At your own risk. Warning. Experimenting with this jig is possible because the Pleiades experiments are with a small battery voltage. For higher voltages it poses lethal risk.
The Pleiades experimental jig was made a few years ago.
The objective was to push as much as possible the low noise limit of electron tubes. Also being able to make instant changes to the circuit. And trying to find possible best conditions for electron tube operation with just a few volts at the anode.
The Pleiades experimental jig consists of an aluminum enclosure or chassis with open bottom and positioned upside down. Not very deep.
Inside it are placed 3 small breadboards.
Neutrik female and male chassis XLR sockets are mounted on the front panel side of the aluminum chassis. Female input on the left side, male output on the right.
A military 500μΑ meter is placed inside the box on the upper side. Everything is mounted logically following the electronic circuit diagram or electronic engineering schematic convention, signal flow from left to right. Effort was put to make the component layout looking as much the same as the schematic itself.
A main basis of inspiration of the Pleiades V6 prepreamplifier is the Neumann U47 prepreamplifier. On its schematic one can see the Neumann U47 using just one VF14 electron tube. A pure, simple, as ingenious as possible class A (electrons flowing all the time) circuit that proved for more than half a century to be the beloved prepreamp inside a microphone preferred by the top artists, producers, listeners on our planet. Georg Neumann made the legendary condenser microphone around 1947, hence the U47 name.
The Pleiades V6 is an as simple as possible circuit with one electron tube triode connected as on the U47 prepreamp. It uses even less voltage on the anode than the 34V of the Neumann, underheated heaters too, and even less components as the output transformer is directly connected to the tube. And because it operates with less than 6V no bulky or complicated power supply is needed. Just a battery supplying both heaters and anode.
On the Pleiades prepreamp experimental jig, 3 white breadboards are used as modules:
In the central one is a 9 pin electron tube base pluged in. (Exchangeable modules can exist for any other socket or tube). This is done by soldering 9 vertical wires and cutting the extra length accordingly so that the tube base can be fully inserted in the breadboard like an IC with circular pin out. So we have instant access to all the pins of the electron tube.
The left hand side bread board is just for the input transformer. So an octal tube base is inserted in a similar way. An octal tube base is inserted on the right hand breadboard and this is for the output transformer. The 3 breadboards touch each other firmly side by side.
It is great fun as one can make changes to the circuit in seconds flat. And it is an analog programable computer for playing with electrons and nature itself in real time while listening to what electrons can do!
This is how the Pleiades V6 microphone pre preamplifier operating with just a few volts came to life.
A Shure 515SB Unidyne B Mexico microphone was hanged from the balcony (pedestrian area) so as to get noise and real voices from people passing by etc.
The objective of the experiment was getting from a preamp so much detail that almost everything could be heard and distiguissd clearly even in mono with as less hiss as possible.
The input transformer was the green 4722 octal peerless used on the Altec 1566A electron tube preamps. It is a 1:30 transformer which proved to be possibly too much for this experiment (it turns out the input impedance of the EF183 in the Pleiades V6 circuits becomes 100kΩ) but did sound great.
Same for the output red 15095 octal transformer.
The low Z output was connected to the mic input of a realistic disco mixer if memory is correct or to a Sony TC-D5 pro switched in mono operation.
The Sennheiser HD580 studio quality headphones were used for precision monitoring.
It had already been observed that an external positive bias on an electron tube cancels the internal negative bias of the electron cloud and therefore makes possible the operation at less than 6V at the anode. Various triode connected tubes such as EF37A (the tube used on the EMI RS61), EF96, 6SK7, 12SK7 were tried and gave impressive anode currents sometimes approaching 1mA? with just 6-12V. The EF183 proved to give a nice anode current and in practice high amplification and nice sound quality.
So she found herself sitting in the middle of the experimental jig.
The military Cole code for tube cabling was used. The thin blue flexible wire was the anode, white for the supressor grid, purple for the screen grid, green for the control grid, yellow for the cathode, brown for the heaters.
The small type 6V lead acid battery was supplying power to everything including of course the heaters.
In series with the + terminal was the primary of the output transformer, then the micro ammeter, then the tube anode connection.
The cathode was connected to ground or -.
The input XLR was connected to the primary side of the input transformer at the octal female base.
A coupling capacitor of 22nF was connected from the secondary to the grid to isolate it from the transformer DC resistance, while passing through the audio signal.
Initially only the anode was connected. There was almost no anode current. Then a few Megohm resistor was connected from Vb=6V to grid and the needle started moving but to a low anode current. Just a few micro amps.
When all the 3, screen grid, suppressor and anode were connected together the current was a healthy one perhaps exceeding 200μA.
It was found that most of the work (electron collection) was done by the screen grid. Just connected on its own it accounted for around 80% of the total current. Then connecting the anode the value of the total was increased a bit.
By listening in real time to the pavement sounds and surrounding ambience it was dramatic how the resistor form + to Control grid made it all work with an amazing detail and low noise.
2 options were in an instant tried. Getting the + from the Vb (V battery) or from the anode.
For some reason the connection to the anode instantly gave an impression of a more natural, lifelike sound. Also extended in HF detail. Subtle important differences.
So this is how a few megohms resistor found herself been connected from the anode to grid and the Pleiades V6 was born.
On this or latter experiments with the Grampian ribbon, a handwound tranformer (on Magnetec 073 core) with just 3mH input inductance to compensate for the proximity effect something interesting was refound. When disconnecting the heater circuit for a while, while the cathode temperature was dropping the gain was increasing instead of decreasing! So a 22Ω 3 Watt wirewound resistor was connected in series in order to find the best temperature for maximum signal extraction, amplification and less noise. The VF14 in the Neumann circuit is underheated too.
Now it is perhaps time to put everything, just 7 components, inside an orange octagon Hammond box with a Bungin battery holder used with a 3.7V li-ion battery, everything point to point hardwired and find the optimum value of Rag and heater voltage.
References:
Neumann U47 schematic
http://www.gyraf.dk/gy_pd/g7/u47.gif
Pleiades V6 schematic - euroelectron
http://euroelectron.blogspot.gr/2017/07/pleiades-v6-schematic.html
On preserving transconductance (at low battery voltage) - euroelectron
http://euroelectron.blogspot.gr/2017/03/on-preserving-transconductance-of.html
Operating features of the Audion - E.H. Armstrong
https://earlyradiohistory.us/1914opr.htm
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