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Fender Cathodyne Phase Inverters

Discussion in 'Amp Tech Center' started by Bendyha, Aug 3, 2017.

  1. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Good point, good question, a longer answer would be fitting, and I will get back to addressing it when I have the time.....hopefully soon. In the mean time I will ponder upon it.
    Short provisional answer; To a large degree, yes, but.....
     
  2. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Draft: "To a large degree, yes, but the substituted tube does needs to be within x% of the spec to avoid damaging y, and your ability to z the power tubes will affect the tone and max amp volume"

    (something like that ^ is my working assumption)
     
  3. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Well it's more about what you see here on the load-line below.....Just because it might work, doesn't mean it is a good idea. One could design a great PI with the 12AT7, but just plugging it into a circuit made for the 12AX7 is not taking advantage of its strengths. The bias point is right down in the low volt / low current region where results can be very unpredictable, and vary hugely with different sample tubes. The output swing looks like it would be quite small one way and mostly unused in the other....so not an improvement. None the less, I have read of people trying this tube swap, and being happy with the results.

    upload_2017-9-22_20-10-34.png
     
  4. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    "results can be very unpredictable, and vary hugely with different sample tubes"

    Good to know, thanks, good explanation, and I don't disagree. In my case, I'm trying to not slam the output tubes too hard (= as hard as with the 12AX7 that was in there), which appears to be what is happening. Anyway with this amp, and this 12AT7 tube, sounds pretty good, to me. If I want more crunchy breakup I'll pop the AX7 back into V4 (this is a PRAA1164)
     
  5. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    As the gain of the phase inverter itself is slightly below unity, I would guess that the larger part of the difference is coming from the other half of the tube that is driving the P.I. whose voltage gain will drop from about 60.5 down to about 53.75 (ignoring the feedback loop)


    ah..I see iv'e just become Tele-Afflicted.
     
  6. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Bendyha-san

    You came to the realization that you are not blind to your self-induced tele affliction, that's a very good sign. A manageable and sustainable state of recovery should you someday chose it should be a straight if not easy path. You alone decide what path to walk.

    ...

    In a PRAA1164, the tube (half) driving the PI tube in V4 is V3b, and V3 is a 12AX7. Assuming I am reading things right (happy to be corrected), V3A is Reverb recovery, V3B is pre-inverter gain.

    And. Because V2A and V2B both drive the Reverb in this amp (tube is now AXAT7, per spec), and am wondering now about V3.

    If I swap it (V3) to an AXAT7, I will have (per citable documents at least), 10x the current to work with, 10 milliamps, compared to a 12AX7. I need to chew on how this tube change might positively (or negatively) affect both reverb recovery and pre-inverter gain feeding V4B (PI). Fun puzzle!
     
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  7. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    * I'm sure plenty of very smart people have figured this ^ out years ago.
     
  8. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Ah yes... my oversight, the other half of the PI tube is not the driver in a Princeton, but the Vibrato oscillator. Does the tube swap give a noticable change to the functioning of the vibrato?

    "Fun puzzle"....thats exactly how I see circuit diagrams, and why I don't get bored of all this stuff. Forget Sudokos, give me a broken amp!
     
  9. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I played through this amp today for a bit and post-tube-swap I did not hear a legitimately noticeable change pro or con to the vibrato. Very subtly, maybe a little? more appealing...but the room was totally different than the last listening session, my body has been whacked by days of 90+F heat, and I have new Tomastik flatwounds as of last night...so a bunch of recognized physical variables are in play. I can say for sure, Vibrato definitely didn't "degrade" post-tube-swap, but Vibrato is another something on my list to tweak because I am not totally loving it right now (I want to slow it wayyyyy down, and increase the depth a bit, I have what are supposedly proven simple recipes for both, we shall see).

    Summary: 12AT7 in V4 is a net positive all told, for my current situation/needs.

    I also swapped BACK to 12AX7 in V4 in case I was imagining it all and instantly got back to what I got what I had before, too-soon breakup for me with this particular guitar (w/ hot-ish humbuckers, plugged into the right (hot) amp input jack).

    There's clearly a lot going on with regard to hot input (from my humbucker axe), so I will soon try everything again with my Telecaster. I have not yet ever done a jazz gig with the Telecaster, maybe I should try that soon.
     
