... to get class-A (curvature) like behavior from a jFET ...

Could someone, **ANYONE**, explain to me what in the hell this sentence fragment ('class -A (curvature) like behavior') means?

What he means is operating in Class A *and* with minimal negative feedback thereby producing lots of 2nd harmonic content as a result because vgs is allowed to span a significant portion of the transfer characteristic.

that's right ...

in analogue signal processing HARMONICS are a product of a non-linear transfer function ... a non-linear transfer function is loosely analogous to a curved mirror at the fun house ... it makes you look fat, thin, distorted ... when the transfer function is linear a sine wave going in comes out a pure sine wave ... when it's non-linear more than one sine-wave comes out - the difference is called Harmonic coloration ... now, the thing to note here is that Harmonics are a direct by-product of a curved transfer function ...

although many folks think that WE ARE ALL after tube emulation what we really are after is a simulation of the non-linear transfer response of a class-A circuit running on tubes - usually triodes since they produce more class-A MOJO per pound of signal ... I'll say it again, we're after class-A curvature, and all the other fun stuff that comes with it ... in other words it's not just about Harmonics production - I think this needs to be made clear since it's really not a very obvious thing ...

to explain this a little, class-A curvature does two things for us ... (I) it provides a dynamically varying addition of class-A harmonics via transfer (provided there's enough signal swing as ALANLAN pointed out) , and (ii) the transfer curvature produces an elongation of the sinewave that results in baseline (bias) shift ... if you've ever stuck a scope on the cathode of the 6V6 in a Fender Champ you know what I'm talking about ... this opens us up to many exciting potential musical doodads to explore ... one is bias shifting of a circuit that follows this one - I'm not sure I want to spill the beans but you might already get my drift of what might happen when you stick this directly in front of the first stage of a guitar tube amp ... it's awesome in a booster setting and I'm really digging playing the circuit implementation I came up with ...

so, in effect we're not just adding harmonics through spectral means, like one can do/approximate with a mixer and clean octaver, but rather we're now doing what a single ended tube circuit does, but at a scaled down voltage ... essentially simulating, and even exaggerating, the curvature of the response ... I'm saying exaggerating because a triode circuit (and not the tube alone) can have at most power 1.5 curvature while a jFET circuit can exhibit power 2 curvature at most (that's the mode we want to be running it if we can - Danyuk shoots for 1.5 emulation in his paper) ... hence jFET circuits can be used to go a little further in terms harmonic coloration than what you can get from a class-A circuit running on a triode ...

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back to the jFET matching thing for a second ... I did an extensive search to see if there's anything solid on the net and pretty much everybody is doing the same non-thing ... I wrote out the math to show where/how the tests are faulty and I'm pretty sure most of the established members on this forum don't want to see that stuff dragged out ...

I'll just say this because it applies to phasors ... the kind of test where a fixed current is drawn into the DUT's, a Vgs voltage is measured and devices are lumped together when close in value will produce a locus of possible points for which devices with Idss and Vp values all produce the same Id/Vgs pair response at the test current level ... it is easy to show that if you change the current test level then devices that matched cannot match any longer unless their Vp and Idss values are identical - which to trained engineers is the only real terms of criteria for matching // the data-sheets are written for people who know and understand this ... to make proper use of the proposed test jigs a second pass needs to be performed at a reference current considerably different than in the first pass ... this can be demonstrated also with simple math ... in other words, if you were to single out seven devices in the first pass of that test and from those seven you singled out a remaining four after dropping the ref current in a second pass ONLY THEN can you claim matched devices throughout their operating ranges and therefore ALSO guarantee that the matching will apply to the VCR aspect of the jFETs ...

what this means is when people are using such a test they are likely yielding a set of devices for which the Vp/Idss values that are distributed along the curve locus (unless the devices where all matched to begin with by high chance) ... and not knowing what the exact values for Idss and Vp are this means that true resistance levels can be all over the map ...

since the equation for the incremental resistance rds = |Vp|/2Idss(1-Vgs/Vp) is strongly dependent on Vp and Idss then the resistance be all over the map just as the Vp/Idss values will be (lying on the locus curve between Vp min and Vp max) ... now there's a happy conclusion to this story ... many of the people who go through the bother of using this false-match method are probably ending up with devices that exhibit resistance level differences that are considerably large without realizing it ... since the shift frequencies are the result of a product of capacitance and channel resistance, then if a builder uses identical cap values the shift frequencies will be staggered according to the channel resistance differences ... well, as Mark Hammer once pointed out, it would produce the same effect as if we had matched channels but staggered cap values - which we all know is how the Univibe does it's thing ... so people are VERY LIKELY (as in Gaussian likelyhood) thinking they're matching their devices when they're just randomly staggering them ... and since staggered phasors seems more pleasing or interesting to the ear it is no surprise hearing them say their phasor sounds great ... it also explains why many (probably the majority) of people who simply finish their phasors by swapping in devices by ear get there without having to go through this trouble ...

~jc