cold bias gain stage

[12AX7]

I don't know of any others, but there are 2 amps i'm aware of use this, the marshall 2204 and the SLO 100. I was curious about it so i decided to experiment since i have a extra unused AX7 in the amp i was using for a loop i eventually removed.  So i added a stage with a 33k unbypassed cathode. I tried it as 2nd stage then 3rd. It seems to work better as the 3rd.  Anyways, i have a few questions if you don't mind...
1-marshall uses  a 10k cathode and soldono a 39k. Is this because the colder/bigger R the better as far as generating asymmetrical clipping and marshall uses less because it only has 3 stages and cant afford to throw away as much gain as the SLO can? If thats not it, why is the SLO's cathode 4 times the size as the marshalls?

2-is there a method to determine of the 4 stages where it is best placed ?

3-the only issue i'm having, and this was there before but not nearly as bad, is that i'm getting a rather big bump in some super high frequency that sounds like 8 or 10k. Like a very pingy glassy top. In a 4 stage setup like this where would you look at as the likely place for snubbers to rid this? It's really only a problem when i roll the volume can for cleans, especially on a strat in the bridge/middle position and tones such as that.

On a side note, i haven't even started to try different values throughout the preamp to compensate for the extra stage and it already sounds quite promising over 3 stages.

[tubeswell]

what does the rest of your pre-amp look like? Got a schematic? And can you provide the voltage details?

[12AX7]

No schematic, but it was previously very close to a 2204 as in the schematic below except that cold bias in that schematic is a regular gain stage with 1.5k/.68uf/100k in my amp. Any other differences are insignificant as they apply to my questions. Then i inserted a new stage thats cold biased with a 33k unbypassed cathode and 100k plate between V1B and V2A in that marshall schematic. Voltages are around 150v at the plates of the regular gain stages and 250v on the cold bias stage. The  nodes are 10k apart like the schematic but the caps are 22uf.

[shooter]

2-is there a method to determine

How "everybody" else did it is a good start.  Scoping your signal as you progress is another, trial n error works

Using something like the tube data sheet, and Merlin's explanation on pre-amp gain goes a long way to understanding the logic. sound, tone, that is subjective to each, individual builder/player and from what I've learned, there are no standard right/wrong

http://www.valvewizard.co.uk/

[12AX7]

I would have copied something directly but it would have meant really ripping everything out and i was able to do it the way i did in minutes to get an idea of how it would sound.   Heres another question....phase. Adding another stage flips the signal phase right? Why does that matter? I mean, there are amps with 2 stages, 3, 4 etc. What is the implication of this? Could it cause phase issues if you run a second amp or do i have to consider this for some reason now that i have added a 4th stage?

[shooter]

phase

this only comes into play if you have 2 channels and want to use both, then phase matters.
If you're using 2 amps, simplest way I've found is hit each amp with a single positive pulse and watch the speaker, if they both "pop" out, they're "phased", if one sucks in and one out, swap + - on the speaker.  I try n design "in phase" @ speaker with input signal.  I believe though 2 amps on a stage with enough distance and as complex as the signal is, there is no real phase once the noise comes out

[12AX7]

I see. Thats good to know, but i use a second amp only as wet effects return so i suppose it wouldn't matter anyways since i use a touch delay normally and thats about it aside from occasional chorus.

On another note, i notice the SLO uses a relatively huge cap as a snubber on the cold bias stage, 1000pf. i tried that and it just killed the cleaner tones when you roll the guitar back to clean up. I guess he designed that channel for nothing but fat all out gain. I may try a 100pf there and at the PI just to round off that very hi top i mentioned thats there when i roll back for cleans. Another question also occurred to me. In general is there a consensus on whether any gain attenuation is best done before or after the cold bias stage? I need to drop a bit to just what i need, maybe with a split load or at the divider after a coupling cap. Just wondering if there is any school of thought about the best place to do that in a pre with a cold bias stage or if it's another one of those to each his own things.

[HotBluePlates]

... Adding another stage flips the signal phase right? Why does that matter? I mean, there are amps with 2 stages, 3, 4 etc. What is the implication of this? Could it cause phase issues if you run a second amp or do i have to consider this for some reason now that i have added a 4th stage?

That Pedal Show demonstrated exactly this issue in a video where they were using two amps.  Getting the two amps out of phase sounds gutless, as most of the bass frequencies cancel.

...  I believe though 2 amps on a stage with enough distance and as complex as the signal is, there is no real phase once the noise comes out

It's still a problem there.  If the sound sources are widely spaced but out of phase, you'll get much of the same bass-cancellation, but if you move relative to the two sets of speakers other frequencies will come in/out of phase (depending on their wavelength).  You get much fewer problems if you start with things in-phase.

... is there a consensus on whether any gain attenuation is best done before or after the cold bias stage? I need to drop a bit to just what i need, maybe with a split load or at the divider after a coupling cap. ...

That's like asking is there a consensus on whether a 0.1µF or 0.001µF is a better coupling cap value... Everything depends on specific application and what you're trying to accomplish.

Your example schematic has voltage dividers (attenuation) both before and after the cold-biased stage.

The best you can do is learn how each of the building blocks work, the impact changes will have, and then conceive an overall goal and proposed build architecture to get you there.  Then breadboard/build/test and see if reality conforms with your expectations.  If it doesn't, use your knowledge of the impact of various changes to alter the original plan and repeat.

[12AX7]

I can certainly be wrong, (and often am ) but i think 2 amps out of phase would only cancel frequencies if the 2nd amp is reproducing the direct signal. But with say delay where you have turned the direct signal off in your effects settings and the 2nd amp is only reproducing the repeats, i don't think there would be phase issue. Further, if the delay is tweaked so it's EQ is darker it would make it even less likely. At least i think this is all correct. Thats how i run the 2nd amp, which i only use because of the lack of a loop. (tried several effects loops but they screwed with the tone)

[sluckey]

Put a polarity reverse switch on one of your amps or speaker cabs. Heck, put a reverse on all of them.

[HotBluePlates]

I can certainly be wrong, (and often am ) but i think 2 amps out of phase would only cancel frequencies if the 2nd amp is reproducing the direct signal. ...

It doesn't have to cancel "everything" to be audible cancellation.

Reverb usually isn't a problem because there's so many additional copies of the signal delayed and at different phases anyway.

Try what Sluckey said, or use any kind of isolation box which provides for polarity-reversal; one way will almost always have more low end, and that's the setting putting everything in the same phase.

[12AX7]

I will if i decide to go on with this. But at this point, while i'm liking the OD tone, i MUST be able to roll my guitar down and get the sort of clean tone that i like and i'm just not getting it no matter what i try. I did before and i think it's the asymmetrical clipping of this new CB stage that isn't good for that because even with the same amount of gain i used before this it doesn't clean up the way i like.  Even without the effects amp it's not near as good as it was with 3 regular gain stages. So unless i can think of something to try that works to get it where i want i'm probably going to go back to 3 regular stages. Does anyone have a SLO or clone, and if so does the lead channel allow a really nice chimey clean tone when you roll your guitar back to 4-5? (assuming of course the gain knob isn't cranked way up)