    Last edited: Sep 24, 2017
  10. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    //also, I changed my avatar pic yesterday from Jerry-as-Santa (not my work), to a glowing Russian 6V6 tube (I took the picture but the subject is obviously not my own, other than getting it to glow). Apologizes or credit where credit is due to RR and anybody else who takes pictures of tubes; once I saw blue I figured I better shoot me some of that.
    //
     
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  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    This reminds me of decade+ of researching breakover angles and tailpiece tensions on acoustic classical and archops, google away to see how some of the elite makers treated this important dimension (for some, breakover angle was THE most important aspect, everything else was subservient. Wow and very thought-provocative).

    So, noting all the variables and opinions, and reconizing we now have 50+ years of study of the Blackface circuits, leads me to ask (limiting to an AA1164 for now):

    - is bias point the super-critical value? as in, pretty much else should be subservient?

    - what is the ideal proposed bias point wrt predictability, swing etc?

    Vigorous debate that appreciated :twisted:
     
    Last edited: Sep 27, 2017
  12. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    I was just reading in a copy of "Radio-Electronics" from September 1959, another article by the great Norman Crowhurst, this one headed "Improving Radio and Phono Amplifiers" an article of his that I had not seen before. Much to my pleasurable surprise, not only is it mostly about cathodyne phase inverters and how to improve them, but it is all based around a 12AX7 driving a pair of 6V6 output tubes in a schematic that is almost identicle to that of our Fender amps. It also covers the Paraphase PI.
    And...it describes the reason for the asymmetrical distortion in cathodyne, has a scope trace showing nipple distortion, tells you how to fix it with output tube grid resistors. Everything that Merlin wrote about....but 50 years earlier!
    Not only that, he covers negative feedback (a thing he wrote much about, and the reason I was reading his articles today)...but also combining it with positive feedback for more gain...if you want it.
    Anyway, here is a copy of the article.
     

    Attached Files:

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  13. Old Tele man

    Old Tele man Friend of Leo's

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    Re-inventing the "electronic" wheel!

    What is "old" is prologue...again.
     
  14. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    nihil sun sole novum
     
  15. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Looking in my files, I found I had read, marked, and forgotten this from Nov. 1957, again from Crowhurst. Here is an snip-out from the facinating five page article about a twin coupled 15W P-P EL84 amplifier. Here are the bits relating to the cathodyne....he does go on to talk more about a balanceable paraphase PI as another option. Again, it is about the development and cure of nipple distortion caused by grid current flowing in the tubes being driven.
     

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  16. peteb

    peteb Friend of Leo's

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    Nice find Bendyha.


    I read it and was hoping to read more detail.



    I agree that it does appear to be talking about the very thing that we have been discussing.
     
  17. danlad

    danlad Tele-Meister

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    And the Supro Thunderbolt had the 6L6 grid stoppers too. Hiding in plain sight!
     
  18. sdcoffeeroaster

    sdcoffeeroaster TDPRI Member

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    Fantastic work Bendyha. I was thinking of doing this to my 5E3 VHT or just adding the master volume control by replacing that 1 meg resistor with a pot as described by Rob. Could I do both? That 680 ohm resistor in your circuit would limit the effectiveness of the master volume wouldn't it? Maybe doing both doesn't make sense. Anyone tried this circuit and care to comment on their thoughts about how it affected their 5E3? I know this can be very subjective as you've stated but it would interesting to hear from anyone trying this one. Thanks again for all your effort and expertise on this subject. This is probably the best thread around for Cathodyne information.

    I picked up a few of these VHT boards for only $10. each. I wish I had bought more, like maybe 10, lol. I am thinking of converting one to a long tail pair PI and stealing one of the parallel stages to make up for the gain lost. Sort of a Brownface clone (but cathode biased) and noticed someone else has already gone done this path here. But these Cathodyne mods look very promising. Thanks again.
     
  19. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    An interesting thought.................but..............you were thinking maybe of something like this;

    upload_2018-1-30_23-8-17.png upload_2018-1-30_23-12-42.png The problem is you would have over 50V of DC across your pot, and that would be scratchy and noisey. Any volume pot would have to isolated from DC, and other than replacing the 0.022µ with two 0.047µ caps in series, and placing the 1M volume in the middle, I see no other method at the moment....but I will ponder upon it.

    If you do try out the fixed bias option, do let us know about your results and thoughts.
     
    Last edited: Jan 30, 2018
  20. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I'm not seeing the 50v DC across the 1M volume pot in the above schematic because the grid leak has been de-bootstrapped--this is a critical problem.

    The 680k resistor in parallel with the 3M3 resistor would set a minimum master volume of around 38% of max volume.
     
    Last edited: Jan 30, 2018
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