12ax7-bypassed vs. unbypassed cathode

[Merlinb]

If you had followed the link on my previous post in this thread and you will see this question posed. I.E yunger asking if unbypassed stages sound sterile?

see below

Ah, so I should be asking Yunger where he heard it. Fair enough.

[tedmich]

i read somewhere that a typical 12ax7 preamp stage with an unbypassed cathode sounds sterile do to negative feedback compared to one that is bypassed. anybody have real-world experience with this?
if this is true, does it make sense to have a larger voltage divider at the end of this stage to bump the overall gain back down to usable levels.....assuming gain was already high on the unbypassed cathode model.

There are many interesting designs with switched caps in the cathode bypass, including the PV Bravo and VTM (which is a bit extreme) The Bravo's Dirty->Ultra switch is a bypass cap which gives a selective boost, I believe.

One of the most useful discussions of this (for me) was by a UK fellow who goes by "Valve Wizard" and his monograph is here:
http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

(I am told this is Merlinb, if so well done.)

also Aiken (of course) but at a higher level than I can sometimes appreciate

[yunger]

i've been playing with different combinations of bypassed/unbypassed and attenuation levels for the past few days. i may not have a professional ear but i detect no sterileness in unbypassed cathodes. actually, i think i like the sound better but that could be due to a number of things. i guess the moral of the story is to try it for yourself and then make a decision.

[Merlinb]

It considerably softens the way the signal clips, which gives a softer, more compressive sound when overdriving. I think most people prefer the tone that way, if they can manage with the reduced gain!

[Guitarist]

I had two of those Ampeg amps and always went with the Bypass mode. I'd go with 5uF vs 22uF.

Too much gain? Slide in a 12dw7 or 12at7 - you'd be surprised how effective that is.

all the best, Evan

[6267]

Yeah, it's definitely one of those things were people tend to prefer one or the other in certain preamp spots. I think it has a lot to do with the sound you're looking for...the difference isn't between "good" and "bad" but between two different sounds that both are pretty musical. Someday I'll try battery bias, probably get some great rock tones that way, since the cathode resistor tends to prevent total overdrive by shifting the bias point.

[daz]

It's kinda hard to say when it's impossible to compare them. at least in my experience with my very limited theory, i can't see how you would compare them because the second you bypass the cathode you change the frequency response, possibly in a huge way depending one the resistors size you're bypassing and the cap value. The way i see it that notion probably came from people who are hearing a flatter response and lesser gain that you get unbypassed. more gain+more harmonics usually, and bypassing with say a 1uf will give it a treble/mid peak that also accentuates guitar tone in a good way. It may seem sterile, but usually when you roll back the amps gain know for less distortion it gets more "sterile" too, right? See what i'm saying?

[Guitarist]

I'm about to to try soldering in some charged, rechargeable batteries on the preamp cathodes. If there was a top ten list of real improvements to do on amps, this might be high up there.

[6267]

Daz, I agree, taking out the cap in a circuit designed with the cap will drive everything a bit less, possibly resulting in a cleaner sound. You can always add another gain stage, lower feedback, or alter a grid resistor here or there to make up for the gain loss.

I wasn't even considering small bypass caps to change frequency response...that is definitely separate from just bypassed/unbypassed, it's really more of a tone-shaping choice. I was thinking enough capacitance to entirely bypass the cathode down to DC or so. Since our ears perceive certain frequencies as louder, and perceive harmonics at times as the fundamental, an "undersized" (i.e. tone changing) cathode bypass cap can play a big role in determining the kind of tone you get.

You can get a similar effect in the selection of coupling caps, but it is a steeper low cut (I think) and it also interacts with different parts of the circuit (like the next grid & anode).

So really this discussion comes in three flavors: Unbypassed, partially bypassed, and bypassed at all frequencies. You can put inductors in the cathode circuit too, although I'm thinking they need to be in series with the cathode resistor for a low boost.

[6267]

Guitarist, tell us how that ends up sounding, I'm interested in hearing. Are you basing the voltage of batteries on the previous grid-cathode voltage? I'm interested in hearing what happens to the tone, clean & overdriven for sure.

[loudthud]

Working mostly on bass amps I've always liked the warmer sound of a fully bypassed cathode on the first stage. If I needed a little treble emphasis, it sounded better to passively do it on the output than to do it on the cathode. The extreme example is grid leak bias. I wouldn't say unbypassed is steril, but it's not as warm. An op-amp is steril.