The Pleiades V6 is in breadboard (experimental board). The EF183 electron vacuum tube is powered by 3.9V and even less voltage for the heaters as there is a wirewound series resistor in the heater circuit of the cathode. An 8ΜΩ anode to grid electron accelerating resistor was used making battery operation possible. The input and output transformers are made using the Magnetec 070 nanocrystalinne ring core and are described on previous euroelectron posts. The usual green Altec input transformer was also pluged in and was somehow bass heavy as its input inductance is 400mH. This should be great for drums but not for voice as voice effort curves, Fletcher Manson Curves etc have to be compensated for flat frequency responce from singer's vocal chords to listener's brain.
Signal path, Setup:
Male singing voice at 0-2in from the microphone - Sennheiser MD 211 N microphone - Pleiades V6 preamplifier - Realistic disco mixer at mic in - Sennheiser HD580
What an amazing microphone! So small yet when it is connected birds on the trees can be heard so clearly.
It is great that this mic has a so smooth, without reasonances curve. And being extending to 20KHz.
Of course underheating reduces the high end of amplification as well giving more presence. Underheating seems to increase the output resistance of the electron tube so external components or even stray capacitances have more effect on the frequency responce.
Of course no pops or s problems even just next to the mic.
With the Altec transformer the sound was somehow bass heavy for voice as expected. By further reducing the heater voltage from 3.9V to 2.5V and thereby the anode current from 90μA to 30μA the sound was big without bass heaviness.
Using the 140mH Pleiades input transformer at the very low heater voltage the sound was in the verge of becoming bass light. It was full at 3.9V heater voltage. And a sweet spot must be between 40μA and 60μA. For some reason an underheated cathode makes a more full and easy perception of sound to listener's brain. Is this a main reason why the U47 sounds the way it does with the VF14 electron tube or radio (TV) valve underheated by Georg Neumann?
Perhaps the Pleiades V6 prototype should have a wirewound cathode temperature adjusting resistor for adjusting for optimum sound depending on singer, song, arrangement, recording environment etc.
The wire wound resistor value for 30μA anode current when Vb=3.9V is 9Ω. So a 10Ω wirewound or multi turn wirewound potentiometer may prove very useful. When an electron tube is underheated, it is very interesting and a joy to wait at least 47? seconds for subtle hiss to change to more hiss to more sound, to bird ambient sound like on expensive cathode underheated electron tube condenser microphone internal preamplifiers.
References:
Pleiades V6 Schematic
Flat frequency responce from actor's, singer's vocal chords to listener's brain Sound Pictures recording and reproducing characteristics - Loye, Morgan - Journal of Motion Pictures sound engineers
On preserving transconductance of electron tubes (on an anode potential as low as 3V) - euroelectron
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