Tube
Life &
Biasing
WARNING: Tubes can become extremely hot during normal operation. Make sure the
amplifier is OFF, and always allow tubes to cool before handling to prevent burns.
P reamp Tubes:
Preamp tubes typically last many years, but fail occasionally. Typically they will become noisy
or “microphonic” before they wear out. This is characterized by ringing, static, or popping
noises.
Changing Preamp Tubes:
V1, V2, V3, V4 (see tube diagram on pg. 12)
To remove tubes, gently pull upward using a very slight circular motion if necessary. When re-
inserting a preamp tube, mind the pin/socket orientation as they are “keyed” to insure proper
installation.
O
utput Tubes:
Output tubes (See tube diagram on pg. 12 ) generally produce a strong output for 6 months to
a year when played regularly, then they may become dull sounding, and/or the amplifier may
begin to loose some power. Output tubes may last much longer depending on how hard the
amp is played. Sometimes they die gracefully, sometimes abruptly causing a fuse to blow,
which in turn protects the amplifier from further damage.
It's sometimes possible to see in plain view which power tube(s) is damaged. If necessary,
remove the output tubes to inspect them by grabbing the plastic base, and gently pulling up in
a shallow circular motion.
Here are things to look for:
•
The shiny silver area on top of tube has turned white (tube has lost vaccuum).
•
Burned spot(s) on the large gray structure inside the tube (tube has “red-plated”/drawn
excess current.
•
The filament is not lighting up (not common, but can happen).
Like preamp tubes, sometimes output tubes can become “microphonic.” This occurs when
some part internal part of the tube becomes physically loose. In turn, this noise then gets
amplified. The noise can be anything from static, a high-pitched ringing, or intermittent
sputtering noises. This is usually exacerbated by vibration from the speaker cabinet. If you
think a tube may be microphonic, try isolating the amplifier from the speaker cabinet to see if
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