Mesa/Boogie II Manuel d'utilisateur télécharger pdf (Page 5)

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SPEAKER JACKS One 8-ohm and two 4-ohm speaker jacks are provided on the 50 Plus. Use the

8-ohm jack for maximum power and clarity with the enclosed 12" speaker (if combo version) or when

using a single MESA one-twelve or four-twelve cabinet. Use the 4-ohm jack(s) when using two 8-ohm

speakers or two 8-ohm cabinets, or when using a single MESA two-twelve cabinet (as these are

4-ohm cabs).

EQ FOCITSWITCH

JACK This jack allows connection of the optional EQ Footswitch for remote

Equalizer switching. It is compatible with any standard, grounding-type switch and should be used

with a shielded, mono (two-conductor) cable. When the EQ Footswitch is in use, the front panel EQ

Switch should generally be set to its center, unlabelled (EQ OUT) position, since the footswitch

cannot fully override the “EQ IN” or “EQ AUTO" modes (although it can be used to turn on

the EQ in

Rhythm mode if “EQ AUTO” has been selected).

CAUTION The .50 Caliber Plus is intended to offer its full range of great sounds at any volume, from

very soft to very loud. And in order to do this its controls must be very powerful. When turning up the

Master and/or Lead Master and playing quite loudly, you should expect to reduce the Volume and

possibly the Treble and/or Presence. Otherwise, you may push the 12AX7 preamp tubes beyond

their reasonable range, and noise, feedback or ringing may occur. This is normal,

however; it does

not indicate a malfunction in the amplifier. It's like having a Ferrari that takes tight corners great…

and also goes 160 miles an hour on straight-aways. Just because it can go fast doesn't mean it can

also take tight corners wide open! Similarly, your .50 Caliber has been designed both to play quite

loudly, and to simulate the characteristics of loud playing at

soft volumes when its preamp gain is

turned up high. But you cannot run both the Volume and the Masters wide open at the same time

without encountering feedback. There will always be an optimal point between the two controls that

balances preamp gain (Volume) against power amp loudness (via the Masters). Like the tires on a

Ferrari, you will have to maintain your 12AX7 preamp tubes (especially “V1” the tube located nearest

the input jack) much more carefully if you insist on "flying around those hairpin turns" with everything

wide open!

MAINTENANCE Excessive noise, feedback, squealing or rattling sounds coming from the speaker

are almost always the result of a deteriorated 12AX7 preamp tube, usually the first (“V1”) or the

second ("V2") tube on the right (as seen from the rear), nearest the input jack. Checking the preamp

tubes in a tube tester is almost useless. The best procedure is to substitute a tube that's known to be

good and noise-free in each possible tube location, one at

a time. Here are some clues to speed up

the process: if the problem appears only in the Lead mode, then the culprit is probably V1, V2, or V3.

(V1 is the tube nearest the input jack; V2 is the “next one out”; etc.) If the problem is in the Rhythm

mode only, then V2 is usually the problem. If the reverb is noisy or has some other problem, replace

V4. (Unlike the other preamp stages, V4 requires a 12AT7-type tube.)

Sometimes the insides of the power tubes themselves can be heard rattling. If the power is weak and

your tone is flat, try replacing the larger power tubes, type 6L6GC. We strongly recommend using

only genuine MESA tubes; in fact, your amplifier's warranty is not valid if you use other brands or

types. Power tubes are like guitar strings; they begin to deteriorate as soon as they're put into use.

You may notice improved performance by replacing the 6L6’s, every few months, depending on

usage. But do not change the 12AX7 or 12AT7 preamp tubes unless a specific problem arises. Their

aging is very slow and you won't likely hear any improvement by changing them unless one has

become noticeably noisy (and in fact you may even incur worse performance if you change a tube

that's currently working fine).

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Mesa/Boogie II Manuel d'utilisateur Page 5

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