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  • #1

    Screen Bypass Caps

    Hi all,

    In twiddling with my last build, I had to replace the EF86 socket, and when the new one was in, I tried the amp without the .1 screen bypass cap in it. The amp had less volume and definitely less distortion on 10, and I didn't notice an increase in hum (which, I thought this cap was there to prevent hum?), so I started looking into various amps that used EF86s. The consensus seems to be that .1uF is the best, and I saw one or two with a .047uF with other pentode preamp tubes.

    My question is, is this a "tweekable" value, like a cathode bypass cap? I admit, as it is with a 1.5k/25uF cathode, .1 screen cap, & 220k plate load, it's a screaming banshee... a happy medium between none & .1 would be nice. So basically, I'm willing to experiment.

    Will anything blow up by me messing around some? Don't wanna sacrifice my Mullard EF86! Though, I did accidentally plug it into my 12AX7-wired PI slot for a few seconds the other day, no seeming damage...

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -
    Tags: None
  • #2
    From the valve wizard: The Valve Wizard -Small Signal Pentode

    The Screen Bypass Capacitor
    The screen bypass capacitor serves a similar purpose as the cathode bypass capacitor: it holds the screen voltage steady to prevent internal negative feedback from reducing gain. However, in the same way as for the cathode, the capacitor can be made small in value to boost higher frequencies, or omitted completely to reduce gain and increase headroom. Many traditional guitar amp designs do not include this capacitor at all.
    In general, the screen bypass capacitor has a greater effect on the gain of the pentode than the cathode bypass capacitor. The screen bypass capacitor should always be connected to the cathode rather than to ground. Yes, I know a lot of circuits connect it to ground and get away with it, but it is bad practice.

    The screen bypass capacitor can be chosen according to:
    Cg2 = 1 / (2 pi f Rg2)
    If we want to pass all audio frequencies without any loss of gain then we might set f to 10Hz or therabouts: <
    Cg2 = 1 / (2 pi × 10 × 390000) = 40.8nF
    A more common standard value is 47nF.

    Comment

    • #3
      If I'm thinking right, without the screen cap, your EF86 is operating in triode-strapped mode, thus the internal NFB is giving you the 'less volume, less distortion'.

      Merlin devotes an entire chapter in his preamp book to playing with the EF86 circuit. A lot of things can be done with the screen to get various responses. So, remarkable, yes! Try different values to see. I don't have the theory at my fingertips to tell you what you're adjusting, but you will affect the 'pentode-ness' of the tube this way.

      edit: D'oh! simulpost.
      If it still won't get loud enough, it's probably broken. - Steve Conner
      If the thing works, stop fixing it. - Enzo
      We need more chaos in music, in art... I'm here to make it. - Justin Thomas
      MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

      Comment

      • #4
        Thanks guys, will twiddle away...

        Eschertron, I've got the screen resistor (1M) in place, so it's not running triode, I don't think. I thought that would mean connecting screen to the plate? But the rest of it makes sense.

        I'm not even going to pretend to understand the Merlin-Math...

        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

        Comment

        • #5
          So what fraction of a milliAmp goes through that 1M resistor? Yes, enough to drop a few volts, certainly. Is the other end of that resistor connected to the plate? If so, the screen is still following the plate (output) signal. The cap filters that signal to reduce the NFB generated by that action. A big enough cap, and the screen voltage doesn't wiggle at all.
          If it still won't get loud enough, it's probably broken. - Steve Conner
          If the thing works, stop fixing it. - Enzo
          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

          Comment

          • #6
            No, the other end of that 1M is at the filter cap node. The 220k & 1M are "in parallel" from the final filter cap. I'll pore over the pentode page @ Merlin's site & try to figure out some correlations.

            Justin
            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

            Comment

            • #7
              Originally posted by Justin Thomas View Post
              No, the other end of that 1M is at the filter cap node. The 220k & 1M are "in parallel" from the final filter cap. I'll pore over the pentode page @ Merlin's site & try to figure out some correlations.

              Justin
              Ah. OK. I'll have to read that chapter, I guess
              If it still won't get loud enough, it's probably broken. - Steve Conner
              If the thing works, stop fixing it. - Enzo
              We need more chaos in music, in art... I'm here to make it. - Justin Thomas
              MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

              Comment

              • #8
                Hey, I don't know <WHY> I did it, I just copied a Matchless DC30 first channel!

                I thought you meant, is that 1M between the pins on the tube socket, in which case, no... I reckon I could try it, when I get another JJ EF86 to risk exploding! I just kinda figured, well, Mark Sampson isn't TOO clueless,I hope.

                Anyway, now I kinda wanna start putting all kinds of switches in! Maybe when I get to some of my wider chassis...

                Justin
                "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                Comment

                • #9
                  OK, so in a blond moment (actually a bald moment, for me) I remembered the small-signal pentode circuit wrong. So the screen grid is connected to HT via a largish resistor, so the DC current at idle kinda finds its own happy voltage drop

                  By omitting the screen grid cap, you forfeit almost 20dB gain by Merlin's account. Choosing a cap value to partially bypass the screen gives a similar result to partial cathode bypass, but with many more dB across the difference.
                  If it still won't get loud enough, it's probably broken. - Steve Conner
                  If the thing works, stop fixing it. - Enzo
                  We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                  MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                  Comment

                  • #10
                    No problem, I'm the guy who put a 100V cap right on the B+line with a pot set to 0R for a dropping resistor. The joys of experimenting with bias supplies. Did you know electrolytic capacitors can sing really high notes right before they explode in your face? And I mixed up the triode parts in my Cathodyne. Of course, I see no reason why taking it right off the plate couldn't work. Maybe not WELL, but, whatever, tubes forgive. And, I put my Mullard EF86 in my 12AX7 LTPI spot and fired it up. Even tried to play through it at full-tilt whackage (I think that's the word we used?).

                    On a more useful level, I may try messing with both caps...

                    Justin
                    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                    Comment

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