Explaining caps - no signal

[überfuzz]

If we take a 12ax7. There are many designs were polarized caps are connected from ground to pin 3 and pin8, with a parallel resister. First of all, if anyone could give me the English term of this cap. Secondly what's the function of it?

[defaced]

Cathode bypass cap
Explanation:

Capacitor Ck is used to bypass the cathode resistance to ground for AC signals, which results in a higher gain. Without Ck, there is negative feedback, or degeneration, which reduces the gain of the stage and increases the output impedance. If Ck is not large in comparison to Rk, it will affect the frequency response of the stage, by introducing a "shelving" response, where the stage gain is boosted at higher frequencies compared to lower frequencies. The "breakpoint" of the frequency response is controlled by the value of Ck, in conjunction with the cathode impedance.

Source: Designing Common-Cathode Amplifiers

See also: The Valve Wizard

[Wilder Amplification]

We call that a cathode bypass cap. As the input signal causes cathode/plate current to alternate at input signal frequency, a cooresponding alternating voltage drop appears across the cathode resistor that is 180* out of phase with the alternating voltage drop that exists across the plate load resistor, which causes the cathode voltage to alternate above/below its static zero signal value. This induces degenerative feedback as the voltage differential between cathode and plate alternates with input signal out of phase relative to the plate signal.

The cathode bypass cap acts like a filter cap that filters this alternating voltage drop out and converts it to steady DC, thereby holding the cathode voltage to a constant value and removing the degenerative feedback. This causes only the plate to have an alternating voltage, which is basically AC with a DC offset that is equal to the static zero signal plate voltage. The coupling cap at the plate blocks the DC plate voltage, which eliminates the DC offset and you end up with pure AC at the other side of the coupling cap, which is the result of the coupling cap charging/discharging along with the alternating plate DC at the plate at a frequency that is equal to the input signal frequency.

Since you've eliminated the alternating cathode voltage and as such the degenerative feedback, you've now maximized the gain of the stage for a given component combination.

If you use a low enough value bypass cap, you can tailor the gain of the stage to a higher frequency pass band as gain is maximized for frequencies above the rolloff frequency of the cathode resistor/bypass cap combination while frequencies below that receive the "unbypassed gain" of the stage as if the cap wasn't there. There is a slope in between the two of course though because nothing is "brick wall".

[überfuzz]

Thanks defaced and wilder. The link you posted gave me a fair idea of how a whole 12AX7 stage works, specially self biased. It actually made me decide that I'm going to design one of the two channel of the current amp build myself.