From: ruffrecords@yahoo.com   
      
   Ian Iveson wrote:   
   > Ian Bell wrote:   
   >   
   >>> I keep making my point about grid current rising as anode   
   >>> voltage falls but no-one's picked up on it.   
   >   
   >> I thought I did in one of my earlier posts where I did   
   >> tests at various operating point. The anode voltage varied   
   >> by over 100V but there was no discernable change in grid   
   >> distortion.   
   >   
   > Oh. Maybe I've lost track.   
   >   
   > I gave some thought to how such a test could be done with DC   
   > without introducing more complicating variables. Alex's idea   
   > of an AC short seemed like a reasonable half-way house,   
   > because it allows a varying Va to be compared with a   
   > constant Va. However, the results are open to several   
   > interpretations. Is it an AC-only effect, or does it also   
   > happen at DC? Is it the low AC load that makes the   
   > difference, or the constant Va, or are they two sides of the   
   > same coin?   
   >   
   > It might be useful to try to reproduce the same effect by   
   > altering the DC conditions, to freeze-frame the point of   
   > lowest Vak and highest Vgk reached in your pre-Alex test.   
   > Does this result in the same grid current as for the AC   
   > case?   
   >   
   > Is that what you've already done? This test, together with   
   > your pre- and post-Alex data, would distinguish between AC   
   > and DC effects.   
   >   
      
      
   I'll check. I did the pre Alex tests at several dc conditions which resulted   
   in a 100V variation in   
   Va and no change is distortion  measured at the grid. Not sure if any of these   
   same conditions   
   occurred when I did Alex's cap test.   
      
   > There are two issues that interest me here. One is the   
   > concept of a "non-linear capacitance" which makes no more   
   > sense to me than a non-linear inductance or resistance. What   
   > can be non-linear about a farad, a henry, or an ohm?   
      
   Well Henries are well known to be non linear, just look at the B-H curve. It   
   is well known that   
   measured inductance varies with signal level in passive equalisers for   
   instance. I am sure Patrick   
   knows of similar effects in output transformers.   
      
   A non linear capacitance might have a capacitance value that varies with   
   applied signal lev el for   
   example. I think someone suggested the tube capacitances might vary due to   
   instantaneous bias   
   changes altering the space charge.   
      
      
   The   
   > concept of linearity applies to relationships, not values.   
      
   Indeed.   
      
   > Is my log pot a non-linear resistance because it's log, or   
   > because it varies with current-induced temperature change,   
   > or because it slowly corrodes?   
   >   
   > If I put an SS rectifier accross a capacitor, does that make   
   > the capacitance non-linear? It could be characterised as   
   > such, and analysis would render a complicated equation to   
   > express that non-linearity. Rather like choosing to analyse   
   > the solar system from the point of view of earth, when   
   > standing on the sun would be so much simpler.   
   >   
   > There are several fixed electrodes in a vacuum. How can the   
   > capacitance between each pair vary? Seems to me a sensible   
   > starting point to stand on the fact it doesn't, and look at   
   > all other variables from the point of view of that   
   > assumption.   
   >   
   > Each interelectrode capacitance is perfect: two fixed plates   
   > in a vacuum. The only thing that could be responsible for   
   > any variation is the mass transport of charge between the   
   > electrodes. The rules governing this transport don't care   
   > about frequency: they apply at DC equally, so how can they   
   > be responsible for an effect that is best described as a   
   > varying capacitance? Surely a much simpler equation would   
   > result if the combination of capacitance and mass transport   
   > were disentangled and characterised as a DC, rectifying   
   > effect, in shunt with a constant capacitance? After all,   
   > that would result in the increase in 2H that you observe.   
   >   
   > The other matter of interest for me is to what extent grids   
   > can be universally characterised using the same equation.   
   > IIRC Duncan Munro's last triode model, including a full grid   
   > model instead of the common diode bodge, treated the grid in   
   > a similar way to the screen of a pentode. Alas, the   
   > explanatory document is no longer available at his site.   
   >   
   > My hypothesis, deliberately contrary to Alex's, is that   
   > there is a constant capacitance, in combination with a   
   > current arising from the transport of charge which varies in   
   > a way which is non-linear with respect to the grid voltage.   
   > The former is an AC effect, the latter DC.   
   >   
   > The DC non-linearity arises, my hypothesis continues, from   
   > the universal properties of a grid, and can be seen as an   
   > elbow in those curves labelled "anode characteristics" in   
   > datasheets. They often show screen grid current, but I   
   > haven't seen one that shows control grid current.   
   >   
   > The major problem with my contention is that it ignores the   
   > rectifying nature of a valve, in that my control grid   
   > characteristic curve would cross from negative to positive   
   > current, and valves don't behave backwards the same way as   
   > forwards. OK, so draw the control grid curve the same shape   
   > as a screen grid but further down, so that it crosses the   
   > voltage axis at the appropriate point. Now chop off the   
   > negative current part. Simple. Maybe.   
   >   
   > In that case you would expect, given constant Vgk, grid   
   > current to be zero at high Vak, remain so as Vak is reduced,   
   > and then shoot up steeply as Vak becomes very low. Where   
   > very low is, I would be interested to find out.   
   >   
   > I'd check this as a farewell tribute to my valve tester, but   
   > it works by mysterious AVO magic, and would only make   
   > everything much more complicated. It uses full-wave AC on   
   > everything except the control grid, which is rectified AC.   
   > Consequently, for my beautiful AVO, the distinction between   
   > AC and DC is always, sadly, moot.   
   >   
   > Ian   
   >   
   >   
      
      
   Cheers   
      
   ian   
      
   --- SoupGate-Win32 v1.05   
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