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.

Tube Mic Pre-Amp

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.

There are two options for line level signals. First if you know that you”ll be using line level signals all the time with the TMP then you can change the two 33k 1% resistors to 1k 1% types and your done. Alternatively if you want the option of line level or low level signals then you can sacrifice the polarity switch and and rewire it here as shown. Notice that the 47k resistors are again of the 1% variety.

Download the schematics and circuit explanation of Tube Mic Pre-Amp circuit:
Tube Mic Pre Amplifier 26.25 KBDownload Circuit Manual

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