12ax7-bypassed vs. unbypassed cathode

[yunger]

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.

[6267]

It's more likely to sound sterile due to operating point. I tend to prefer the tone of unbypassed in my builds, the gain can be made up elsewhere. It's easy to try both, you could even include a switch.

[yunger]

can you describe the difference in tone?

[6267]

I think that any statement of that sort will be subjective...I've never built the same amp twice, and so when I've tried it there is no way to seperate say the V1 bias conditions, B+ available, PSU sag, etc from what might be the "unbypassed cathode" sound. In my experience, I just felt that bypassed resulted in more gain, especially at high frequency, than I was looking for. I find the tone a bit mellower overall without the cap, but with a richer overdrive. Like I said though, try it, it's not hard to put a switch or pot between the cap and the cathode. What might sound like heaven to me might sound bad to you, we have different subjectivities.

[yunger]

i've got a feeling that i'm going to be spending a lot of time tweeking once my parts arrive. i'll have to hear for myself. i was wondering though, if i attenuated the bypassed stage to end up with the same gain as an unbypassed, would the sound be the same between the two? or would the unbypassed still be mellower?

[tubeswell]

Build it with the bypass cap, but make the cap switchable out. That way you got both worlds.

[6267]

I don't think the sounds will be the same, but I've never tried it. If you do, post back, that sounds like an interesting comparison. Electrically, they *should* be different, but I don't know if that translates into audibly different. Most of the time I'm trying to remove early preamp gain, not add it, but I'm not going for preamp distortion generally. I'd imagine you'd get more difference in tone by changing the anode and cathode resistors, though.

[yunger]

i found this chart while searching for information. it looks like the frequency response would be the same while using a 22uF bypass cap with an increase in gain. since i already have too much gain in my design there's no need to bypass. although, i hear that bypassing reduces hum.
i'll still try it both ways just to hear for myself.

[txstrat]

A bypass cap "fattens" the tone to some degree.
The Fender Vibroking amps have a switchable bypass cap of 22uF and it's actually called "FAT".
http://www.schematicheaven.com/fende...ing_manual.pdf

[tubeswell]

The bypass cap increases gain (as you hinted at the start). It does this by holding the cathode at a constant voltage (where it would otherwise follow the rise and fall of the plate voltage slightly, causing a reduction in cathode-to-plate voltage, and thereby reducing gain). The different values of bypass cap will affect the frequency(s) which are held constant. Smaller caps will only hold the higher frequency swings constant, thereby 'boosting' the highs. Bigger caps hold all the audible frequencies constant.

You can have a shelved response whereby you put a resistor in series with ground side of the bypass cap, and then another smaller cap in parallel, which only lets through highs - that way you get a partial boost of say lows/mids and a full boost of highs. Alternatively you can put a pot wired as a variable resistor on the ground side of the bypass cap, and dial in and out of your preferred freq boost.

[Suusi M]

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.

Does an Ampeg V4 sound sterile with its gain switch at minimum?

The gain swich does 0db -3db -6db on the input stage by this a clever manipulation of this method

see attached schematic

[Merlinb]

Does an Ampeg V4 sound sterile with its gain switch at minimum?

The gain swich does 0db -3db -6db on the input stage by this a clever manipulation of this method

see attached schematic

I don't think unbypassed ever has to sound sterile. Where did you hear that??

[Suusi M]

I don't think unbypassed ever has to sound sterile. Where did you hear that??

it is the question posed in the originating post on this thread 12ax7-bypassed vs. unbypassed cathode I just answered a question with a question.

In my opinion the Ampeg V4A is one of the best designed amplifiers ever made.

[Merlinb]

it is the question posed in the originating post on this thread 12ax7-bypassed vs. unbypassed cathode I just answered a question with a question.

In my opinion the Ampeg V4A is one of the best designed amplifiers ever made.

I know you asked that, what I meant was, who told you that unbypassed stages sound sterile? It's a pretty vague statement.

[Suusi M]

I know you asked that, what I meant was, who told you that unbypassed stages sound sterile? It's a pretty vague statement.

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

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.