In summary 2 electron tubes operating in class A push pull drive the output transformer.
By varying the DC balance between the electron tubes there may be a DC magnetisation component on the output transformer core. The inductance change may vary the low frequency response.
An example microphone head amp or booster preamp with an ECC82 operating at less than 12V may as follows:
This Pleiades amp is derived from the Pleiadss (-2) that originally had an UCC85 later changed to ECC82 operating with 12V and the Pleiades 2 using an ECC82 single ended with Pleiades bias for even lower voltage battery operation .
The microphone drives the primary of the input trasformer.
The secondary drives each control grid.
A resistor of the order of a few Megohms is connected to Vb (voltage battery supply) which can be less than 12V.
The other side of the resistor is connected to the center tap of a potentiometer. Each other end of the potentiometer to each grid. This creates the suitable Pleiades bias for operation at anode voltage of less than 12V while the pot adjusts the above mentioned balance.
The cathodes are connected to minus ground.
The center tap of the output transformer primary is connected to less than +12V.
The other terminals of the primary are connected to each anode of the ECC82 or 12AU7.
The secondary is the output of the booster preamplifier.
A battery of less than 12V in series with a fuse can supply both the anode and heater circuits in a simple, uncomplicated, and low noise way. The light weight and battery operation creates portability.
Everything is balanced and geometrically symmetrical. Even both sections of the electron tube operate in a controlled balanced way. Controlling the balance of each electro tube section can adjust the low frequency response to aid flat frequency response from producer's brain to listener's brain.
The input and output transformer can be wound relatively simply using for example a nanocrystalinne ring core for each. The Magnetec 070 is a suitable brilliant example.
A typical turn ratio is 60:600 (60 turns to 600 turns).
A 1:10 ratio creates a 10 squared impedance transformation ie x 100.
So a mic of 200Ω becomes 20KΩ to generously drive the grids.
The impedance looking at the grids should be of the order of 100KΩ due to the biasing arrangement from anode to grid.
A typical electron tube anode output impedance at such conditions may be 30KΩ.
So the output transformer will feed the outside world with 300Ω.
The heater voltage can be quite less than 12V. Underheating can give even less noise (see Neumann U47 where the VF14 is deliberately underheated by Georg Neumann). In addition variable underheating can adjust the anode output resistance and hence the low frequency response by voltage drop due to loading by the primary of the output transformer. See relevant posts on the Pleiades V6 head amp.
Further helpful information can be found on the Pleiades V6 posts. The Pleiades V6 is a single or single ended electron tube battery operated booster studio microphone preamplifier.
Variable gain should be possible without thump by varying bias with a side chain circuit as this is really a first stage of an RCA BA-6A limiter battery operated. 6SK7 or 12SK7 electron tubes triode connected can be used. And of course EF183 electron tubes triode operated can be used with a battery voltage as low as 4V feeding the heaters too.
This amplifier may be made with 2 JFETS, K117 for example. Centre tap on the output transformer primary can give phantom power capability. See Pleiades schematics. If the Magnetec 073 magnetic nanocrystalinne Nanoperm cores are used with some luck everything may fit inside Neutrik barrel XLR adaptor modules.
References:
Pleiades V6 schematic
The Pleiades bias - euroelectron
Applied Electronics - T.S. Gray - MIT
Flat frequency response from producer's to listener's brain, Sound Picture Recording and Reproducing Characteristics - Lowe, Morgan - Joural of the society of motion pictures sound engineers
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