Bingo! An electron tube microphone booster preamplifier operating with a AA 1.2V and a 9V battery

AT your risk. After countless experiments spanning almost 45 years assuming i started at around 7 years old... i used an electron tube for audio no one in their right mind would choose to use... A pentagrid ie heptode electron tube usually used for frequency convertion at rf receivers. Wanting very low energy consumption the D series of battery tubes was chosen. So there it is DK91 found waiting at Pleiades lab. Pentagrid tubes have 5 (pente in Hellenic) grids. So the 1st should be very near the cathode, and the second too. Using Wago connectors 1.25V was supplied at pin 1 and 7, ie the direct heaters or cathode. Then 9V was connected to grid 2 which is also internaly connected to grid 4. The current meter immediatly measured about 30 microamperes which is amazing at such low Vb and grid 1 at space potential (ie connected to nothing). With Vb=18V Ig2 increased to 500 microamperes which is aaamaaazzzinggg! Greatly incouraged, today, a Sowter special quality octal base 1:5 input transformer was connected to grid 1 trough a 0.010 microfatad capacitor. 9 volts through an output transformer. A 8.2 Megohm resistor connected to grid 1 for biasing (unconnected means grid at space potential, then if needed it can be connected to the negative terminal of the 1.2V battery or the positive terminal for even more anode (g2) in this case current. Connecting this resistor to the anode was found to be unecessary. The output transformer was connected to the mic input of Sony TC-D5 PRO. It worked nicely! The sensitivity was such that even touching the mic cable produced whistling sound, before connecting the SM58 mic. Mic was used at very soft voice 24in from mouth which is a nice distance for good quality sound, 30 degrees above head. WIth grid at space potential sound was about right with much high frequency detail. With resistor connected to Vf minus there was more bass heaviness as the output current increases and the tube plate resistance decreases, and with R connected to Vf positive side, even more bass for the same reason. (This effect naturaly comes from interaction of the output impedance of the electron tube with the input inductance of the output transformer dictating the low frequency cutoff. As frequency is decreased said inuctance becomes more of a short circuit so bass is reduced the higher the output impedance of the driving electron tube) Yesterday the same electron tube was used to directly drive the Sennheser Hd580 headphones through said output transformer, electron tube driven by the usual Sony CD walkman kine out. The headphones could be driven even with Vb 9V, more with 18V but the sound was very nice at 9V with R connected to the negative terminal of the 1.2V battery. Song used was To France - Mike Oldfield But back to today's mic expewriment. After disconnecting mic a 0 ohms specialy made male Neutrik XLR was connected. hiss droped to almost zero. Same withy a 200ohms XLR. Difference cannot be heard yet as there was a lot of hum picked up by the non mumetal protected output transformer. It would be nice to have such a pre preamp in a nice box with octal bvases for shielded input and output transformers. Powere consumption is only 1.2V at 50mA, ie the heater power supply plus 9V at say 50 or more microamperes ie the anode power supply. So little more than 60 milliwatts! If more such electron tubes are used in order to be able to drive say HD580 headphones then more power would naturaly be needed as the output electron tube may need more anode current or anode voltage, please protect very carefully your precious ears from high sound presure levels.