Can someone educate me on transconductance?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by elkym, Oct 23, 2008.

  1. elkym

    elkym Member

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    I understand something of the different tube types...

    I'm now trying to understand measurements taken of tubes...

    for example: just got a couple of 12at7's. each of these was 'measured' at 3000/3200, 3200/3200, and 3700/3900. I have no clue what this means... and I'm not sure what to Google to find the answer... can you help me?
     
  2. elkym

    elkym Member

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  3. shaneygoo

    shaneygoo Member

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    Vacuum tubes
    For vacuum tubes, transconductance is defined as the change in the plate(anode)/cathode current divided by the corresponding change in the grid/cathode voltage, with a constant plate(anode)/cathode voltage. Typical values of gm for a small-signal vacuum tube are 1 to 10 millisiemens.

    from wikipedia...

    so i would say it depends what you're looking for... a clean amp? you probably want them to be consistent ratios, dirty or unique sounding amp, go for asymetrical values...
     
  4. elkym

    elkym Member

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  5. elkym

    elkym Member

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    When you say assymetrical, versus matched, is there a rule of thumb or anything you guys could share as to how much of an assymetrical does what? And is a perfect match necessary to be cleaner, or just a really close match?
     
  6. shaneygoo

    shaneygoo Member

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    here's your tube

    http://en.wikipedia.org/wiki/12AT7

    "Compared to the 12AX7, the 12AT7 has somewhat lower voltage gain, but higher transconductance and plate current, which makes it suitable for high frequency applications."

    so that will be slightly cleaner and brighter sounding tube
     
  7. donnyjaguar

    donnyjaguar Member

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    If you look at the mu curves for triodes, you'll notice they are all the same for the given amplification factor. Ie the curve for a 6SN7 is the same as a 6CG7. Transconductance is only relevant, IMO, for tetrodes and pentodes where the output signal is dependent on more than one input.

    The 12AU7 has lower gain so the miller capacitance will be lower, hence it has less attenuation at higher frequencies. If you use a 12AX7 or a 12AU7 in a cathode follower configuration there is no difference because the gain is unity anyway.

    I guess my point is to not get hung up in the minutiae of tubes.
     
  8. elkym

    elkym Member

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    Thanks again, people. I like your 'title' Old Tele Man-- how do you get one of those?
     
  9. 59Vampire

    59Vampire Silver Supporting Member

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  10. tritone1

    tritone1 Member

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    millisiemens? I'm hearing new terms from the wisest ones, but how do they compare to micromhos? I could look it up, because I'm sure of the people who know something at all of electronics, I am in the minority who doesn't know of millisiemens.
    There was another term in this strand that was new to me. I'll check on it.

    Thanks
     
  11. tritone1

    tritone1 Member

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    miller capacitance? must me interelectrode capacitance of some sort. I'll look it up ,too.
     
  12. Prairie Dawg

    Prairie Dawg Member

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    If I may be permitted to indulge in a bit of speculation, I'd say that mS and its progeny are primarily in use these days so some people can claim they invented the entire idea.

    Why, then, take a unit of measurement that becomes an advertisement for an electric gear company every time it's uttered? That'd be like changing the word "automobile" to "Chevrolet".

    For an interesting slice of transoceanic paranoia, see the entry at

    http://en.wikipedia.org/wiki/Siemens_(unit)

    in particular the last few sentences.

    As you can see "S" became an international standard in 1971, and if these fellows have their way they'll rewrite everything that says ohms or mHos on it.

    The satire light is off. You may now move about the cabin.

    :rotflmao
     
  13. Wakarusa

    Wakarusa Member

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    Miller capacitance is actually one of the interelectrode capacitances (plate to grid in this case) multiplied by one plus the stage gain.
     

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