Tube Maximum Grid Resistance and grid leak Resitance generally

Discussion in 'Amp Tech Center' started by separateness, Feb 17, 2020.

  1. separateness

    separateness TDPRI Member

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    Looking through the data sheet for the 5751 I noticed the maximum rating for 'Grid Resistance' was 0.5MΩ. Does this refer to the largest value of grid leak resistor one can use this this tube? If not, can any one tell me what it refers to? What happens if you exceed it? 500kΩ seems like a woefully small value for setting the input impedance of a gain stage.

    Relatedly, can any one tell me what the upper limit on a grid leak resistor is and the logic involved in arriving at this value? I have seen them as high as 5.6MΩ. Isn't higher = better, generally? What do you lose by increasing this value?
     

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  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The grid resistance limit is one that is often exceeded in amplifier design with seemingly little consequence. A higher value places less load on the upstream gain stage but it also affects overdrive behavior by increasing the upstream coupling cap's discharge time.

    If a grid stopper resistor is placed between the grid and the grid leak resistor the stopper's resistance has to be included in the grid resistance. If you place the stopper on the upstream side of the grid leak resistor to avoid this situation it will form a voltage divider with the grid leak resistor (which can be useful in high gain preamps).

    Grid leak biased triodes use very large grid leaks, like 5M. They use electrons captured by the grid to generate the bais voltage (voltage diff between the cathode and grid).

    I have read that exceeding the grid resistance limit for power tubes can supposedly lead to bias runaway but I don't understand the mechanism for that to happen. Too much grid resistance would cause a buildup of captured electrons on the grid which would slow the flow of electrons from cathode to plate.
     
  3. Ten Over

    Ten Over Tele-Holic

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    I believe that the stated max grid circuit resistance only applies when the cathode is grounded. Once you put some resistance between the cathode and ground the grid current drops dramatically and grid leak bias effects essentially disappear. With a typical Fender gain stage utilizing a 100K plate resistance and a 1.5K cathode resistance, a 10M grid leak has a negligible effect on the biasing of a 12AX7.
     
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  4. separateness

    separateness TDPRI Member

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    Thank you, fellas. Intuitively it seemed to me that increasing the value would have at least no catastrophic effects, which seems to be the case. I am trying to squeeze every bit of voltage I can out of a tone stack is why I ask. I suppose I will have to see for myself if it is worth the change in overdrive characteristics indicated by robrob.
     
  5. the fatch

    the fatch TDPRI Member

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    The grid, sitting next to the cathode, becomes hot and emits a small number of electrons which flow to ground in the grid leak driving the grid more positive, reducing the negative bias. If the resistance of the path to ground is too great it can result in thermal runaway where the tube passes more plate current, the grid becomes hotter which increases the grid current which increases the plate current... until the tube red-plates and dies. With cathode bias, the decrease in bias is partially cancelled by the increase in bias developed by the increased current in the cathode resistor allowing a larger value of grid leak.
     
  6. separateness

    separateness TDPRI Member

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    Okay so this is normally only a factor in grid leak biasing, which I do not use.

    As a follow up question: If I have a common cathode stage, then a tone stack (no volume control) and then another common cathode as pictured, I can omit an Rg, as the tone control pots provide a path to ground for the grid, correct?
     

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  7. Ten Over

    Ten Over Tele-Holic

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    The grid is mostly open space, but there are solid surfaces. Some of the electrons from the space charge which have been attracted by the plate's electrostatic field collect on these solid surfaces. These electrons flow from the grid to ground through the grid leak resistor. Electron flow in that direction makes the grid end of the grid leak resistor negative with respect to the ground end by definition.
     
  8. trobbins

    trobbins TDPRI Member

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    I would recommend you comply with a manufacturer datasheet max allowable grid resistance. The max resistance spec does not relate to grid-leak bias design, except to indicate where grid leak bias may start influencing operation. Valves go gassy and grids become polluted over time, and the datasheet spec aligns with a useful service lifetime for the valve whereby other performance specs will still hold up. For new valves you may get away with using a higher leak resistance, but in general that is poor design.

