bypass cap values

[trem]

I am trying to get a rope around bypass caps. I understand it plays a role in tube bias and stability. But what determines the size of the capacitance? The Fender 59 Bassman/Marshall/JTM-45 used some caps that varied greatly in value on that first preamp tube; approaching 330mf if I am not mistaken.
But is this a good place to make changes in regards to tone shaping? I have often changed the value of coupling and tone stack caps to adjust the voicing of an amplifier. But does the same hold true for the cathode bypass cap?..... I have an Epi Valve Jr. head. I have done a few "mods" to it, but it still seems a bit muddy. I was thinking of lowering the value of the bypass cap on V1B to help with that. Usually with a mod this small, I would just go ahead and do it, then see what I hear. But I started to think if it worked, maybe I just got lucky, or maybe it works good in a SE amp like the Valve Jr., but would not be as successful in more powerful class A/B amps.
So, again, that is my question. Generally speaking, is the cat bypass cap a good place to adjust tone, or am I better advised to keep my hands off that part of the circuit? Sorry for the long question.
Thank You

[tedmich]

I am trying to get a rope around bypass caps. I understand it plays a role in tube bias and stability. But what determines the size of the capacitance? The Fender 59 Bassman/Marshall/JTM-45 used some caps that varied greatly in value on that first preamp tube; approaching 330mf if I am not mistaken.
But is this a good place to make changes in regards to tone shaping? I have often changed the value of coupling and tone stack caps to adjust the voicing of an amplifier. But does the same hold true for the cathode bypass cap?..... I have an Epi Valve Jr. head. I have done a few "mods" to it, but it still seems a bit muddy. I was thinking of lowering the value of the bypass cap on V1B to help with that. Usually with a mod this small, I would just go ahead and do it, then see what I hear. But I started to think if it worked, maybe I just got lucky, or maybe it works good in a SE amp like the Valve Jr., but would not be as successful in more powerful class A/B amps.
So, again, that is my question. Generally speaking, is the cat bypass cap a good place to adjust tone, or am I better advised to keep my hands off that part of the circuit? Sorry for the long question.
Thank You

A lot of tone tweaking happens at cath bypass, resistors and caps. Check the Valve Wizards chapter on it, its free for now!
http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

[defaced]

I was just looking there to see how Merlin describes it. The cathode bypass section starts on page 25.

For adjusting it, it is irrelevant what "kind" of amp it is. Meaning, a tiny class A amp or a monstrous high gain class AB1 amp both will be affected by any adjustment made in the preamp, including cathode bypass caps.

This is a nice visual tool of the frequency response curve as a function of cathode bypass cap size. The Valve Wizard -Cathode Follower
So if you think it sounds muddy, try lowering the value.

[mr.jetski]

after you go there, hit the home button. there is more to dig out of his site. great info!!

[trem]

Thanks for the link guys. I have his web site in my favorites, but I didn't think to check there for some reason. I could swear.... seems like the more I learn, the less I know.
Thanks Again