and Tweed Bassman) use a CF instead of a gain stage for driving the tone stack while producing a rather unique overdrive (hot-rodded Marshalls usually swap this to a gain stage instead for more drive); Dumble used CF's after the PI in some of his Steel String Singers to give the power amp more clean drive; and almost all tube-buffer FX loops use a CF to match the outgoing impedance of the SEND circuit (coupled with a Gain Stage for boosting the RETURN signal).
     

  3. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Lots of things:

    1) provides impedance isolation (hi-Z input to low-Z output)
    2) provides power gain (but not voltage gain)
    3) provides current gain (but not voltage gain)
    4) provides one-HALF of Cathodyne PI output (the in-phase half)
     
    Wyatt likes this.

  4. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Hey old tele man, I hope all is well!

    Are you sure about #2 and 3 above?



    I thought current gain and the resulting power gain is only in the realm of the power tubes.



    pre amp tubes can either gain voltage, not gain voltage, or even lose voltage but it's not their place to be growin the power.

    I could be wrong but I thought that was the basis of it all.
     

  5. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    Old Tele man speak truth!

    Your thought wrong, you basis wrong. You missunderstand power.
     
    RLee77 and aerhed like this.

  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    The 12AU7 makes a pretty good power tube for low output amps. It's all in the supporting circuitry.
     

  7. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    The cathode follower in its DC coupled form also can add some interesting overdrive flavor when pushed. It provides progressively greater compression and even order harmonics as the signal level rises. I hear its effects strongly in one of my amps; as gain starts to get towards max the overdrive gets actually less harsh/aggressive and more fat and thick.

    For more reading:

    http://www.valvewizard.co.uk/dccf.html

    There's a couple of pics in the link that show the asymmetrical compression effect.
     
    Commodore 64 likes this.

  8. 3-Chord-Genius

    3-Chord-Genius Friend of Leo's

    Apr 3, 2015
    Winchester, VA
    I don't know but whenever I see a word with "cath" in it I think of a catheter in my weenis, and I absolutely don't like that. I'm cringing.
     

  9. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Ok, a preamp type tube could be wired like a power tube, and then it would be a power tube.



    But, I'm talking about stock fender tube amps. The reason they call the power tubes the power tubes is because they are in the power section of the amp and that's where the power is generated.

    Everything I've ever read about tube amps states that the voltage gain takes place in the preamp including the PI, and the gain in current and power takes place in the power tubes.
     

  10. Ed Storer

    Ed Storer Tele-Holic Ad Free Member

    830
    May 27, 2010
    Seattle WA
    Thanks for the responses. BTW my one cathode biased amp is the only one with a cathode follower.
     

  11. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Ed Storer, which amp is that that has a cathode follower and cathode biasing of the power tubes? ...curious I am....
     

  12. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Power is voltage x amps: P = V × A.

    Cathode follower provides current gain, but not voltage gain; in fact, it has a slight voltage loss (Vi/Vo < 1.00), no better than µ/(µ+1).
     
    Last edited: Sep 12, 2017

  13. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I'd bet it's something like an AC30 or Matchless Lightning. The Brilliant channel of the AC30TB has a DCCF driven tone stack.
     

  14. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    Gain with a following load is usually quite a bit down on the "slightly less than 1" gain that is usually accredited to the CF.
    For a good basic understanding, you can't get a better description than from Norman Crowhurst.

    Try his 1954 article in Dec "Radio Electronics" p.50-51
    Radio-Electronics/50s/1954/Radio-Electronics-1954-12.pdf

    And the follow-up in May 1958 p.50-53
    Radio-Electronics/50s/1958/Radio-Electronics-1958-05.pdf


    Other good reading from his English equivalent - Cathode Ray (the pseudonym of M G Scroggie)
    Wireless World nov. 1945
    Wireless-World-1945/The%20Cathode%20Follower.pdf

    and his follow-up on CF biasing and loadlines in june 1955 p.292-296 (p.42-46)
    Wireless-World/50s/Wireless-World-1955-06.pdf

    and later about CF distortion in may 1961 p.261-265 ( p.25-29)
    Wireless-World/60s/Wireless-World-1961-05.pdf

    That should do for now, .....but as Old Tele man pointed out, a CF is half of a Split-load PI, hence its cathodyne by-name, and a lot of understanding about the one can be usefull in understanding the other.
     
    Last edited: Sep 12, 2017
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  15. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    Yup. I use a 12AU7 in one of my Fender Gretas as the output tube. Stock was a 12AT7. Any similar tube will work as an output tube - tone can vary, but they definitely can be "power" tubes.
     
    Last edited: Sep 13, 2017

  16. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    Sidebar - The never-ending debate isn't new! From a Zenith ad in the 1958 Radio Electronics:

    "3 reasons why there are NO printed circuits in a Zenith TV chassis"

    I swear - it's there!

    :lol:
     

  17. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    The purpose of the preamp tubes? Raise voltage.


    The purpose of the power tubes? The Signal voltage from the preamp is imprinted on the high current flow thru the power tube, greatly increasing the current and power.



    How is it known for sure that the power tubes supply the power and not the preamp tubes?



    Think about it.




    What does the power tube need in the way of signal to operate? VOLTAGE ONLY
     

  18. Endless Mike

    Endless Mike Friend of Leo's

    Nov 2, 2016
    Arlington, Texas
    So he speaks truth to power?
     
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  19. Endless Mike

    Endless Mike Friend of Leo's

    Nov 2, 2016
    Arlington, Texas
    It's like the paparazzi, it follows cathodes around and takes their pictures.
     

  20. Ten Over

    Ten Over Tele-Meister

    184
    May 13, 2015
    Central California
    No need to stress-out about it, you are right about the function of the preamp and the power amp. The fact that a cathode follower can provide current gain and, therefore, power gain is irrelevant. In fact, the current gain is a severe annoyance when designing buffers and there is the need to balance current consumption with the desired performance. The current gain from a cathode follower buffer in the preamp is in no way used for the power output of the amplifier.
     
    Last edited: Sep 13, 2017

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