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RE: definitions

"The amount of capacitance may be inadequate for full range performance......"

Yes, that is what I meant.

When a cathode is bypassed to the point that the lowest frequency of interest is unaffected by the filter (amplitude and phase), I call that fully bypassed.

When the cathode is bypassed but the -3db point is higher than a decade below the lowest frequency of interest, I call that partially bypassed.

A fully bypassed cathode will prevent local current feedback from occurring down to the lowest frequency of interest.

A partially bypassed cathode will allow local current feedback to occur below a certain frequency depending on the value of the cap but not above that frequency.

This will give less gain in the lower frequencies and increase the plate resistance of the tube at those lower frequencies thus changing the damping factor of the amplifier if it's an output tube.

My use of the terms, fully bypassed and partially bypassed, can be found all over the internet. Rightly or wrongly.

BTW It is not the cathode resistor alone that is being bypassed but the cathode resistor in parallel with the cathode impedance, r.

The value of the impedance of the cathode of the tube is (the plate resistance + the plate load) / (the mu + 1)

That number is in parallel with the value of the cathode resistor to give us r.

r is what is what we are attempting to bypass.

This can make a huge difference. In the case of Dennis' amp it's not his 5k ohm cathode resistor that needs to be bypassed but that 5k ohm resistor in parallel with the 634 ohm cathode impedance. So 563 ohms is what needs to be bypassed.

56uf would get him 5Hz with 50Hz phase and amplitude correct.

141uf would get him 2Hz with 20Hz phase and amplitude correct.

Understand that I'm just talking about the effects of this filter, not the amplifier as a whole.

I believe Dennis uses 15uf and that will not fully bypass (by anyone's definition) 563 ohms.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"


Edits: 11/17/14 11/17/14 11/17/14
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