Hi guys,
Now we are getting into some fun tube stuff.
The typical triode has cathode resistance. It is generally between 200 and 1K ohms, depending on the transconductance (Gm). This is calculated by a simple formula, 1/Gm. So if your triode has a Gm of 1600 µmho, or µSiemens (the 12AX7 or 6SL7), then the cathode resistance is 1/0.0016 or 625 ohms.
The calculation for gain in a stage of amplification is simply Rp/Rk (some will quote more elaborate and esoteric equations, but it can be simplified into this one). This is total resistance, which means Rp is the plate resistor in parallel with the plate resistance in parallel with the resistance of the following stage. Rk is the cathode resistance in series with the cathode resistor. So for a typical 12AX7 stage with a 100k resistor and a next stage of 470K, with a 1K cathode resistor, the gain would be about 32k/1625 or 19. With a bypass capacitor, the gain now becomes 32K/625 or 51, which seems significant numerically, but audibly amounts to about 3 or 4 decibels difference. Not much.
However, if you have a global NFB in that circuit, you now have some more gain to play with, and some more NFB for the frequency response to add to. IOW, deeper bass and higher highs. And less peaks throughout.
One thing you have to ask yourself: Am I hearing deeper bass, or more bass?
The reason I ask is because when going from one quality of anything audio to another, one of lesser quality may sound more bassy but may be limited in range, whereas the better quality one may sound less bassy, but you can get 10 Hz out of it with equal force as 100Hz.
As for larger caps in the bypass, or interstage for that matter, I have an opinion. If one were to rearrange the triode amplifier such that the plate were the input and the cathode the output, it would look like a rectifier. So the bypass cap will be a filter.
In essence that is what it does. With low frequency sound, the larger caps fill up to the higher voltage of the signals positive peaks. What happens then? For a brief moment the tube goes into cut off, or close to it. In a word, distortion. Also, the tubes characteristics are no longer constant.
With musical instrument amps this is more of a problem, because the pluck of that string introduces a very high amplitude very low frequency signal. So for a moment right after that pluck, you hear nothing, then the sound comes as if someone raised the volume quickly.
For high fidelity it is bad because we want out tube amps to sing properly all the time. It is sort of a testimony for fixed bias. Now that is another thread altogether.
Gabe