Tube Noise & Microphonics - Mesa/Boogie RECTO-VERB 25 Owner's Manual

TUBE NOISE & MICROPHONICS
You may occasionally experience some form of tube noise or microphonics. Cer-
tainly no cause for alarm, this quirky behavior comes with the territory and the Tone. Much like changing a light bulb, you don't
need a technician to cure these types of minor user serviceable annoyances and in fact, you'll be amazed at how easy it is to
cure tube problems...by simply swapping out a pre-amp or power tube!
First may we suggest that you set the amplifier up on something so that you can get to the tubes comfortably without having to
bend down. It also helps to have adequate lighting as you will need to see the tube sockets clearly to swap tubes. Use caution
and common sense when touching the tubes after the amplifier has been on as they may be extremely hot! If they are hot and
you don't want to wait for them to cool off, try grasping them with a rag and also note that the glass down around the bulbous
silvery tip is considerably less hot which makes it easier to handle. Gently rock the tube back and forth as you pull it away from
its socket.
DIAGNOSING POWER TUBE FAILURE
There are two main types of tube faults: shorts and noise. Both large and
small tubes may fall prey to either of these problems but diagnosis and remedy is usually simple.
If a fuse blows, the problem is most likely a shorted power tube and shorts can either be mild or severe. In a mildly shorted tube
the electron flow has overcome the control grid and excess current flows to the plate. You will usually hear the amp become
distorted and begin to hum slightly. If this occurs, quickly look at the power tubes as you switch the amp to STANDBY and try
to identify one as glowing red hot. It is likely that two of a pair will be glowing since the "shorted" tube will pull down the bias
for its adjacent mates, but one tube may be glowing hotter — and that one is the culprit. The other two are often fine — unless
they've been glowing bright red for several minutes.
Because there is no physical short inside the tube (just electrons rioting out of control) merely switching to STANDBY for a few
moments then back to ON will usually cure the problem...at least temporarily. Watch the tubes carefully now. Should the problem
recur, the intermittent tube will visibly start to over heat before the others and thus it can be identified. It should be replaced with
one from the same color batch, shown on its label. Call us and we will send one out to you.
The severe short is not nearly so benign. In the worst cases, a major arcing short occurs between the plate and the cathode
with visible lightning inside the glass and a major noise through the speaker. If this is seen to happen, IMMEDIATELY turn the
amp to STANDBY. By this time the fuse probably will have blown. Such a short is usually caused by a physical breakdown inside
the tube including contaminate coming loose or physical contact (or near contact) between the elements. Replace it and the
fuse with the proper slo-blo type and power up the amp using the power up procedure as we described earlier in this manual.
TUBE NOISE
Often caused by contamination within in a tube, the culprit can usually be identified, and by lightly tapping
on 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 prob-
ably found the problem. To confirm a noisy power tube, merely put the Recto on Standby, remove it from its socket and turn
it back on. It will cause no damage to run the Recto 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 war-
ranty 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.
PAGE 16