Grid leak bias

Discussion in 'Amp Tech Center' started by tobyk, Jun 16, 2021.

  1. tobyk

    tobyk TDPRI Member

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    To those of you who have tried grid leak bias – is it worth trying out? Does it differ in tone and/or touch sensitivity?
     
  2. '64 Tele

    '64 Tele Tele-Afflicted

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    I liked their 2nd album better than the first
     
  3. tobyk

    tobyk TDPRI Member

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    Yeah ok, anyone else?
     
  4. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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  5. tobyk

    tobyk TDPRI Member

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    Thanks, yes I ment for input stage. I heard it’s more gritty. Anyone?
     
  6. FenderLover

    FenderLover Poster Extraordinaire

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    Yes, it takes less input amplitude to overload the stage. When you hear, "Does NOT take pedals well", the grid leak bias would be that circuit. It can be a lot of fun for just plugging straight it. Someone here has a 5E3 with two jacks wired normal with a cathode bias triode, the other two jacks go to the grid leak bias triode. Brilliant.
     
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  7. dan40

    dan40 Friend of Leo's

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    I have tried it a few times but it was always too much of a "splatty" sound for me to enjoy it.
     
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  8. nvilletele

    nvilletele Friend of Leo's

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    Well, since you asked, an in an effort to be equally unhelpful as the post to which you were reacting, I’ll just say that none of us can completely escape our inherent biases.
     
  9. ArcticWhite

    ArcticWhite Tele-Afflicted

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    I built a hybrid Gibson GA-20T /40.
    One of my four inputs uses a grid leak bias. (The GA-20T had it on the mic channel.)
    As others have noted, it's pretty splatty (great description BTW) . I haven't tried a pedal with it yet.
    It's pretty simple to do two different input jack designs into one 12AX7 preamp. And easily changed back if you don't like it.
    I enjoy monkeying around with this sort of stuff, so I'd recommend trying it yourself to see if you like it.
    It tried on a Silvertone 1482 clone and it was pretty bad.
     
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  10. tobyk

    tobyk TDPRI Member

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    Thanks, I have a schematic for a switch, guess I’ll try it. You can’t just change the input though, the tube’s cathode needs to be grounded for it to work properly, I think..
     
  11. tobyk

    tobyk TDPRI Member

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    But if I didn’t want that overload, wouldn’t a simple grid resistor fix that? Or perhaps then I might just as well use cathode bias.
     
  12. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Yes, I realize that. I didn't put mine on a switch, I'm talking about using half of the 12AX7 input tube, like this.

    20210617_041908.jpg
     
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  13. printer2

    printer2 Poster Extraordinaire

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    The thing is it have less dynamic range than cathode bias. It would be better with lower output pickups (or if you turn down the volume control, what a concept).
     
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  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You’ll have read this, but Merlin's comment always makes me think it might not be worthwhile...

    “Since the grid current is normally less than a microamp, Rg must be very large to generate any useful bias. This method is almost never used in modern circuits as it is not very predictable and results in
    excessive noise and distortion...”
     
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  15. tobyk

    tobyk TDPRI Member

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    ’Not predictable’ and ’harmonic distortion in the first preamp tube’ still feels like something I would like for some old school blues sounds, along with the touch sensitivity..
     
  16. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I did a 5B5 Pro-inspired build years ago, complete with grid-leak biased inputs. It taught/reinforced a handful of things, and was worth it for that alone:

    * Yes, the bias is unpredictable. Especially if you try to build it with old paper capacitors from an organ that turn out to be leaky...
    * Yes, noisy. The ~5 Meg grid leak resistor contributes plenty of hiss if you're using old carbon comps. More subtly, grid leak bias typically uses a larger plate resistor (I think to limit the current so as to avoid upsetting the bias), so that will contribute more resistor noise than the usual 100k. Finally, this bias scheme doesn't seem to reject heater hum as well as cathode bias.
    * Yes, less input headroom. Having only a fraction of a volt of bias means a hot guitar pickup (or microphone) could be enough to clip the input stage. But some people dig that!

    There's a lot of emphasis on "best practices" in these amp forums, which is generally good, but I find it fun to explore the old ways of doing things, too. There's always some degree of "historical reenactment" whenever you build an amp with vacuum tubes. :)
     
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  17. tobyk

    tobyk TDPRI Member

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    And how about the tone and feel of the grid leak? Any difference?
     
  18. dkevin

    dkevin Tele-Meister

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    I found a Supro 6616 and loved the all-around tone of the amp. I mean, from low volume to dimed, this amp sounded really good. I wondered why my smaller Supro 6606 has such a harsh, blatty overdriven sound as compared to the 6616. When I studied the two schematics, I found that the 6606 had a 12AX7 grid-leak biased and the 6616 had the same tube but with cathode bias. I converted the 6606 to the 6616 style preamp and found that I really liked the tones. I cannot explain in EE terms why it should be so, but in these two amps using 12AX7 preamp tubes, the cathode bias arrangement was preferable to the grid-leak bias arrangement in my opinion.
     

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  19. tobyk

    tobyk TDPRI Member

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    Thanks for your input, and I’ll add those schematics to my collection!
     
  20. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    It might have sounded a little more "lively" due to the gain, and perhaps a little more "relaxed" by comparison after I converted to cathode bias, but it was kind of subtle to me unless I was really driving the input. (To confound things, I ended up getting rid of the two-channel design and used both triodes of the 6SC7 in parallel at the input. I like it!)

    I would assume that grid-leak bias was attractive earlier in the tube era since it wrings about as much gain as possible out of that stage, with a minimum of parts.
     
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