BIASING THE KOMET
19
The Komet
19
is a cathode biased amplifier. An internal resistor sets the amplifier’s power
tube bias. A bias adjustment should not be necessary. To guarantee your amplifier will operate
properly in conjunction with the supplied voltages and current draw of the Komet
19
, we
recommend that when you replace your
power tubes, that you purchase a
matched
set of
EL84/6BQ5
s
labeled by the manufacturer with a low
IP
number
(or a low plate voltage / current
draw number)
or
grade and
labeled as well with a
transconductance
(Gm)
matching numbers.
Having matched power tubes ensures that all of the power tubes will operate and react equally
within the amplifier. Matched power tubes will also guarantee better harmonics, sonic
performance and extended tube life. Most reliable and reputable audio tube vendors and n.o.s.
tube dealers sell their power tubes in matched, output power or plate voltage / current draw,
numerically rated sets.
You should be able to request a certain IP
(plate voltage /current draw)
grade or range from your
tube vendor.
Some tube vendors refer to their power tubes for their " break up "
characteristics
by labeling their sets with terms like: "
soft
", "
medium
" or "
hard
". Usually, power tubes labeled
"
soft
" are the low plate current drawing type. Many vacuum tube vendors use their own numeric
measurement system. Some examples:
the Sovtek
EL84 grade for the Komet
19
is an IP
number range from
# 12
through #
15
. The original Groove Tube
rating scale for the
Songwriter
30
would be a grade
# 1
through
# 3
.
Note
: the 12AX7 pre amp tubes in your
amplifier are internally self biased and need no adjustment.
WARNING
:
Never remove any of the four EL84/6BQ5 power tubes in order to attempt
cutting the power of the amplifier in half. The Komet
19’s
EL84 power tube circuitry shares
only one cathode resistor. Removal of one or more of the power tubes will cause the other
EL84
s
to catastrophically malfunction and fail.
Under-biased Power Tubes
:
EL84
/
6BQ5
power tubes that are under biased,
(or drawing too much current)
, will usually create
the following symptom: a low, background
"
hum
"
sound coming from the amp, which may get
louder as time passes. You should inspect the plates of the power tubes
(the flat grey colored,
metallic, square section inside the
center
of the power tube/s).
Make sure your amplifier is on.
Do
not
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
"
red
"
or are
"
cherry colored
"
from the
center
, (see reference photo on following page)
, then the power tubes are under biased and
must be
replaced with a
lower
(
IP
)
plate voltage / current drawing
- set. If not replaced, they will fail
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
(s)
has turned a bright, frosted white
color, or occasionally - completely burned off.
Copyright 1999-2017, Komet
Amplification. Group, L.L.C. May not be reproduced in any form without express written consent from Komet
Amplification.
