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Old 9th March 2009, 03:15 PM   #1
rtill is offline rtill  United States
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Default cap question....

what does this cap do?
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Old 9th March 2009, 06:22 PM   #2
DougL is offline DougL  United States
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The cap * Av adds to the miller capacitance of the tube.
I have seen it used by Norman Koren (sp) to limit the HF input to an amplifier.

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Old 9th March 2009, 06:32 PM   #3
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Might be used to "filter" RF frequencies from getting amplified?

Or as Low Pass filter but not the best way to do it.
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Old 9th March 2009, 06:38 PM   #4
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For an accurate figure of what it "actually does" we would need to know the Grid resistor value, the gain of that stage and the tubes input and G-K capacitances. If you can calc the actual miller of the tube then add the 22pF you can calc the LP F3 point using the grid leak resistance.
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Old 9th March 2009, 07:12 PM   #5
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Default Re: cap question....

Originally posted by rtill
what does this cap do?
it adds an additional 22pF to the VT's reverse transfer capacitance. As to why you would want to do that, there are any number of reasons. If this is an audio amp's VAS, it will increase the Miller capacitance to roll off the high frequency response. You'd want to do that to either enhance the stability under gNFB if excessive open loop gain is causing instability, and/or to reduce an otherwise excessive bandwidth. To amplify audio, you do not need a high frequency cutoff of, let's say, 200KHz. (You're more likely to see this done with solid state amps since transistors are natural high gain devices, whereas VT's are not. Also, SS amps require a lot more gNFB in order to linearize these highly nonlinear active devices. That, in turn, can lead to excessive BWs.)

You would also add that capacitor if this is part of a "Miller" oscillator that's operating at low frequencies. If the proposed operating frequency is too low, the VT may not have enough Crt in order to provide enough feedback to sustain stable oscillations.
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