Canelina the cat and her friend the patient tortoise who for obvious reasons winds the ring core transformers are very proud for the Pleiades V6 microphone booster or front end pre amplifier.
By looking at the schematics the V6 can be seen to be derived from the world class RCA BA-2C and Neumann U47 front end amplifiers, hopefully inheriting the best gene code from its mum U47 and dad BA-2C.
An evolution difference is that the little kid V6 is operating by just one 4V battery powering both heaters and anode. So you can take the kid to the beach, forest etc.
The Pleiades V6 is open source. For anyone who wishes to build, experiment, improve. Please take all safety and health precautions when experimenting with electronic circuits.
Describing the V6 in reference to U47 and BA-2C.
The ribbon, moving coil microphone, condenser microphone is connected to the primary of the input transformer as on BA-2C. The V6 input transformer is wound on the Magnetec Nanoperm 070 ring core. The number of primary turns dictates the primary inductance which in turn can be used to compensate for bass heaviness or not of the mics used.
The stepped up voltage which is of higher impedance will drive the grid through a capacitor as the condenser capsule on Neumann U47. The capacitor is used so that the grid is free to assume the DC voltage it wants. At very high input signal voltages the rectification cathode, grid diode effect will make the grid more negative. So there may be a voltage gain change as on a compressor as on U47 for very large signals.
All the family, BA-2C, U47, V6 have the electron tubes triode connected. Anode or plate, suppressor grid and screen grid connected together.
The U47 uses lower voltage on the anode than the BA-2C. The V6 much less.
The U47 has a lower cathode resistor than the BA-2C with Neumann's trick of passing the heater current through it to create a given voltage drop. The Pleiades V6 has an even lower cathode resistor. In fact it is almost 0. So no bypass electrolytic capacitor again is needed.
The cathode resistor is reduced to 0 so that the negative bias is decreased. This makes low anode voltage operation possible. But there is another trick on the V6 to make even less anode voltage operation possible. The electron tube internal bias is still negative (very easy to verify by an external voltmeter). This is because the heated cathode has lost boiling electrons to the cloud and the remaining lonely protons in the cathode atoms are positive. So the cathode is more positive wrt grid or the grid more negative wrt cathode. An external biasing resistor of typically 8MΩ from anode to grid further decreases the negative bias. The electrons are now free to travel to the anode by only 4V anode potential. The current can be much more than 100-200μA and we have a normal transconducatnce and low anode resistance since Va is low and Ia relatively large. Gain is dictated by the geometry of the electron tube.
On U47 the heaters of the VF14 are deliberately underheated for low noise, low temperature in all electrodes therefore low secondary electron emission or even photo emission effects. And most importantly of all, for better sound. On the V6 the heater voltage is even more reduced to less than 3V (compared to 6.3V). This is possible by the very low anode voltage operation. And who needs more billions billions of electrons than are needed to carry the music from cathode to anode? The very low heater voltage apart from the above mentioned advantages keeps power consumption very low. To a typical 150mA making battery operation a true joy.
The signal from the anode does not pass trough an anode resistor. But for less thermal noise and higher anode voltage through the output transformer secondary. Just like BA-2C.
The output transformer is again wound on a Magnetec 070 nanocrystalinne tape wound core. For the number of turns used or inductances please refer to a previous euroelectron post. The output is from the secondary winding, again balanced, low Z, typically 300Ω ready to feed generously with low noise and big natural sound a low Z microphone preamolifier. Just as one would connect a U47.
So there it is. V6, the child of U47 and BA-2C. Made with 2 signal transformers, 1 EF183 electron tube, 1 resistor, 1 22nF capacitor, 1 battery. A pure and simple class A operation (electrons flowing all the time) amplifier circuit.
The battery gives complete absence of all other sorts of AC hum electromagnetic noise.
The 4V is always connected to the anode and a switch just supplies the heaters providing the most quiet and smooth turn off or on.
Everything can be built and fit inside an octagon orange box.
So far favorable operating conditions are. Vb=3.9V. Rag=8MΩ. Cc=22nF. Heater voltage such that Ia is typically 50μA. Greater or higher Vh gives a whole range of sound palettes and the prototype may have a 10Ω variable wirewound resistor in series with the heater circuit. The transformers used on the Pleiades breadboard jig developing stage are 1:10 with 140mH primary for the input transformer and 10:1 with 800mH secondary inductance for the output transformer.
The output impedance at pins 2,3 is typically 300Ω. The grid from the way it is biased presents an impedance of aproximately 100kΩ. The amplifier is so low noise that the equivalent input noise resistance looking at the grid should typically be 2.5KΩ. So referring to input pins 2,3 the equivalent noise resistance is only 25Ω which is great even for 25Ω microphones. The input impedance at pins 2,3 is 100KΩ:10:10 or 1KΩ. Transformers of smaller turn ratio can be used too. More stages can be added and an interstage low crossection core interstage transformer can be used for analog simulation of analog magnetic tape recording with saturation on peaks or instantaneous peak limiting. Recording, playback preemphasis, deemphasis (with the interstage transformer in effect being 2 magnetic heads kissing each other in the air gap) is therefore possible, this may be the other child, possibly called Pleiades V7.
References:
Neumann U47 schamatic
RCA BA-2C schematic
Pleiades V6 schematic
On Preserving Transconductance of Electron Tubes at Anode Potential as Low as 3 Volts - euroelectron
Operating Features of the Audion - E.H. Armstrong
What a cool project! Do you have any audio samples?
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