Komet Aero 33 Owner's Manual page 9

Some tube vendors refer to their power tubes for their "break up" characteristics by labeling their
sets with terms like:
"
low / lower plate current drawing type. Many vacuum tube vendors use their own numeric
measurement system. Some examples: the Sovtek
number range from # 12 through # 15. The original Groove Tube
would be a grade # 1 through # 3.
WARNING: Never remove any of the four EL84/6BQ5 power tubes in order to attempt cutting
the power of the amplifier in half. The Aero
cathode resistor. Removal of one or more of the power tubes will cause the other EL84
catastrophically malfunction and fail. Note: the 12AX7 pre amp tubes in your amplifier are
internally self biased and need no adjustment.
Under-biased Power Tubes:
EL84/6BQ5 power tubes that are under biased,
the following symptom: a low, background
louder as time passes. You should inspect the plates of the power tubes
metallic, square section inside the center of the power tube/s).
confuse the yellow / orange filament glow on the top and bottom of the power tubes for the
center plates.
If the plates in the center of the tube are glowing a dull
center
, (see reference photo on next page)
replaced with a lower
and short out in short order, and the H.T. fuse will blow. Another sign of under-biased power
" "
tubes - the descriptive, colored lettering/labeling on the tube
color, or occasionally - completely burned off
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soft , medium or hard
" " " "

, then the power tubes are under biased and must be
plate voltage / current drawing - set. If not replaced, they will fail
(IP)
. Usually, power tubes labeled
"
 EL84 grade for the Aero
's EL84 power tube circuitry shares only one
33
(or drawing too much current)
hum sound coming from the amp, which may get
" "
Make sure your amplifier is on. Do not
or are
"red"
has turned a bright, frosted white
(s)
are of the
"soft"
 is an
33
 rating scale for the Aero
to
s
, will usually create
(the flat grey colored,
from the
"cherry colored"
IP

33