[Amps] 4CX1500B project - swamped grid

[Bill Fuqua]

   Sorry, looked up the wrong tube in the wee hours on the morning.
But, that was not my real point.
    In an amplifier with  a plate tank circuit with Q>10 the Miller 
capacitance is not
going to change much. Miller Capacitance is where we have the effect of a 
much larger
capacitance on the input of an amplifier than the real input capacitance,
This is because of the feedback capacitance and the voltage across it is 
much greater than
just the input voltage. The voltage across the feedback capacitance will be 
the output voltage plus
the input voltage ( in the case of a inverting amplifier). This magnifies 
the effective capacitance by
the amplifier's gain plus 1.
     If the output voltage continues to oscillate, even if the amplifier is 
not functioning at that moment,
this AC voltage is still across the feedback capacitance and the current 
continues to flow. The
class room analysis of Miller Capacitance is not only a class A amplifier 
but one with a
simple resistor as load, not a tuned circuit (Flywheel).
     Look at it this way, Take two synchronized signal generators. One 
driving one end of the
capacitor and one driving other. If you have one adjusted to one volt and 
the other zero  and you had a capacitive
reactance of 1 Ohm, you would have 1 Amp of current. Now adjust the other 
to 1 volt and have its phase set 180 degrees
from the other. Then across the capacitor you would have 2 volts. Now even 
though signal generator #1
is 1 Volt RMS the current thru the capacitor is 2 Amps. Set #2 to 100 
volts, then the current is 101 Amps.
The capacitive reactance as signal generator #1 sees it is 1/101 of the 
actual capacitance.
Or the capacitance that signal generator #1 sees it is 101 times the actual 
capacitance.
     In an amplifier with resonant circuits on the output the sinewave 
continues to even when the active device is
inactive for any portion of the cycle.
     It is interesting that even though reactance seen by the "tube" is 
resistive the reactive components play a big
part in these circuits. There is nothing magic about amplification that 
magnifies the feedback capacitance, it is  just
the mean by which another voltage is applied to the "output" side of the 
capacitor and the reactive parts fill in
the gaps of the sine wave that would otherwise exist produce harmonics.
      Miller effect applies not only to just capacitance but to any 
feedback impedance and to inverting and
non-inverting amplifiers ( just have to change sign). The case of a 
non-inverting amplifier is even more interesting.
      This just occurred to me when I looked at just the final couple of 
comments on the topic. I have been too
busy dealing with other stuff and getting things ready our Club's Hamfest 
that I have not been following the thread.

73
Bill wa4lav


At 10:10 AM 8/12/2012 -0700, you wrote:
>ORIGINAL MESSAGE:
>On Sun, 12 Aug 2012 01:51:55 -0400, WA4LAV wrote:
>
> >. That is shunted by the
> >tube's input capacitance, 15.7pF.
>
>REPLY:
>Where did that figure come from?
>
>According to the Eimac 4CX1500B data sheet, the input capacitance in grounded
>cathode mode is between 75 and 88 pF.
>
>73, Bill W6WRT
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