Too much electron tube pull up or Pleiades bias can be a bad thing

It was tried in this serup, signal path:


http://euroelectron.blogspot.com/2018/07/md441-or-md211-to-pleides-v6-with.html


Essentially the signal path is a Pleiades V6 with a 7586 Nuvistor electron tube the plate of which is powered by 1.2V.


When the pull up resistor or Pleiades bias resistor from anode to grid is 6MΩ the sound is great a bit bass light. Anode current is of the order of 1-2μA? (Question marks are used to denote very rough measurements of high inaccuracy).


When Rag is 1.8MΩ the sound is great, a bit bass heavy. Anode current is of the order of 4-5μA?


When Rag is 1 MΩ although the the anode current is more than 20μA the gain drops dramatically.


Even more dramatic for lower pull up resistors such as 470KΩ.


So higher anode current in no way means higher gain or better sound.


A potentiometer of 4.7MΩ in series with 1 MΩ for protecting the electron tube was connected in parallel with the 6MΩ resistor. Continuous adjustment confirmed the above observation. The best sound was when the reries combination was 4.7MΩ+1MΩ. So at approx 3ΜΩ Pleiades bias resistor from anode to grid the amplifier souned great.


A pull up resistor was also tried not from anode to grid but from 3.9V to grid. Using 1.8MΩ the sound was great, slightly louder than when 1.8MΩ is connected to 1.2V anode (plate).