5F2a grid leak resistor

Discussion in 'Shock Brother's DIY Amps' started by Mongo Park, May 29, 2020.

  1. Mongo Park

    Mongo Park Tele-Holic

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    I was looking at the 5F2a it has a grid leak resistor(220K) but no grid stopper resistor. There is discussion of grid stopper values and how it interacts with the grid leak resistor, but I did not find much about the grid leak resistor on its own.
    I know some of the signal going to the power tube is taken out to ground by the grid leak resistor. What happens if the grid leak resistor is increased in value, does this overload the power tube. Is there some change in the amp without damaging the power tube.

    [​IMG]


    Cheers Ron.
     
  2. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Power tubes have a maximum grid to ground resistance. Tube amps typically violate that by a factor of ~3 to 4...

    Something to do with gas current and runaway.

    Edit: Found something http://ken-gilbert.com/grid-current-in-tubes
     
    Last edited: May 29, 2020
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  3. Mongo Park

    Mongo Park Tele-Holic

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    Thanks for the reply, I will read the link.
     
  4. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    The 6V6 datasheet shows a max of 500k for the grid leak resistor. Typical values are ~150k to 220k.

    I don't hear much difference but the lower values are generally said to give more grit.

    I think the grid leak resistor is most often looked at as part of an RC high pass filter in conjunction with the coupling cap. (If you look up RC filter calculator on the internet, you can plug in the R and C values and find the cut-off frequency.)

    The power tube grid stopper in fender type circuits is a low value (1k5) and is there to help prevent parasitic oscillation from very high frequencies. Larger values can be used to help prevent blocking distortion or to reduce high frequencies but that is not how fender used them.
     
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  5. Mongo Park

    Mongo Park Tele-Holic

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    This is starting to make some sense. I was think backwards as in a higher Bali would pass less to ground. I will give it a real world try and see if a 150k and see if it sounds different.
     
  6. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    6V6 cathode bias maximum grid-to-cathode (not grid to ground as I said earlier) is indeed 500k. For fixed bias it is only 100k. 'Guitar Amplifier Power Amps' by Richard Kuehnel uses the Fender Showman AB763 as an example where the grid-to-cathode resistance is 442k for a 6L6GC, for which the limit is only 100k (page 108).

    The RC filter is a low (edit: high, not low) pass filter with the -3dB point at

    f = 1 / (2 x pi x (Rg + Ro) x Cg)

    Rg : grid leak resistance
    Ro : preamp output impedance

    which for (typical) values of Rg = 220k, Ro = 38k and Cg = 0.02uF results in f(-3dB) = 31 Hz.

    Edit: I thought I knew better than Lowerleftcoast, but I screwed up...
     
    Last edited: May 30, 2020
  7. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    In this case the RC filter is high pass. The corner frequency would be ~31Hz. Signal above the corner frequency would pass through as signal. The lower Hz signal would be attenuated at 6db per octave.
    hi pass.gif
    A low pass filter would have the position of the capacitor and resistor reversed. The corner frequency would use the same calculation. In the above example the corner frequency would still be ~31Hz. With a low pass filter the signal below the corner frequency would pass through as signal. The higher Hz signal would be attenuated at 6db per octave.
    low pass.gif
     
  8. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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  9. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    No worries, I mess up too. I am not so sure we are answering the OP questions though.
    I will try again.
    In a cathode bias amp the grid to ground resistor provides a reference to ground for the 6V6 grid. If it is too large the tube can start to run away, which would cause damage. According to the 6V6 datasheet 500k is the max suggested value.

    A very large grid stopper can have the signal be affected by the previous tube. The coupling capacitor along with the capacitance within the previous tube and the plate resistor form an RC filter. That filter will have some of the signal go to the B+ side. (Essentially this happens in the Fender Tweed 5E3 circuit when both volumes are turned up.) I don't find it very musical. OMMV. A large grid stopper will not damage the amp.
     
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  10. Mongo Park

    Mongo Park Tele-Holic

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    Ok I am going to have to read this over a few time for it to sink in. This is another step to learning how this all fits together, and how one part must be correct for the next part to work properly. Thanks for sharing your immense wisdom.
     
  11. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Maybe you are talking about Tom. I never thought I had immense wisdom. I keep telling myself I should have paid more attention when I was a kid.

    I think we are all just trying to learn a little more each day. Fun stuff...

    :cool::cool::cool:
     
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