Electro-Voice 635a to Nuvistor 7586 at 1.3V plate potential

Signal path, setup:


Male singing voice at 1-6in - Electro-Voice 635a - Pleiades V6 headamp with Altec 4722 input transformer, Cc=22nF, Nuvistor 7586 - Sony TC-D5 Pro at XLR mic in, mono mode - Sennheiser HD580


(On all these experiments there is a phase reversal XLR adaptor as the particular TC-D5 Pro seems to phase reverse from mic in to headphone out). Also when the mic is connected directly to the great Sony TC-D5 the sound is nice but too bass heavy (Flether Munson, voice effort curves uncompensated?), and of low sensitivity, the rec level potentiometer has to be at the full clockwise position and the modulation VU level is still not high.





This is the schematic used but with above described changes.


Initially the 7586 Nuvistor electron tube was operated at 3.9V including heater and anode.


The sound was somewhat bass heavy, and treble heavy. Inserting a Pleiades (130Ω,40mH) filter in cascade after the mic made matters worse as there is already a presence curve on the 635a. This Pleiades filter operates constructively with MD441, MD211 etc.


Then the heater supply was kept the same but plate supply was changed to 1.3V.


Anode current dropped from 30μA to something so small that the 0-500μA analog meter could not show. But the sound was great.


Bass heaviness gone. Presence. A big sound. Perhaps a bit bass deficient.


On both plate supply cases noise was low. The ticking clock in a far away room could be head through the headphones. Also a short circuit input XLR reduced hiss a lot indicating that the headamp could read thermal noise resistance.


The pull up resistor from anode to grid or Pleiades bias resistor was usually 6MΩ.


1.7MΩ made the anode current needle move and added some bass register to the voice. For some reason the 6MΩ option sounded more clear and more natural.


There is an edge or aggressiveness using the excellent Electro-Voice 635a with this setup which may be a good thing for some types of music.


At 3.9V plate potential the pull up resistor could be increased to 18MΩ with a reduction of plate current from 30μA to a smaller value but the sound was easy and interesting.


So, would a 2DV4 Nuvistor electron tube which already has a heater rating of 2V operate with just a 1.3V battery supping both heaters and plate? 1.3V should provide great underheating of the cathode as does 3.9V compared to 6.3V. But will the electron tube operate at 1.3V anode potential?


See also:


The Pleiades bias - euroelectron


Operating features of the Audion - E. H. Armstrong