Turn the bias control clockwise to approximately 30mV on the
meter ( 30mA of bias current )
.
Now check the other tube. The power tubes should be within
5
m
A
of each other after 10 to 20
minutes of warming up. Unfortunately, you do not have a matched pair of power tubes if they
are farther than 5mA apart. Plug a guitar into the amp and begin to play. Turn the bias control
half a turn at a time (
in either direction
) and listen again. Always check your voltmeter and make
sure you stay between
18
m
V to 30
mV
Please read the whole bias chapter before you start
experimenting.
(
Note:
it is common for the
mV
readings to fluctuate somewhat when measuring the
power tube bias over a period of a few hours. This is due to the fact that some power tubes "
drift
"
somewhat. Again, this is not un-common and not a problem with your amplifier ).
This is what you may notice: generally speaking, lower bias current settings
( 18mV to 25mV )
will yield more headroom and note separation. There will be more clean volume available and
as the amp breaks up, the distortion can be slightly harsher and grittier. The treble strings will
sound thinner. Power chords, especially with emphasis on the bass strings, will sound clearer
and more defined, with more “
space
” between the notes. As you increase the bias current,
( past
25mV to 33mV )
, the treble strings will sound fatter and “
sing
” more. The amp will distort more
readily and with more harmonic content. The bass will stay consistent and strong. Also,
listen
to the clarity
of the pick attack, the “
envelope
” of the note and the sustain characteristics. Try
playing double stops and chords and listen to the blend of notes and their harmonic overtones.
As you experiment with different bias settings, you’ll find certain “
sweet spots
“ where things
just fall into place. The bass to treble balance is just right and the amp sparkles with harmonics.
At that point you have biased your amp perfectly for the tubes your using, the guitar your
playing, your playing style, your string gauge, your speakers, etc. Don’t forget to carefully lock
the bias control’s brake lever at this point. In order to be able to replicate your setting, just make
a note of the dial setting of the bias pot. You can do this for any number of different tube sets and
types. This will allow you to simply plug in a set of tubes, set the dial to the noted setting and be
done. Instant tube change and perfect bias without a meter!
What is the safe bias setting range? Use these guidelines: the lowest usable bias current
( in our
opinion )
is somewhere around
10
m
A
( mV on the meter ).
This depends on the power tube type
and manufacturer, among other things. Going lower will cause the amp to first cut in and out and
finally quit working all together. It will sound like a fuzz box with a dying battery. At this point
the amp is “
over biased
”
( very low bias current )
. No damage will occur; the sound of the
amplifier is just unusable.
In most amplifiers, biasing above
50
m
A +
will cause the plates of the power tubes to glow red.
The plates are the large gray or black metal parts inside the tube. In this condition the amp is
described as “
under biased
”
( too much bias current )
. You do not want this to happen. An under
biased amplifier can damage the tubes right away and kill them within minutes. So keep an eye
on the power tubes' plates and back off the bias control immediately should they start to glow.
Again, -
we recommend that you do not bias this amplifier higher than
30
m
V
with the stock, or
modern made power tubes,
( non - n.o.s. )
to guarantee an efficient life span of your power tubes.
Copyright 2000-2012, Riverfront Music, L.L.C. d.b.a. Komet
Amplification. May not be reproduced in any form without express written
consent from Riverfront Music, L.L.C. d.b.a. Komet
Amplification.