By-Pass Cap

[trem]

In a basic 12AX7 gain stage for example.....
With an AC input to the grid, I understand how that effects the bias of the tube. As the signal goes more Positive, plate current increases and plate voltage decreases. As the input signal goes Negative, the opposite occurs.
What I do not understand is the AC source at the cathode. Where the bypass cap is located in parallel with the cathode resistor. Where is that AC source.? How is AC occurring at the cathode, so the cap can bypass it.? Where is that AC coming from.?
Thank You

[Chuck H]

Very good question. Without going too long (and only regurgitating what I've read in laymans terms) the function of a tube is less than perfect. There is a certain amount of electrons build up in the envelope. The phenomenon seems to be proportional to gain or current and is predictable. This can serve to regulate tube function to some degree but they also impress some NFB on the output by virtue of interaction. This is a useful function depending on the application. For maximum gain it's best to eliminate this very local NFB by sending it to ground. Since capacitors block DC the tubes bias isn't affected though gain is increased. Sometimes at the expense of THD content at the plate output. Usually not a concearn with guitar amps.

[pdf64]

The way I understanding it is that signals applied to the grid also appear at the cathode (and visa versa) which will cause NFB unless bypassed to ground.
Sometimes this can be very useful, eg cathode follower providing a buffering function. Pete.

[es345]

Well,

hopefully I have understood your question in the right way:

Scenario is: triode with cathodes bias resistor and "bypass capacitor", grid connected to ground via e.g. 1M Ohm

The following is a qualitative description which helps to understand:

1. Assume for a moment no bypass capacitor.

first no signal. The cathode current will set according to the load line, the current through the cathode resistor creates a positiv voltage from cathode to ground or the other way around a negativ voltage from grid to cathode.

Now inject an AC signal at the grid. Then the cathode voltage will follow the AC so that the difference between cathode an grid basically is constant (cathode follower effect). It is a negative feedback.

2. Now connect a bypass cap.

with no AC signal there is no difference at all to the scenario before, because the resistance of the capacitor at 0 Hz is infinite.
If you now inject an AC Signal to the grid with raising frequency there will be the point when the AC resistance of the capacitor (Rc= 1/(2*3,14*frequency) ) will become lower than the cathode resistor. If you further increase the frequency is AC resistance of the capacitor is going near 0 compared to the ctahode resistor. Then you do not have a negative feedback anymore because the capacitor "bypasses" the resistor for this frequency and the amplification at that frequency is higher than without the bypass cap.

The size of the bypass cap depends what you want. In a HiFi Gain stage you would set the point where R cathode is equal th R capacitor to ~5 Hz, for a lead overdrive gain stage it might be some hundred Hz.

[trem]

Hang in there with me....I am a bit slow.
Are you guys saying the AC source for the cathode is the same as the gird.....the AC signal from the guitar.?
I am having a hard time grasping the path of travel. How does the AC signal that is at the grid (12AX7) get to the cathode.?
Thank You VERY Much
this has bothered me for a long time. I have always been too embarrassed to ask.

[es345]

Again scenario 1: Cathode bias resistor without bypass cap

the following happens simultaneously, sequence is only for understanding

0. in the starting point you have a given voltage difference between the grid and the cathode based on the operation point.
1. you increase the voltage at the grid
2. this results in an increase of the current at the cathode.
3. when the cathode current increases, the voltage of the cathode will increase until the new balance is there between grid and cathode.
4. this balance results in nearly the same voltage difference as before, in other words the cathode voltage is following the grid voltage --> cathode follower.

The situation changes with the bypass cap as described above.

Edit: don't think that there is an "AC source" at the cathode. The only source is at the grid, at the cathode without the bypass cap you have a (possible) output signal with no amplification but lower impedance

[trem]

OK....I re-read, and printed both of you posts. I do understand it while I read, but it will be awhile before I can recall this process from memory.
Kind of basic electronics when drawn on paper, but it sure is not (not to me anyway) very intuitive.
But I do grasp your explanation(s). They are very clear. I am somebody who really benefits from a classroom, teacher, and chalk board.
Thanks for clearing up the mystery. It has nagged at me for a long time. I could see the current from the power supply, through the chassis, and a voltage across the cat resistor....but I could never figure where the by-passed AC was coming from.
Thanks Again
I really appreciate it.

[Enzo]

Let me try to state it simply.

You understand the current through the plate resistor varies by the grid voltage. That same current flows through the tube and the cathode resistor. Current has to have a complete circuit between power supply and ground for any to flow.

SO when current through the plate resistor increases, the voltage drop across the resistor increases - that is why the plate voltage drops. You know that. But the same current also flows through the cathode resistor. Remember, the tube just acts like a valve controlling current flowing through the plate and cathode resistors between powr and ground. They are in series through the tube.

When current increases through that cathode resistor, then by Ohm's Law, the voltage across it increases. Since the one end of the cathode resistor is grounded, that means the cathode end of it must increase, get more positive.