grid of the Cathode Follower would be at the B+ voltage and its cathode at ground potential, which might damage the tube by exposing it to cathode stripping. (Large positive grid to cathode voltages can cause the grid to exert so great an electrostatic pull on the cathode that portions of the cathode are literally stripped away and fly onto the plate, thus damaging the tube.) At only ten cents, the addition of a diode makes a great bargain.

7. Which tube?
For octal fans, the 6J5, 6AH4, 6SN7, 6BL7, 6BX7 and triode connected 6V6 would work just fine. For 9 pin fans, the 6FQ7, 6C4, 12B4, 12AU7, 12BH7, 5687, 7119, 5693 and triode connected 6BQ5 would do the job. The dual triodes would certainly make for an easier job of building the line stage. 

8. How much plate voltage and current to use?
Of course the answer depends on which tube is used. A good starting point would be "more than you would think at first glance." In general, tubes sound best when driven hard, as it usually means moving their operation above their non-linear cutoff region. The harder they are used, however, the shorter their lifetime.

9. Which sort of power supply should be used?
A line stage amplifier does not require the supremely low noise DC power supply that a phono stage would. Nonetheless, the CCDGC amplifier has a poorer PSRR figure than other circuit topologies, so a very clean B+ would be preferred. Any of the power supply circuits from July's circuit of the month would do the job nicely.

10. Design Examples