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06-11-2023, 07:40 PM
Hi Guys
As you will expect the answer to this question is in TUT The Ultimate Tone (first volume).
"Current" is the flow of electrons. All currents flow in loops. The DC current through the tube flows up from ground, through Rk, through the tube from cathode to plate, through the plate resistor to the positive supply node, then back to ground through the power supply. The DC current is always in motion but it is a steady value.
When there is signal at the grid of the tube, the current through the tube is hindered or enhanced, so the signal is superimposed on the DC. Without a cathode bypass cap Ck, the signal at the grid has to work against Vk, which provides feedback to linearise the output BUT it also keeps the voltage gain from being as high as it could be. Maybe you want this?
Ck allows the varying current caused by the AC signal to literally "bypass" Rk - go around it. This allows Vk to remain constant and keeps the operating point stable. meanwhile, the Ck current adds to the plate current and there is increased amplification where Ck has influence. This influence is frequency-dependent using the standard RC equation. By changing the value of Ck, gain can be enhanced above the RC frequency.
As you will expect the answer to this question is in TUT The Ultimate Tone (first volume).
"Current" is the flow of electrons. All currents flow in loops. The DC current through the tube flows up from ground, through Rk, through the tube from cathode to plate, through the plate resistor to the positive supply node, then back to ground through the power supply. The DC current is always in motion but it is a steady value.
When there is signal at the grid of the tube, the current through the tube is hindered or enhanced, so the signal is superimposed on the DC. Without a cathode bypass cap Ck, the signal at the grid has to work against Vk, which provides feedback to linearise the output BUT it also keeps the voltage gain from being as high as it could be. Maybe you want this?
Ck allows the varying current caused by the AC signal to literally "bypass" Rk - go around it. This allows Vk to remain constant and keeps the operating point stable. meanwhile, the Ck current adds to the plate current and there is increased amplification where Ck has influence. This influence is frequency-dependent using the standard RC equation. By changing the value of Ck, gain can be enhanced above the RC frequency.
