Cathode bypass capacitor

[Champ81]

I had a question regarding the cathode bypass capacitor.  Whether in preamp or power amp sections.  I know and have witnessed how it affects the tone of the amp and increase gain. From my current knowledge it compensates for the frequency loss due to the resistor allowing certain frequencies to pass through depending on the value.  My understanding is the cathode resistor value determines how hot or cold the tube is biased. So essentially the amount of electrons that emit from the cathode. Higher value resistors emit less and vice versa. The grid is sort of like a valve that controls how much of the electrons go from cathode to anode (the plate).   

If I got that part correct I don't understand how a cathode is associated with the frequencies it let's through.  Because isn't it the grid where the guitar signal goes into?

I cannot visually understand it. I have heard from others a DC current flows from the ground through the cathode resistor to the cathode of the tube. Bit how van current flow from ground because the ground there is no current? I know ground is all relative but this is circuit ground to the chassis.

[K O'Connor]

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.

[Champ81]

That makes alot more sense now. The signal actually goes through the bypass cap.  Without it the signal only sees the Vk which would block some or most of the signal frequencies.

[DCarrer]

Hey @Champ81, I do love your enthusiasm however you, this forum and KoC might be better served if you invested in at least one of the TUT books. They are invaluable to novices and professionals alike.

[Champ81]

I have TUT 2,3,4 and 6. 
I don't have the first one. Some of the things I understand in terms of what they do. For instance the question regarding the bypass cap. I know what it does and why it's used. But not how it does what it does. 

 But alot of it goes over my head to be honest. Maybe it is time to invest in TUT 1 if that will be an easier book for me to start from and start from fundamentals. Maybe the ones i have are too advanced and i jumped the gun. But either way I have learned alot. Just from this forum alone and I think I'm more knowledgeable than I was a couple of months ago.

[K O'Connor]

Hi Guys

TUT assumes a high skill level inasmuch as : you should be able to read a schematic; you should understand basic component function; you should understand RC interactions; you should have safety awareness when working with live circuits. TUT was our first book, so we had to make some assumptions about the reader to keep the book to a manageable size.

TUT3 is the usual "gateway" into the TUT-series. You can build an amp using the layout drawings and eventually learn to relate that to the schematic.

TUT and TUT5 detail how things work in a preamp, TUT5 particularly goes into much more depth.

Generally, there is simple and advanced information in each volume, so there will be something you can grasp and build from regardless of your interests and skill.

[mooreamps]

I once bought all of Kevin's TUT's. I read all of them, very carefully.

-g