    Often a wired in resistor is used to minimise noise and crackle that can occur over time as the pot wiper(s) becomes noisy, especially where there is some grid leak current passing through the pot wiper - so in general it is poor design to just rely on the pot for that situation.
     
  9. Jon Snell

    Jon Snell Tele-Meister

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    Some designs use a wasted grid circuit. Screenshot 2020-02-20 at 16.33.11.png
    These are 5M6 resistors with no cathode resistor. Widely used in the earlier days.
     
  10. separateness

    separateness TDPRI Member

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    I note, too, that the Ampeg B-15 uses 5.6MΩ grid leak resistors for its first stage, but it uses an 6SL7 input tube. I am not sure if this tube has a widely different max grid resistor value or if the fellas at Ampeg just disregarded it. I notice that this spec is only rarely found on data sheets for whatever reason.

    Fair enough and thank you for your guidance. I might yet do some experimenting to see what I can get away with. This is the first amp I've built where I am doing any sort of intentional design. I've gone about it rather haphazardly but I have learned a good bit. I find that getting to know design requires me to trial and error a lot of things. It is one thing to understand things on paper but another to hear the effects.

    As an aside and for any who come after me I provide this example. I built this amp up largely based on a few well-trodden designs. It worked quite well and sounds wonderful for bass. Lovely, full clean tone. But then I thought 'well what if I want it to growl a little' so I started looking at various other topologies, plotting out different load lines and bias points, I moved the volume control after the tone stack recovery stage. I had that (recovery) triode biased on the saturation end.. then I tried it on the cutoff side and decided (for now) that I rather liked that better. Now it sounds great and I can make it growl when I dig in. Unfortunately I am an overly curious person and can never leave well enough alone so here I am, trying to see what else I can get away with. All this is to say that I take your advice well but I fear my own personal failures will force me to try it anyway, just to see or rather hear.
     
  11. printer2

    printer2 Poster Extraordinaire

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    Which sums it up well. The designers gave a value that will work with most tubes, they are variable after all. The manufacturer using the tube does not want warranty returns, their radio is a piece of... ..you get what I mean. So some specs can be bent, some not. This is one that is regularly bent. As far as using it in grid leak, there are two things to consider. The actual warning is being used as a feature, the grid is self biased. Using this high a value will shift the bias point in a cathode biased application. But in grid biased form, we need another part, the capacitor at the input. Without it the change and therefore voltage will discharge into out guitar volume control and pickups.

    So say we have a Fender normal 1M input resistor, too high according to the datasheet. But parallel it up with the guitar volume pot, we are bang on. The stage after the tonestack. Add up the pot values, even the Bassman, 250k + 1M + 25k, 1.275M. Not horribly far away from the spec. A little bent at the one extreme, normally not too bad. But when you think that amp designers expected you to operate the amp cleanly (in the 50's and 60's), well we are bad people that don't behave nicely, who like the amp's sound when it behaves not very nicely.
     
  12. Ten Over

    Ten Over Tele-Holic

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    No it doesn't. The electrons that are emitted when the grid gets hot are not flowing through the grid leak resistor because, well, they've been emitted from the grid. Out into a vacuum. Guess where they go.

    It can easily be demonstrated that more electrons are being collected by the grid than are being emitted by measuring the voltage across the grid leak resistor. If the grid was being driven more positive because more electrons were emitted than collected, grid leak bias wouldn't work.
     
  13. Ten Over

    Ten Over Tele-Holic

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    It occurs to me that there is one instance where the magnitude of the grid leak resistor has a big impact and that's when the effective grid voltage becomes positive. In earlier times when they said "fixed bias" they meant a battery hooked to the grid and the cathode grounded. The battery presents zero resistance to the grid so that grid current doesn't affect the voltage and the tube will keep operating with positive grid voltage swings. But if you put a resistor in series with the grid battery, then grid current will alter the grid voltage and the output will no longer be directly proportional to the input. The larger the grid leak gets, the more positive input signals get rounded off at the output.
     
