The Pleiades V6 is an one electron tube battery powered pre preamplifier or booster pre preamp designed to extract as much information as possible from a microphone while sounding correct to the listener's brain.
It is based on the legendary Neumann U47 amplifier and attempts to improve if that could be possible on the simplicity, purity and excellence of this deliberately underheated state of the art VF14 electron tube circuit.
The Pleiades V6 is to be connected to moving coil or ribbon microphones through its input transformer, but connection to any other type of microphone may be possible too.
Referring to the Pleiades V6 schematic.
This test was carried out on the Pleiades experimental jig. The jig is as follows. Three small breadboards are connected one after the other from left to right. One for the input transformer with octal base. One for the EF183 electron tube. One for the octal base output transformer.
The input transformer for this test is the green Altec Peerless 4722. Step up ratio is 1:30. Primary inductance is aproximately 400mH.
The electron tube is powered by 3 1.2V rechargeable AA batteries. So normally the heater voltage and anode voltage are about 3.9V. The electron accelerating resistor used from anode to grid Rag is 8ΜΩ. The tube is quietly powered on by connecting the lead to heater supply as the anode circuit is always connected to the battery. An available option is to pass the heater current through a wirewound rheostat resistor thereby further reducing the temperature of the cathode from the already relative small temperature from the 3.9V instead of 6.3V. When the tube cathode is heated by 3.9V the anode current is 100μA. When the rheostat was connected the heater voltage was such that the anode current dropped to 40μA. The tube still operated in a great way and perhaps more natural sounding with still easier sound. It seems electron tubes or electrons like anyone do not like to be forced to potentials etc. See later comments.
The output transformer connected is the version 2 Pleiades output trasformer. It is wound on Magnetec 070 nanocrystalinne ring core. 672 turns for the primary giving an inductance of 80 Henries or 80H and 70 turns for the secondary (output of prepreamp) giving an inductance of 800mH.
Signal path, setup:
male voice - mic at 1-12in - Pleiades V6 - 10KΩ impedance mic input of Realistic Disco mixer, (mono button pressed) - Sennheiser HD580 precision headphones
It was a bit strange that when gently tapping the mic there was a click sound on the headphones even before applying heater voltage. The electron tubes seems to operate from the anode voltage supplied. Is this operation happening by capacitance or a few electrons available, or by photo emission?)
Firstly the AKG D130 was connected. It took time for sound to come. Perhaps more than 40 seconds.
There was sound even before the 40μA point. As the 40μA anode current was reached the sound was already full and a lot of detail could be heard from sound sources outside of the building, dozens of meters away.
Reducing the fader and singing to the mic the sound was very full. Perhaps a bit bass heavy with great mid and treble detail. When the heater voltage was increased to 3.9V, the anode current went up to 100μA and the bassiness increased as expected from the output transformer being driven by a lower electron tube output impedance.
It was then decided to change to the Electro-Voice 635A which sounds less bassy than the D130.
It was fantastic too. Better in frequency balance to listener's brain. At 100μA the sound was full, slightly bass heavy.
At 40μA the sound was amazing. Everything sounding with a natural spectral balance to listener's brain. The sound was easy. With scary treble detail without any harshness. As if there was no microphone or amplifier or headphones. As if the sound was guided by a perfect frequency responce acoustic tube from mouth to ears.
When a terminating load of 800Ω was connected to V6's output the sound was more hi-fi with the wrong sense of the word. There was more treble and bass extension as expected and the result was worse as expected to listener's brain. The gentler roll offs when the amplifier is terminated with 10KΩ seem to compensate voice effort and Fletcher-Munson curves making for a pleasing natural result to brain perception.
When the mic was disconnected from the female XLR it was a joy to hear how quiet with so little hiss and no noise like hum the battery powered Pleiades V6 pre preamplifier is.
Once the mic is connected it is sticking how immediately all ambient noise from outside the building can be heard through the double glassing windows.
References:
Pleiades V6 schematic
Neumann U47 schematic
On preserving transconductance of electron tubes (with an anode voltage as low as 3 volts) - euroelectron blogspot
(Flat frequency responce from singer's, actor's vocal chords to listener's brain) Sound Pictures recording and reproducing Characteristics - Loye, Morgan - Journal of the Motion Pictures sound engineers
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