Comparison of 1H4, DC90, DC96, DF97 electron tubes with open grid and low anode voltage

At your risk. Please take all safety precautions. Summary: For mic, headphone tests DC90 sounded best and amazing at just Ia=20 microamperes at Va=18V. This could not be repeated as subsequently anode current increased to 40uA due to unknown systematic error. DF96 was least microphonic. Signal path: male singing voice - reverberant small room - Shure VP64 at 1-12 in - Pleiades V6 1H4 - Realistic disco mixer at mic in - Sennheiser HD580 - listener's, singers' brain). An octal to noval base adaptor was made to make posible the quick insertion of D family electron tubes to the Pleiades V6 1H4 octal base. All electron tubes open grid. 1H4: Sounds great and low noise as always with Ia=20uA at Va=12V. DC90:Ia=1.5uA at Va=12V, low noise, low gain, nice midband edge. Ia=22uA at Va=18V, amazing balanced sound. Ia=240uA at Va=25V, no listening to avoid magnetisation of output transformer. It seems low battery consumption RIAA le Pasific Walthers type pres are possible with electron tubes. To recap amazing sound at 18V, 22 microamperes. Experience makes things easiser to proceed. Vb=12V, Ia=30uAwith 5.6Mohm from anode to grid. VP64 mic did not sound very nice. Possible transformer, mic loading by decrease of input impedance. bass heaviness, undefined, boring sound. DC96: Vb=12V, Ia about 0. Rag=5M6, Ia becomes 7uA. Too much grit edge, not a great sound. Less gain at 5uA. At open grid, Vb=19V, Ia=4uA. Vb=25V, Ia=60uA, very bass heavy, grit edge, very low noise. Back to DC90: Vb=18V, Ia=40uA, why? Interminent heater problem? VP64 sound is nice but not as good as at 20mA. Possible cause bass heaviness by lower ra interaction with Lp of output transformer (anode resistance, primary inductane interaction). DF97 anode to screen grid: Vb=18V, Ia=5uA. Vb=25V, Ia=25u 62ua in one minute. Nice sound , very low noise, a bit bass heavy, no microphony, less bass heavy when begining at 20uA. Back to DC90: Vb=18V, Ia=40uA, Vb=25V, Ia+370uA, Vb=12V, Ia=2.6uA, intimate telephonic sound, still low noise but not perfect. Back to 1H4: Vb=12V, Ia=25uA, some microphony including bass one. very nice sound, a bit bass heavy but presence, 25uA sounds great on mic. Back to Pleiades Siemens AC151 VI open base: great sound, low noise, some flicker bass noise, when including WE 1:2 input transformer bass noise almost absent. A bit metalic but stangly smooth sound, too much gain, fraction of cost or power consumption.