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Calling all Merlinites... Gain of Cathodyne

Discussion in 'Shock Brother's DIY Amps' started by andrewRneumann, Aug 27, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Holic

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    Turn with me to DTPGB, Chapter 8, Verse 2.

    Merlin states that the gain to the anode is the same as an ordinary unbypassed gain stage.

    Then he states that the gain to the cathode is the same as for a cathode follower, and that's where the confusion starts.

    Since the equations are similar and Ra and Rk are chosen to be equal for the cathodyne, he gives one equation for the gain of either the anode or the cathode. The problem is that the denominator of the cathode gain equation is not the same as the denominator for the anode gain equation, so I don't see how he arrives at Eq 8.1 for both anode and cathode.

    I pondered this for a while and then realized that there IS a difference between the cathodyne cathode gain and cathode follower gain--the presence of an anode load resistor, Ra. If we take that into account, and add a term for it in the denominator of the cathodyne cathode gain, then Eq. 8.1 makes total sense to me. But that's not how it's presented in the book.

    Is this an error or am I missing something?
     
  2. cottontails1959

    cottontails1959 Tele-Meister

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  3. andrewRneumann

    andrewRneumann Tele-Holic

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    Ok, the pdf looks like it solves my question. I keep telling myself to get that second book... this is the second time someone has shown me it has corrected/better info than the first. Thanks!
     
  4. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Might this make Merlin a mere mortal after all?

    I have the both books. I might even be that other person that showed you other info (power tube heater wiring?). But I can't make myself read the hifi book as lots is literally the same as the guitar one. Except for the changes, obviously...
     
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  5. andrewRneumann

    andrewRneumann Tele-Holic

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    Yeah that was you! Well the book is on its way. The library ever grows. I’m going to do a side-by-side and see if there’s anything else that evolved since 2012.
     
  6. Ten Over

    Ten Over Tele-Holic

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    It's hard to follow the math when it isn't there.

    Gain Cathodyne JPG.JPG
     
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  7. cottontails1959

    cottontails1959 Tele-Meister

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    :cool: freakin' awesome, thanks! I can follow that.

    What throws me off in Merlin's DVPGB book on page 144 ... he says, "But for normal purposes, Rk = Ra. Hence, if we call the load at the anode or cathode R, the gain to either output becomes A = mu(R/(ra+R(mu+2))) ... ignoring minus sign."

    I read that, then, as him saying eq 1.5 = eq 6.1 ... substituting in R for Ra and Rk ... but clearly they are not, as the denominators are different.
     
  8. andrewRneumann

    andrewRneumann Tele-Holic

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    Thus the OP!
     
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  9. tubeswell

    tubeswell Friend of Leo's

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    You don't need math to understand that in a balanced cathodyne, the cathode signal swing follows (and is the same amplitude as) the grid signal swing, and so is about unity gain. The plate signal swing is the same amplitude as the cathode signal swing (albeit that's it's 180 degrees out of phase). Therefore, the plate signal swing is also about unity gain.

    The effect of 100% cathode current feedback means that there can be no positive gain at the plate. (whereas in a 'conventional' inverting gain stage, the plate load is bigger and the cathode load is a fraction of the plate load)
     
    Last edited: Aug 31, 2020
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  10. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    If you like the maths , chew on this;
     

    Attached Files:

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  11. tubegeek

    tubegeek Friend of Leo's

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    Oh jeeze, Linear Audio... shît just got REAL.
     
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  12. XTRXTR

    XTRXTR TDPRI Member

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    Cathodyne PI) The total load resistance is divided into two parts and shared between the anode and the cathode, So from the input perspective the gain is the same to each of the two (anode and cathode) and the gain to each is near unity. The math Ten Over posted works out. The handwritten part is the derivation of the printed equation for gain "A" I don't claim how each of the terms Rp, Rk, Rb, ra, Rt were derived but if he sets Rp=Rk=R and Vk=Vp=Voutput the math is correct, though there are bits he doesn't state which make it a little confusing to follow, like when he shows how he factored out R separately from the rest of the equation seemingly leaving out "ra" and then its there again. But it all works and is correct.
     
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