12AX7 Stage 1 + 2 Biasing

[jordan86]

I was recently comparing a 6G14 schematic to an AB763, and noticed they bias the first two stages differently (that’s a blonde vs blackface showman). Wondering if some more experienced builders would offer their insights regarding bias your first two stages. I understand for a typical 12AX7 that 1.5K is considered a center bias point for the cathode R. Not "warm" like 820R, nor "cold" like 2.7K. I also understand that the wave form is flipped at the preceding stage to either emphasize or negate asymmetrical clipping. I just don't have any experience as to how that translates tone wise in relation to gain, headroom, etc....

All things being equal, would these two scenarios produce very different results or would they net a similar outcome? I know option 1 is a little more "marshally" if we can speak in such (un)helpful generalities....

Option 1:
- First stage: 2.7K cathode bias resistor
- Second stage: 820R cathode bias resistor

Option 2:
- First stage: 1.5K cathode bias resistor
- Second stage: 1.5K cathode bias resistor

[shooter]

I call those ??'s dealers choice, it's our amp, circuit is easy to "mod" and play/hear.  just use gatorclips, jumper until "we have a winner, Dooooor #3 please"

[SILVERGUN]

Here's some good reading to support your thirst for knowledge:
Microsoft Word - Fundamentals.doc (valvewizard.co.uk)

I just don't have any experience as to how that translates tone wise in relation to gain, headroom, etc....

Think of headroom as the ability of a stage to pass the largest signal possible before clipping. Anything other than that would be less headroom.
For headroom, you would want a high anode voltage combined with a center bias point. This would be a great way to have a nice clean tone.

If I were you, I would change my definition and usage of the term "gain" to reflect it's actual meaning = voltage gain. (not distortion)
When you start to blur the terms you are inviting in opinion to interfere with the facts of voltage gain.
I say this because I respect that you are really trying to learn.

IMHO your 2 scenarios alone create a negligible difference that depends a lot on your anode voltage. A single gain stage is made up of 5 or more negligible differences.

TONE comes from the way you alter the frequency response from stage to stage. Bypass caps, coupling caps, tone stacks, etc.
One factor to consider is that the "Marshally" version has an un-bypassed 820r, so that plays into the TONE of that stage more than the actual bias point.

[AmberB]

Another thing to consider is the relationship between the cathode resistor and the plate resistor.  If you leave the cathode resistor the same, say 1.5K, and change the plate resistor, that will change the operating point and the headroom.
There seems to be a more or less 100 to 1 relationship between the cathode resistor for the best balance of headroom and operating point, which I'm guessing could be considered about the same thing.  1.5 k cathode, 100K plate.  2.2K cathode, 220K plate, and so on.  By changing both resistors while keeping this relationship, you change the actual amplification output of the tube.  The RCA tube manual has some charts in the back that show this relationship.