This little microphone headamp or booster amplifier is powered by a 9V battery supplying both heaters and plate circuit.
The configuration is just like a long tail pair but the tail is short as no common cathode resitor is used.
Both cathodes are connected to ground. When heater voltage is suppliedto an an electron tube the grid becomes negative by the electron cloud or space charge missing from the cathode. For low voltage plate operation we do not want to make the grid even more negative by cathode bias as the electron tube might already be in cutoff. On the contrary transconuctance, gain, output impedance, linearity, class A bias point, headroom would benefit from an even less negative bias on the grid. So a pull up or Pleiades bias resistor is used.
Heaters are connected in series at 12.6V configuration but deliberately or conveniently supplied by 9V. This creates less electron cloud, less cathode temperature, less anode internal resistance thermal noise, less thermo and photo emission from other electrodes. Also less plate supply creates less secondary emission effects, less positive ions. So less noise. Shot noise should be non existent as the plate voltage is small and there is a corresponding space charge. See previous posts.
Ingredients or components:
A 9V battery
A 9V battery holder for example Bulgin
A fuse and fuse holder in series with battery holder for safety
One ECC82 or 12AU7 electron tube
A noval (9 pin) valve socket
An input transformer with center taped secondary
An output transformer with centre taped primary
A resistor
An enclosure
Male and female XLR chassis socket
Preparation:
Time is allowed to decide the best and simplest position of all the components. Then the box and component securing takes place. Star washers are used to secure or lock fastening nuts. XLRs, electron tube socket etc.
From input to output:
Female mic in XLR pin 1 is connected to chassis XLR pin. this may be used as star point where all grounds, -, shields, etc are connected.
Pins 2, 3 are connected to the primary of the input step up transformer.
Each of the cecindary leads is connected to each grid.
The center tap is connected through a few Megohms variable resistor in series with another resistor to +9V. The series resitor is used to avoid over bias giving a too high anode current. Microphone voltages are so small that the game can be played in micro amps or hundred of micro amps etc. This resistor combination provides the right pull up bias. If we do not do this connection the amplifier would still work and gain can be increased or controlled by light emission on the electron tube. Is a compressor possible?
The cathodes are joined together and connected to - supply.
The 12AU7 anodes are connected to each output transformer primary lead.
The center tap is always connected to +9V to avoid clicks or noise.
The battery holder is connected in series with a fuse holder for safety and 9V supply is taken after the fuse.
The secondary leads of the step down output transformer are connected to pins 2 of male XLR mic out. Pin 1 is again connected to chassis as explained by Jsnsen on their articles on elimination of pin 1 interface problems.
A switch disconnects or supplies power only to the heaters for gradual noisiless on off.
There are many higher or lower than 9V rechargeable 9V type batteries to very quickly experiment with the best underheating for best sound, less noise, max signal to noise ratio.
See also:
Pleiades V(-1) with ECC82, UCC85
The Pleiades bias
Operating features of the Audion - E. H. Armstrong
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