The 6BS8 has the same pinouts as the 12AX7, and they are both twin triodes. However, the 6BS8 has a maximum plate voltage of only 150 volts compared to 330 for the 12AX7. If your amp has more than 150 volts on the plate you would be exceeding the tube's maximum rating.
Also, the 12AX7 has an amplification factor of 100, compared to 36 for the 6BS8. In that respect the 6BS8 would be more like a 12AY7 (which has an amplification factor of 44).
The 6BS8 has different plate resistance, transconductance, and plate current than either the 12AY7 or 12AX7. I did not compare the load curves closely. Heater current is a tad higher with the 6BS8 but probably OK.
In my opinion, it may be possible to tweak a preamp circuit to use the 6BS8 provided the plate voltage isn't excessive (that could be a deal breaker right there). However, the plate resistor would be changed, and the cathode resistor also changed to bias the tube correctly for the different current draw. The 6BS8 was designed as a cascade amplifier for VHF.
I don't think you will achieve option D (the great and undiscovered tonal bliss), you may well drive the 6BS8 into cutoff. I'd stick to the 12AX7.
Last edited by nashvillebill : 08-20-2013 at 08:32 AM.