Why does the gain increase on the Pleiades V6 when the cathode temperature further drops?

Referring to the Pleiades V6 schematic.


Possible reasons.


The internal nature negative bias by the electron cloud is reduced.


The EF183 normally operates at 6.3V heater voltage.


On the Pleiades V6 the EF183 heater voltage is 3.9V as is the anode voltage.


But when the heater is disconnected from 3.9V the gain increases by 3dB when the temperature is such that the anode current had dropped from 100μA to 40μA!


When the V6 prepreamp is terminated by 300Ω instead of 10ΚΩ this increase in gain does not occur.


A possible explanation is as follows:


At 40μA the output impedance after the output transformer has increased from 300Ω to 800Ω.


Therefore when the V6 is terminated by 300Ω the increased voltage drop cancels the increase of gain.