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A broken or improperly seated vacuum tube can present an electrical shock hazard, which
can result in serious injury or death! The following instructions in this section for tube
replacement or substitution are for experienced electronics technicians only.
To replace a tube, unplug the line cord, disconnect all the inputs, disconnect the speakers, and wait 30
minutes. Remove the top cover, which is held in place with 14 Philips-head machine screws (4 on each
side, 6 on top). To remove a tube, pull straight up while gently rocking very slightly from side-to-side.
Never twist the tube! Replace tubes with the same type number. The 5751 is a low-noise, low-drift,
lower-gain version of the 12AX7. If you cannot find a 5751, a 12AX7 may be used. JJ Electronics currently
manufactures an excellent 5751.
If you are replacing one of the 5751 input tubes, you will have to perform an offset adjustment after
replacement. Using the tool provided (or a potentiometer adjustment tool with an insulated handle –
NOT a screwdriver with a conductive metal shaft), set the offset trimmer to the center of its adjustment
range. Connect a voltmeter (set to the +/- 1V range or higher) to the output, turn on the amplifier, and
wait for it to warm up. Adjust the offset trimmer until the voltmeter reads zero, then move to a more
sensitive scale. You should be able to adjust the offset to +/- 0.02V or less, although it is normal for the
offset to drift within a +/- 0.05V range. Allow the amplifier to operate for at least 30 minutes before
making the final offset adjustment.
Tube Substitutions
Some experienced audiophiles like to experiment with different vacuum tubes to change the sound
character of their equipment. Feel free, but be sure to replace tubes only with known-good tubes of the
same type. Although unlikely, use of other tube types could possibly damage the amplifier and void the
warranty. The only exceptions to the use of different tube types are the 12BH7, which can be replaced
with a 5714 or a 12AU7, and the 5751, which can be replaced with a 12AX7. It is very unlikely that you
will find a tube with less drift than the JJ 5751 supplied with the Minotaur (if you do, please let us know).
New, old-stock (NOS) tubes tend to have higher noise and drift. The harmonic character of the amplifier
is primarily determined by the 12BH7 driver tube. Note that if you replace one of the 5751 tubes, you
will have to perform an offset adjustment (see above).
