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the glass, you will probably hear the noise change. Hearing some noise through the speakers while tapping
on the 12AX7's is normal however. And the one nearer the INPUT will always sound louder because its output
is being further amplified by the second 12AX7.
The power tubes should be all but quiet when they are tapped. If crackling or hissing changes with the tapping,
you have probably found the problem. To confirm a noisy power tube, merely put the amplifier on Standby,
remove it from its socket and turn it back on. It will cause no damage to run the amplifier briefly with one power
tube missing. You may notice a slight background hum, however, as the push-pull becomes unbalanced.
Whenever you are trying to diagnose a suspect tube, keep your other hand on the POWER and STANDBY
switches ready to shut them off instantly in the unlikely case you provoke a major short.
If you think you've located a problem tube but aren't sure, we recommend substituting the suspect with a new
one just to be sure of your diagnoses. You will be doing yourself and us a big favor by just following the simple
guidelines previously mentioned regarding tube replacement. You'll probably be successful with much less
effort than is required to disconnect everything and haul the unit to a technician who will basically perform
the same simple tests. If the tubes are still within their six-month warranty period, we will happily send you a
replacement. Just note the color designation on the tube label so that we can send you the appropriate match.
SPEAKER IMPEDANCE MATCHING & HOOK-UP GUIDE
IMPEDANCE
Wiring up speakers to provide the most effective load and making sure that all of them are in phase will help in
creating the best sound possible. This is not too difficult, as long as you understand a few things about loading
and how to connect your speakers to provide an optimal resistive load.
MESA/Boogie amplifiers can handle 4 and 8 ohms effectively. Never run below 4 ohms in a tube amplifier
unless you are absolutely certain that the system can handle it properly; this can cause damage to the Output
transformer. A few amplifiers can handle 2 ohms effectively without damaging them ( for example the MESA'S
Bass 400+ ). You can always have a higher resistance ( 16 ohms, for example ) without damaging results, but
too low of a resistance will likely cause problems.
MIS-MATCHING
When running a higher resistance ( for example: 8 ohm output into 16 ohm cabinet ), a slightly different feel
and response will be eminent. A slight mismatch can provide a darker smoother tone with a little less output
and attack. This response is a result of the amplifier running a bit cooler. Sometimes when using more than
one cabinet a mismatch will be the only option.
WHAT IS MY CABINETS IMPEDANCE?
If you have only a single speaker, you just match that single speakers impedance to the amplifier, and you
are done. In many cases, you will have a number of speakers, and then you must calculate the "load" that the
amplifier will need to support. There are generally three ways to wire multiple speakers together. They are as
follows:
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