the circuit. A red LED next to the fuse will indicate that the fuse is blown. You can keep playing
with just a small reduction in performance and still get through the gig. If this happened in the
past, you would need to take the amp to a repair shop. They would then hold it for ransom while
you figured out how to raise enough money to pay them to fix it. No more. The amp will now tell
you if there is a shorted output tube and which one it is! With the power off, simply remove the
power tube and replace the indicated fuse with a FAST BLO 315mA or 400mA. NEVER EVER
use anything but a FAST BLO 315mA or 400mA fuse. If you do have the misfortune of “blowing”
an output tube, we strongly suggest replacing the set. Chances are if one tube blows, it is an
indication that others may follow. Remember, your amp will only perform as well as your weakest
tube.
Special Note:
You should always carry spare power tubes, fuses, a flat blade screwdriver and your voltmeter
with you. If a tube fails at a gig, you could be back up and running in a matter of minutes. You
wouldn’t drive your car without a spare tire, right? Try that with any other amp. In a pinch you may
also continue to use the MOD100, with somewhat reduced performance, even with one or two
blown tubes. No additional damage to the amp will occur. Do service the amp as soon as
possible, of course. For optimum performance, we recommend replacing power tubes in matched
sets.
Advanced theory (for those who care):
Those of you with electronic knowledge may notice we are referring to current draw but are
making measurements in millivolts. Ohms law states that I=E/R or current (I) equals voltage (E)
divided by resistance (R). Inside the amp are one ohm resistors in the cathodes of the output
tubes. The external test points allow access to those resistors. When you measure across those
resistors at the rear panel test points, you are reading the DC voltage drop across a one ohm
resistor. Referring to ohms law, if R=1 in the formula, then I = E or current equals voltage. So
when you read for example, 30mV you are also seeing the equivalent value of current or 30mA.
You may have also figured out by now that since you have separate adjustments for each tube, it
may be possible to use alternate tube types at the same time. Your suspicion is correct. You
“tweakers” can combine different types by installing one type in the outside pair of sockets and
another type in the inside pair. Be sure to adjust each tube for the proper range. This way you
can, in theory, combine the characteristics of the different types. How cool is that?
WARNING: DO NOT be tempted to run your tubes hotter than the maximum values in the
chart. You may find it sounds really cool as you destroy your expensive tubes and possibly
damage your amp, of course voiding your warranty! Also, in case you haven’t found out the hard
way yet, power tubes get extremely hot (as high as 800 degrees)!!!! NEVER touch the tubes while
the amp is on. Always allow at least 5 minutes for the tubes to cool before touching them after
turning the amp off.
We hope this information is informative as well as educational. Please keep this manual with you
for reference. Don’t hesitate to contact us with any questions or comments.
PAGE 6
Internal DIP switch settings:
Inside the MOD 100 chassis, you will find a dip switch for setting up the midi/channel switching
parameters. The amplifier is shipped set up to receive program change data on midi channel one.
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