michael_nipomo wrote:This question may have been answered before, but I can not seem to find it: Why not use both triode sections of the 12AX7? I have read that there are advantages to doing that!
Hi Michael,
The positives would be:
- lower output Z for certain applications (but not really here since the phase splitter input Z is already very high)
- better PSRR which really isn't a benefit in this design - maybe in a SE
- more gain (again, not needed)
The negatives would be:
- lower input Z for the amplifier by half
- mismatched triodes less linear
- usually less linear output (according to the tube curves) in similar circuits
- the Rp and Rk values would need to be changed
Since the positives seem to not help in this design and the negatives could really hurt the linearity, the sides aren't paralleled. Indeed, the Eico HF87 used half a 12AX7 as its gain stage (using both halves in a stereo amp).
I did make an allowance for a compromise with the layout. With the diytube Poseidon board you can cut the 'X' at V1 and rotate the socket at V1 180 degrees when mounting it. Then V1 will use triode #2. With a monoblock pair you can have one amp using triode #1 and the other #2. Every six months you could rotate the 12AX7s. Too clever by - err - half?