From: apogosso@tpg.com.au
"Ian Iveson" wrote in message
news:bb5lo.201100$ki7.63243@newsfe24.ams2...
> Alex wrote:
>
> Alec wrote:
>
>> Out of curiosity, could you short out the plate load of 12AX7? (AC
>> component by a 10uF from plate to ground.) If the distortion reduces, it
>> is Miller effect amplifying the distortion of the tube. If the distortion
>> remains about the same -- then it is some kind of weird non-linear
>> conduction of the grid. Grid current should not appear with the bias more
>> than --1.4...1.5V, I would think.
>
> Good idea!
>
> I'm fascinated by the idea of non-linear capacitance. A cold valve has
> measurable capacitance between pairs of electrodes. Considering the
> dielectric is a vacuum, these should be linear. When the valve is
> operating, gain between electrodes appears to multiply capacitance, so
> that if the gain is non-linear, so the capacitance also appears
> non-linear. So far, all this assumes that the capacitance is, itself,
> linear.
>
> In addition to the non-linear effects of linear capacitance, there are
> non-linear effects that are not frequency-dependent and that don't
> necessarily impinge on gain. Standing grid current is an example. Since it
> varies with voltage, grid current may result in distortion if it is
> allowed to effect grid voltage, as Ian's experiments demonstrate, maybe.
>
> So what is non-linear capacitance? Is it merely one or both of these
> effects, lumped together for convenience perhaps? Or is it something else?
> Just thinking of an obvious example of a capacitance that is said to be
> non-linear, such as a ceramic, where the non-linearity arises from a bulk
> transport of electrons in the dielectric. In a valve we have moving
> electrons, obviously. What is called capacitance, and what we call
> something else, may be no more than a point of view.
>
>> And what if you reduce the heater voltage?
>
> What's your thinking? What in general is the effect of reduced heater
> current? My guess was that it's related to perveance, and that it should
> therefore make all the curves flatter. Just a guess. Also, if the grid is
> very close to the cathode, then reducing the cloud of
> thermionically-excited electrons may have a greater effect on grid current
> than one might otherwise expect, especially if they don't see the anode as
> a particularly attractive destination when Vak is low.
>
> It would be easier for me, and possibly others if others exist, if Ian
> would talk in terms of DC voltage and current so we don't have to
> translate RMS and those singularly unhelpful dB thingies. If we are to
> distinguish between DC and AC effects, and relate the two, we need DC and
> AC voltages and currents that are directly comparable. Otherwise we could
> eventually discover the error is in his arithmetic.
>
> Ian
Grid-to-plate capacitance is pretty linear. However, since the gain is
non-linear, I thought that the Miller effect related virtual capacitance
referred to grid also becomes non-linear.
But there is another idea which crossed my mind. Cathode-to-grid capacitance
is inherently non-linear. It is a known fact. In fact, a vacuum diode (or
cathode-to-grid section of a triode) works as a varicap or varactor. If the
negative bias is high, then the electon cloud is all kept at bay at the
cathode surface. When bias voltage reduces, hotter electrons are able to
travel closer to the grid (or diode plate). The electron cloud is sort of a
conductive media. Thus the cloud comes closer to the grid wires, increasing
grid-to-cathode capacitance. This varactor effect at the grid is always
there regardless of the plate load.
Looking forward to Ian Bell's test results.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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