You need to be careful about equating a JFET and a vacuum tube directly. Yes, they are very similar, but there are some very important physical differences. Because the frequency changes for one reason with a tube, that does not mean that the change is caused by exactly the same thing in a JFET.
As far as the cause being "Miller Effect", it is likely that gain change may be the dominant effect in a tube. But, in a JFET, it is likely to be both. To call one more "Miller Effect" than the other is false. Both are. Because Miller Capacitance depends on BOTH gain and internal inter-electrode capacitance plus any circuit capacitance. Inter-electrode C WILL change significantly with a JFET (and even more in a MOSFET) with electrode voltage. In fact, it will change as the signal voltage swings up and down so that it is time varying at the signal frequency. This makes detailed analysis quite complex. But, suffice it to say, the frequency change with supply voltage is real. Further, the frequency change is due to changes in Miller Capacitance. Finally, change in Miller Capacitance is due to gain change AND due to changes in Cdg.?
From: "Chris Maness" To: LTspice@... Sent: Monday, December 23, 2013 2:47:35 PM Subject: Re: [LTspice] RE: JFET Model
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So would the change in gain due the change in B+ be the most likely cause of f_o change for a Clapp oscillator with a valve state amplifier?? The reason for the inquiry is at this point mostly academic.? I love learning things as I work my way through fixing an old radio.? If indeed that is the case, then it would be the Miller effect indirectly.
