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SM-500 USER GUIDE • 11
VACUUM TUBE (VALVE)
SWR installs a specially selected 12AX7 dual triode tube in the preamp section of every SM-500.
When it becomes time to replace it, we recommend that you do so with a similar high quality product.
This tube will need replacing only if it becomes noisey or microphonic (sounds like glass tinkling in
the background of certain notes), or completely fails (resulting in no output).
RACK MOUNTING INSTRUCTIONS
To preserve the beauty and reliability of your amplifier, we recommend that you install your amplifier
in a rack case. The SM-500 is rackmountable, requiring no additional parts or accessories other than
rack screws and the rack case itself.
The SM-500 takes up two full rack spaces (3 1/2"). If the rack in which you mount the SM-500
requires that the rubber feet on the bottom of the chassis be removed, please remember to
REPLACE the screws, as they help to reinforce the chassis.
The SM-500 should be mounted as close to the bottom of the rack case as possible. The height of
the rubber feet was chosen so that when you slide the unit in the bottom of a rack case, the rack
mounting holes on the front panel should line up with the mounting holes of the rack rail. This pre-
vents the SM-500 from flexing downward if the rack case is dropped. If you must mount the SM-500
in an area of the rack other than the bottom space, a piece of wood or similar solid material should
be installed between the bottom of the rack case and the bottom of the amplifier to prevent flexing
of the amplifier’s chassis. Severe or constant flexing of the chassis can damage the amplifier and is
not covered under the SM-500’s warranty.
Don’t neglect your amp after it’s been installed in a rack case. Continuous transportation and vibra-
tion can cause screws to become loose, both on the SM-500 and with your rack case rails. We rec-
ommend that at least once a month you remove the SM-500 from the case and tighten all outside
screws (especially on the front panel), and wipe off the outside of the chassis with a damp cloth.
Then check all the connections in your rack case and reinstall the unit.
POWER-ON TRANSIENT
When the SM-500’s Power switch is moved to the “On” position, you will notice a turn-on transient
heard as a “thud” through your speakers. This will not harm speakers made by SWR, however, you
may connect your speaker cable to the SM-500 after powering up if you choose. Just make sure you
are not playing through the unit when you make the connection, as it could cause a speaker fuse to
blow. Eliminating this transient would require a component called a relay. SWR chose not to incorpo-
rate this type of component due to the fact that relays degrade signal quality and often fail, causing
the unit to have no output, and requiring a trip to a local service center.
A FEW WORDS CONCERNING HEAT
One of the most asked questions about our amplifiers is why they tend to get warmer than other
amps. The chassis of your amplifier can get quite warm during normal usage. This is especially true
if you are using a 4 ohm total impedance in the Bridge mode. This is because a 4 ohm impedance
(or 2 ohms per channel in Stereo mode) introduces the least efficient condition of the unit (in other
words, power drawn from the outlet in relation to power produced in the speakers).
The difference in these two figures can be quite high, resulting in the equivalent of a high wattage
light bulb in a metal box (which would obviously get quite hot).
Most musical instrument amplifiers on the market today use steel for their chassis which, in most
cases, is considerably cheaper than aluminum and does not conduct heat as well as aluminum. The
SM-500 uses an all aluminum chassis because it has less impurities than steel, is less susceptible
to rust and is a better conductor of heat. This results in the chassis acting as a heatsink drawing