... With an adjustable fixed biased 12ax7, I would add the possibility of changing the tone of the tube which would act as a shelving EQ , combined with a rotary switch containing different caps, let's say from .1 to 33uF. ...
There is no cathode resistor when using fixed bias, so there's nothing for a cathode bypass cap to work against to make EQ changes. Instead, your description sounds more like switchable bypass caps for a standard cathode-biased preamp tube.
In such a case, the coupling cap at the output of a fixed-biased triode is the only place to do bass tone-shaping. Series grid resistance could be added to cause treble roll-off, with the penalty that it may add noise.
It seems possible to have a large cathode resistance, but also a fixed bias voltage to the grid, which together set the operating point of a preamp tube. The only advantage which made me think of this approach is that a smaller bypass cap can be used for the same bass roll-off, which might allow the use of film caps in place of electrolytics. The very big downside is to implement this, a lot more voltage is dropped across the cathode resistance, which means you need more supply voltage available. The extra parts/hassle don't seem worth the gain.
... If a 12ax7 acts as a power tube, a fixed bias would give more headroom and we could use a 12au7 to get more headroom out of it, in order to use it as the 1 st preamp tube, taking advantage of it's particular tone. ...
As Willabe says, you could use a variable cathode resistor (pot, in series with a fixed resistor to define a lower limit of Rk) to change a preamp tube's bias & operating point. You could change how fast distortion occurs, and whether plate current cutoff or grid current is the factor contributing to distortion.
The likelihood is that this will
reduce headroom compared to a typical operating point. To actually be able to raise headroom, you need to raise the supply voltage, along with a corresponding change of bias (which would be mostly automatic in cathode bias).
A different way to reduce distortion/gain if that is the goal would be to have a variable local feedback loop around the preamp gain stage (say, from plate to grid). Gain could be easily varied from unity up to the normal gain of the stage.
Bass shelving EQ is more easily done with cathode bias, and a pot in series with the cathode bypass cap (variable defeat of a different form of local negative feedback). Or you could have/add a switch to select different cathode bypass caps to shift the bass roll-off frequency.
Other problems of fixed bias are that because it is not automatic, you now
have to monitor and adjust it over time as the tube ages. You also need to adjust it every time you replace a tube to get the same operation you had before.
If you had a cathode biased stage with no bypass cap, the local negative feedback keep the tube operation consistent as it ages, and reduces sonic/performance variations when swapping tubes. Some old tube gear (outside of audio) purposely used negative feedback to keep long-term performance predictable in spite of tube aging. You lose this feature if you switch from cathode bias to fixed bias, unless you add in more parts (over those required for the bias supply) to add back in the feedback.
So for fixed bias you'll need to add a small negative voltage supply or a battery (for non-adjustable fixed bias). The cost will exceed that of a single resistor (plus maybe a bypass cap), yet not really give you anything you didn't have before, while also being fiddly and needing regular monitoring/adjustment. These are the reasons you rarely see fixed bias in a preamp circuit (if you have specific interest in fixed-bias phase inverters, we can explain why the designers chose those on rare occasions).
Topic: 12ax7 fixed bias ? (Read 8389 times)