BIASING THE KOMET
SONGWRITER
30
The Komet
Songwriter
30
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 Songwriter
, 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 Songwriter
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
Songwriter
30’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)
, usually have the
following symptoms: a low, background "
hum
" sound from the amp/speaker cabinet, which
may slowly get louder over time. You can easily inspect your amplifier for this problem.
Unplug and remove your guitar cable from the amplifier’s input. Carefully position your
amplifier where it can be easily and safely accessed from the rear
(with speakers connected)
.
Turn the amplifier on, engage the standby to the on position after 60 seconds. Remove the
back panel so as to inspect the
plates (the flat grey colored, metallic, square section inside the
center
of the power tube/s)
of the four power tubes
(V3 through V6
).
(continued)
Copyright 1999-2017, Komet
Amplification. Group, L.L.C. May not be reproduced in any form without express written consent from Komet
Amplification.
