It is the Pleiades V1 operating with a Mullard CV2269 electrometer tube.
(The idea of using an electrometer electron tube comes from Sean Davies, disc cutting equipment realization specialist and not only. He had mentioned his use of electrometer tubes as a condenser microphone front end amplifier.)
(For more details on component connections of the Pleiades V1 see older euroelectron posts by Google search).
http://euroelectron.blogspot.com/2015/10/pleiades-electra-13v-electron-tube-pre.html
It is not really positive bias. The pull up grid voltage is positive but the grid is still less than 0. There is some external grid to positive supply current flowing. See also:
http://euroelectron.blogspot.com/2018/07/playing-with-electrons-with-cv2269-tube.html
It is called V1 because it was the first Pleiades microphone booster electronic valve or electron tube one operating with just a low voltage battery inside a small aluminum box. Made more than 10 years ago. Sound quality approved by gramma Eleni before she passed away.
1.2V supplies both direct heater and anode (plate).
The low Z in is stepped up by a military Freed input transformer of 140mH primary inductance. The step up ratio is about 1:40.
The secondary is directly connected to grid.
But the other side of the secondary is capacitor coupled to gnd (Western Electric way). So this grid is still free from DC upsetting conditions. The capacitor is a MKT Philips 39nF.
A 60MΩ resistor from grid is connected to the more positive terminal of the heater, ie 1.2V. It was not realized at that time this was Pleiades bias. It was just found by chance. The anode was accidentally directly connected to heater or cathode and the amplifier still worked. In fact the anode was connected to the more positive terminal of the heater again. By simple breadboard experiment the anode current was found to be greater when the 60MΩ return was connected to the more positive heater terminal. A tiny
micro ammeter (from JVC Nivico cassette recorder VU meter) was connected in series with anode. It may be worth again to reduce the Pleiades bias resistor to increase anode current. At high levels of audio input the bias changes by the grid rectification effect. So the gain is reduced. This effect was first observed while sending large voltage signals to the input of the VF14 electron tube amplifier of the Neumann U47 with the capsule removed. See older posts.
There is a anode load resistor of 27KΩ, and a 1μF coupling capacitor to high Z out.
If an output transformer is directly connected to the anode there will,be must less voltage drop of the already tiny 1.2V battery voltage. (Always use a fuse in series with batteries for safety as the current can be huge in case of short circuit).
So how does it sound?
Great and clear removing all the bass heaviness of a top class directional mic used at a small distance. There is not gain. Just character change towards a much flatter frequency response from producer's brain to listener's brain.
Signal path, setup:
Male singing voice at 2in - Shure Unidyne III - Pleiades V1 - Canford red 1/4in to XLR step down impedance converter - Sony TC-D5 Pro XLR mic in (mono mode switch on) - Sennheiser HD580
With attention to phase polarity again, anode followers are phase reversing.
Power consumption is ridiculously low. The electron tube is ridiculously microphonic. A slight touch lead to a nice sinewave tine ringing of about one minute. Possibly as there is no air inside. It sounds nice and could be sampled and played at any frequency.
Of course the Pleiades V1 can be directly connected to hi Z mic input such as Tascam Porta for example. (While connecting a ridiculously low inductor 4.7mH for extreme high pass, the song Tezcatlipoca - renewablemusic channel - YouTube was recorded. The V1 was used for vocals and the singer was using a semi broken MD421 singing near the acoustic cavity or a metal plant watering can).
https://m.youtube.com/watch?v=2RD02ATDPAk
It can also be used directly connected to electric bass, guitar, synth etc. An inductor in series with a resitor then connected in parallel with input should reduce bass heaviness for correct bass ear-brain perception.
The CV2269 is equivalent to ME1401 and 4065. The ME1402 may be a great choice since it is a tetrode. The screen grid is closer to the cathode than the anode. So if triode connected there should be more anode current as the screen should collect most of the electrons, see Pleiades V6 or V4 or V5 used with the EF183. The Pleiades V2, V3 with the 12AU7 or ECC82. The latter options require 4V heater supply. They consume much more heater current, they give much more gain. If Nuvistors such as 2DS4, 2CW4, 2DV4 are used it should be possible to operate both heaters and anode with an AA 1.2V battery at moderate gain.
A low anode supply should give us readily the electron tube gentle overtone production [Hamm]. The rail suply is small so we may get it without the need to use 2 stages of amplification. See also the Pleiades K117 JFET operating with typical 200mV while playing electric bass.
The electron tube anode or JFET drain or bipolar transistor collector can be powered directly by a device such as iPad, iPhone or other device that can power electrets. The load resistor is in the device itself. (At your own risk. electron tubes of course need heater voltage which can be externally supplied. Always include a fuse in series to a battery for safety.
Further reading:
Flat frequency response from producer's brain to listener's brain, Sound Picture Rrcording and Reproducing Characteristics - D. P. Lowe, K. F. Morgan - JSMPE
The Pleiads Bias - euroelectron
Tubes vs Transistors (vs operational amplifiers) Is there an audible difference? - Russel O. Hamm - JAES
Operating Features of the Audion - E. H. Armstrong
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