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Hum and grid-leak bias

Discussion in 'Amp Tech Center' started by andrewRneumann, Aug 17, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Holic

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    I was looking at some old schematics and followed a rabbit-hole of learning about grid-leak bias.

    RDH4 says.... 7D6B0C98-3EB7-40D0-9019-89CB3847553B.jpeg

    Can anyone provide persuasive arguments about why grid leak bias is more susceptible to hum problems? I have searched a lot of different forums and came up with some different potential answers, but nothing that convinced me one way or another.
     
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I know you're looking for explicit reasons it's susceptible to hum. I wonder if the answer is in Merlin's discussion. Like maybe the fact the forward grid current is very small, less than 1 microamp (which is why you're talking about those massive 5 or 10M resistors). Merlin does point out this combination is also highly unpredictable and prone to distortion -- picture trying to tie a ship's hawser around a human hair.

    "...another form of biasing called grid-leak bias or contact bias. It was mentioned above that no current flows in the grid when it is very negative, but if the grid is only slightly negative –-between 0V and about –1V say –- a few electrons leaving the space charge will succeed in hitting the grid, thereby generating a tiny current that flows (conventionally) into the grid, as indicated by the arrow. This forward grid current will generate a small negative voltage across the grid leak resistor that is sufficient to bias the valve by a small amount. 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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  3. andrewRneumann

    andrewRneumann Tele-Holic

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    Yes I read Merlin’s paragraph on it as part of my research. I’m not sure if by “noise” he meant hum, but maybe. As far as distortion goes, I chalk that up to the fact that the valve will be biased somewhere between -0.5V and -1.0V most likely. Those positive signal swings will be damped significantly as more grid current flows. But still... why hum? I don’t think it has anything to do with heater-cathode leakage because the cathode is directly grounded and should stay at 0V. So where else does the hum sneak in? If it’s coming from ripple on the plate, then that could affect any preamp tube regardless of bias and is more of a power supply issue than a bias issue. Still stumped...
     
  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You don't think it has to do with the tiny size of that current, like <1 *microamp*? Seems like even a tiny amount of EMF could perturb 1 microamp...

    Don't worry, I'm not out to convince you. This is just a WAG based on a hunch around a poorly founded intuition. But you're right, I guess I'm also saying hum is only one of the problems here. :)
     
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  5. tubeswell

    tubeswell Friend of Leo's

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    With grid leak bias, you need a large resistance (~5M) for the grid leak resistor to develop the bias voltage. This is (needs to be) an unbypassed resistor, so its prone to noise. Made worse because of the high S:N ratio if its an input stage.

    Whereas with cathode bias, you can either clamp the bias voltage - with a cathode bypass cap, or if unbypassed, cathode current feedback improves fidelity/reduces noise.

    Similar story with fixed bias - the grid voltage is referenced to a 'clamped' VDC source and the grid leak pathway to that source is kept relatively low resistance (between 250k to 1M)
     
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  6. trobbins

    trobbins TDPRI Member

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    I reckon Merlin's comment was about noise due to the grid leak resistance also effectively being a noise generator (any resistor can be theoretically represented as a 'pure' resistance in series with a noise voltage supply, or the equivalent parallel representation that has a noise current generator), where the noise voltage increases with resistor value. In an application, there is another circuit path due to the input signal, so the total situation may not be noisy per se - especially for low impedance signal sources like a 50 ohm microphone, but it may get noisy with a pup with high impedance pots and tone circuitry.

    I reckon the common reference to 'hum', when specifically described as hum, relates to the practical susceptibility of such an input stage to capacitive coupling of nearby heater voltages over to this quite high impedance grid node. High impedance grid circuitry is the bane of many pre-amplifier stage hum and accidental feedback issues, especially for long cable runs over to volume and tone pots. For such a grid leak input stage, the management of hum would likely need at least a humdinger pot to balance the capacitive coupling from nearby heater wires to any grid wiring, and may need some electrostatic shielding, and choosing a valve type that maximises the separation between grid pin and heater pins (the 6AU6 has a nice manufacturer article on how to use a screen as anode, and so obtain shield terminals on either side of the grid terminal - https://www.dalmura.com.au/static/6AU6pAWV.pdf).
     
