Battery Powered Vacuum Tube Microphone Pre-Amplifier for the Purpose of “bootlegging” a Concert

Summary

This is a two stage, two channel (pre-and post gain adjustable) battery powered tube pre-amplifier.  Microphones or other sources can be plugged into this unit.  It is very portable and easy to use.  This unit runs on 8 AA batteries and should last anywhere from 2 to 4 hours.  It has an internal high voltage power supply that is regulated to 80 volts or so using zener diodes.  I have received many inquires asking for the schematic to this pre-amp.  This is built out of a pair of 12AX7’s (West German ‘Computer Tubes’) and an HV supply made out of a 555 timer, a TDA series 2 Watt audio amp chip, and an old military surplus audio transformer (see grey can on top of chassis).

PUBLICATIONS

G. L. Charvat, "Portable tube preamp," Audio Express Magazine, March 2010, pp. 16-19.

Schematics and Notes

Caution!  High voltage.  Do not attempt to build anything shown here unless you have taken proper high voltage training.  The high voltages present in these tube designs are deadly.

Notes on the battery powered vacuum tube pre-amplifier.