The sound of the Nuvistor 7586 is great at 1.2V anode supply

Summary:


With the MD211 N it sounds more like When I Fall in Love - Nat King Cole at 1.2V than at 4V anode supply voltage.


Rumor has it that for some applications even 4V to the anode may be considered a lot.


Today less than 4V anode supply experiments were done using the MD211 microphone feeding the V6 head amp with the 7586 electron tube.


The range of voltage on listening tests was practically all the infinte values from 4V to -70mV while the power supplying 220μF was discharging by supplying power to the anode circuit of Pleiades V6. It gives about 2-3 minutes from full sound to silence. Why -70mV? Because heated cathode misses electrons to the electron cloud and becomes positive. It is not only that the potential difference of grid wrt cathode becomes -700mV, the anode gets negative too wrt cathode!


The Pleiades V6 head amp schematic used is almost:





But Cc is 22nF, electron tube is Nuvistor 7586, Rheater=10Ω, Pleiades bias Rag=1.8MΩ


Signal path, setup:


Male singing voice at 1-2in - Sennheiser MD211 N - Pleiades (130Ω,40mH) filter - Pleiades V6 - Sony TC-D5 Pro - Sennheiser HD580


At 4V, Ia=25μA the sound is full and perhaps somewhat bass heavy.


As Va is reduced the sound changes especially bass heaviness is reduced. Anode current is dropping rapidly but sound output drops slowly. An almost infinite palette of sounds is available.


At 1.2V the sound is more When I Fall in Love. The sound jumps at you. Gain is less but still great enough. The Sony recording level is say increased from 3/10 to 5 or 6/10.


At 500mV anode voltage supply the sound is like a good quality telephone.


For some strange reason at 250mV the sound gets body again and sounds colored like ADT (Artificial Double Tracking) with small time delay. Phasy. Like the oldies song Crazy Love.


An electron tube head amp operating with just a AA 1.2V battery supplying both heaters and anode is certainly possible if a low heater Nuvistor is used such as 2DS4, 2CW4 or 2DV4. With 1.2V they will be possibly underheated in the right way.


Note: The 1.8MΩ Pleiades bias resitor making operation possible at such ridiculously low anode voltage was chosen by chance.


With a 6MΩ resitor the sound was as expected thinner. But it did sound better at say 4V.


More experiments and optimization is needed including selecting the right heater voltage ie cathode temperature.


Even at 250mV the Pleiadss V6 had some gain compared to connecting the mic line (disconnected from the V6) and directly connected to Sony TC-D5 Pro.