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  7. andrewRneumann

    andrewRneumann Tele-Holic

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    This is basically applies to everything I post on TDPRI. I always have to clear my throat and effectively say "I don't have much confidence that what I'm saying is actually true." As long is everyone realizes that, and a bunch of ideas get thrown around with good separated from bad by the "hive mind" of the forum, I'm cool with it.

    When you put it that way, it's more convincing. :) But I still have questions. Isn't any grid susceptible to this? The typical 1M grid leak is still pretty large and any EMF could easily get into the grid and "rattle" things around. The fact that it's a small current doesn't change much in my mind. The center bias point of a standard gain stage has a tiny amount of reverse grid current flowing in it theoretically all the time (thus one reason we need a grid leak resistor), but we don't consider that as a source of noise or hum.

    If this is the first stage and a guitar is connected, then there is another path to ground... through the source impedance of the guitar. (Or if nothing is connected, directly to ground through the shunt at the jack.) I admittedly have no clue what the source impedance of a guitar and cable is at 60Hz or 120Hz... but if it was down in the 10k range like is typically quoted for guitar source impedance, then what is the difference between 1M and 5M grid-leak in comparison?

    @trobbins , I see you made similar comments while I stepped away from the computer mid response. I will digest what you said, thank you!
     
  8. andrewRneumann

    andrewRneumann Tele-Holic

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    @trobbins I'm going to spend some time with your article on hum. I skimmed it and it looks really interesting.

    Maybe a succinct answer is something along the lines of “The control grid has a relatively high impedance to ground and therefore is more susceptible to building up stray noise and hum voltages.” I can imagine at an intermediate stage where we are using a small coupling capacitor to limit bass response (high reactance at 60Hz) the impedance could get pretty high. I think I’m getting some clarity here.
     
  9. tubeswell

    tubeswell Friend of Leo's

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    If it’s the input stage, there needs to be a DC blocking cap in series with the grid and the input jack. So there is no other DC path for grid leak, except through the grid leak resistor. Otherwise the grid leak bias wouldn’t work.
     
  10. Jon Snell

    Jon Snell Tele-Holic

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    The reason is quite simple; the extremely high impedance allows pickup of noise and hum.
    Here is an example of a 'grid leak' in use on a VOX Echo unit. The valve is an ECC83. Note the leak resistor connects directly to the cathode and chassis point.
    Screenshot 2020-08-18 at 19.18.13.png the circuit uses a high value resistor, 4.3M, that allows the grid to float slightly negative instead of allowing the cathode voltage to rise. Otherwise the grid impedance will only be in the order of 1M say, it is now over 4M. An EF86 for instance would command a 10 - 22M grid leak, due to its effective input impedance.
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, high impedance grid. The same reason hum goes crazy if the grid leak resistor loses its ground connection and floats.

    I used to wonder why we used a grid leak at all. If a high input impedance relative to output impedance is good why not make it infinite with no grid leak at all? This thread answers that question.
     
  12. andrewRneumann

    andrewRneumann Tele-Holic

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    I also wonder why on some of these old designs you would see one input cathode biased and then another input grid-leak biased. They must have thought it was good for something, but I’m not sure what.
     
  13. trobbins

    trobbins TDPRI Member

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    When the input signal was quite small, and the impedance presented by input signal source was quite low, like a moving coil (dynamic) mic or even a ribbon mic, then the low MIC impedance shunts the grid leak resistance, effectively suppressing any intrinsic noise from the grid leak resistor. Of course there was still a need to screen the input stage from any extraneous noise or hum pickup, which was why some PA amps located the preamp tube and input socket in a separated metal enclosure in the furthest corner of the chassis. If 6J7's etc were used (ie. pre 1950) then they were sometimes seen mounted in their own metal enclosure on top of the chassis and with microphonic suppressing rubber grommet mounting.

    [​IMG] [​IMG]
     
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