    Last edited: Feb 21, 2020
  14. printer2

    printer2 Poster Extraordinaire

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    I have tried reading your post a number of times, it does not make sense to me. It might be because electrons are emitted by the cathode, the grid does not emit electrons. I am having a problem seeing the grid emitting electrons, maybe too many years viewing it a different way.

    The grid has to be negative as compared to the cathode otherwise the tube passes full current. At 0V all the electrons boiled off the cathode are being drawn to the anode. A negative grid sets up an electric field that, since it is closer to the cathode relative to the distance of the anode, a small voltage reduces the flow of electrons. As the electrons whizz by the grid some electrons hit the grid and stick, the accumulation of electrons cause the grid to become negative enough to reduce the current flow. The high grid leak resistance is low enough to let some of the electrons to leak to ground but high enough to keep enough electrons on the grid to bias it where we want it. Without the leak resistor will the accumulation of electrons on the grid cut off all flow? I don't know, maybe close to all flow.



    Putting a resistor in parallel with the battery will change nothing, the battery resistance is minimal and current from the battery will flow through the resistor and back to the battery. The minuscule current from the grid will be hardly noticed. Look at a fixed bias amplifier, there is always series resistance in the circuit, the grid leak resistor. It is not parallel.
     
  15. Ten Over

    Ten Over Tele-Holic

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    the fatch said in his very first sentence that the grid emits electrons and you said that he summed it up well.
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If the grid is hot enough to boil off electrons then they would flow from ground, through the grid leak resistor, to the grid and then boil off and go to the plate--same as the electrons boiling off the cathode. The boiled off "space charge" electrons are not attracted to ground, they are attracted to the plate. If electrons boiled off the grid then there would be a positive voltage on the grid of a grid biased gain stage.

    What really happens to the grid is demonstrated by the grid leak bias gain stage. It has a grounded cathode and 5.5M grid leak resistor which develops about 1.3v of negative voltage on the grid--remember, a negative voltage is an excess of electrons. The excess electrons are sourced from the space charge electrons flowing from the cathode to the plate. Some of them hit the grid wire and are captured by it and they stack up on the grid due to the very large grid resistor. These stacked up "excess" electrons repel space charge electrons and slow the flow of electrons through the tube. This is the opposite effect of the "hot grid boiling off electrons". Electrons aren't boiled off the grid, they are captured by it.
     
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  17. Ten Over

    Ten Over Tele-Holic

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    Electrons are, indeed, boiled off of the grid when it gets hot enough. I'm saying that way more electrons are captured than boiled off under the conditions that we are talking about.
     
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  18. trobbins

    trobbins TDPRI Member

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    The interesting aspect of grid bias is that the grid coupling capacitor builds up a bias voltage as signal is passed to the grid. When signal starts, or stops there is a ramp up and down of bias with a time contant that relates to that grid coupling cap. As such the attack and decay dynamics of playing guitar are different than for a typical input stage.

    The anode idle voltage shifts according to the grid bias level, so design of where that voltage sits on the loadline is also different from typical stage.
     
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  19. Jon Snell

    Jon Snell Tele-Meister

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    WE had, back in the 60s, no end of trouble with valve flip flops used as counters in out 'computer' at Decca. They were set up using high value grid leak resistors and when the valves got old, they became unbalanced.
    Here is a good bit of reading for you on the way thermionic emision works.
    The grid, by the way, does not emit electrons, the cathode does. The grid is the tap, like a water flow tap, to allow a specified amount of electrons to flow. I hope you find it interesting.

    http://www.r-type.org/articles/art-025.htm
     
  20. the fatch

    the fatch TDPRI Member

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    Electrons are emitted from the hot grid and travel to the plate and are replaced by electrons flowing from ground through the grid leak, i.e. conventional current flowing from grid to ground developing a positive voltage at the grid. Grid leak bias depends on electrons from the cathode hitting the grid and flowing to ground, i.e. a conventional current flowing from ground to grid developing a negative voltage at the grid.
     
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