The following diagram is the circuit diagram of tube mic pre amplifier 12AX7. This circuit is little hard to built. You must have an intermediate or advanced skills to build this circuit.
All capacitors with value of 33uF are 16V, while the all others are 50V unless marked otherwise. Resistors marked with “#” are 1% metalfilm resistor type.
The “Drive” LED indicates how hard the tube is being driven. The “Blend” control allows for a mixing of SS and tube coloration. Symmetry controls the relative amounts of even and odd harmonics, CCW the Tube Mic Pre-Amp may sound punchier, while CW it may sound warmer. The 12VAC needed for pin 5 of the 12AX7 can be obtained from point G while pin 4 should be connected to point A.
Download the schematics and explanation of Tube Mic Pre-Amp circuit: