Biasing - Egnater Renegade 112 Owner's Manual

Bias: Power Tube Bias
What is bias? Simply put, it is a circuit
inside the power amplifier section that
controls the "idle current" that flows
through the power tubes. Much like the
idle speed on a car. There is an optimum
setting where the engine (amplifier) is
running (idling) fast (hot) enough to keep
it from stalling (distorting) but not too fast
(hot) to cause excessive wear and over-
heating. Get it?
Why don't all amplifiers have bias or
idle current adjustments?
Most do have some provision for that
but typically involve removing the amp
chassis from the box, exposing you to
very dangerous high voltage. Special test
equipment and knowledge of amp circuits
and tubes is also needed. Not a skill most
musicians possess and shouldn't need to.
Why would I want to adjust the bias?
All power tubes are different. They each
have unique sonic and electrical character-
istics. The Renegade amplifier is shipped
with a pair of 6L6/5881 tubes and a pair
of EL34 tubes, but is designed to accept
a variety of different tube types. EL34,
6L6, 5881, 6cA7, 6550, KT66 and KT77
are among the many possible choices. Be-
cause they are all different, each requires
different bias settings for safety, reliability
and optimum performance.
Please read the following instructions
on how to use this cool feature.......
You will need a decent quality digital
voltmeter capable of measuring in the
100 to 200 millivolts DC range. This is a
very basic type of meter available at any
electronic supply house or Radio Shack.
They typically cost anywhere from $10 to
$25, about the cost of one bias adjustment
from your local amp tech.
You will also need a small, flat blade
screwdriver to turn the adjustment con-
trols that are recessed inside the grommets
below each tube label on the rear panel.
1) Turn the amp on, standby switch
in the PLAY position. All controls all the
way down. Turn the meter on and set for
reading DC millivolts. Consult the meter
instructions for how to do this properly.
12
RENEGADE
®
Since all meters are different, it is ex-
tremely important that you thoroughly
understand what you are looking at on the
meter display.
2) Insert the black (negative) test
lead into the panel hole labeled (-)
COMMON.
3) Notice there are two identical
sections to the left and right of the com-
mon terminal.
4) First step is to insert the red
(positive) meter lead into the EL34 test
point (+) hole.
5) with your flat blade screwdriver,
turn the BIAS ADJUST control to obtain a
correct reading from the chart below.
6) Repeat this procedure for the
6L6 tubes.
7) Now allow the amp to warm
up for about 10 minutes and retweak so
the readings are within the range of the
recommended reading.
Displays differ from one meter to the
next. Some may indicate, for example,
60.0 for 60 millivolts. Others may show
.060 for 60 millivolts. Knowing how your
meter works if of the utmost importance.
You should always check the bias readings
whenever you replace output tubes and
readjust if needed. Since we've made it so
simple, there is no reason not to.
Additional features of the "POWER
TUBE BIAS SECTION"
Fast Blo fuses. One per output tube pair.
In the event of a power tube failure, the
corresponding fuse will open protect-
ing the amp from additional damage by
effectively removing the failed tube from
the circuit. You can keep playing with just
a small reduction in performance and still
get through the gig. If this happened in
the past, you would need to take the amp
to a repair shop. They would then hold it
for ransom while you figured out how to
raise enough money to pay them to fix it.
No more. The amp will protect itself from
the potential damage and you can continue
to play.
Read the quick trouble shooting pro-
cedure below if you suspect a tube failure:
If you notice a sudden loss of power/
®
volume or degradation in tone, you may
have a blown power tube. Not really that
uncommon these days. While playing the
amp on either channel, rotate the TUBE
MIX knob from one extreme to the other.
If you have a blown tube (and correspond-
ing blown fuse) you will notice and severe
volume loss on either the EL34 side or the
6L6 side. Once you determine that one of
the tubes pairs is dead, turn the power off,
unplug the power cord and wait at least
5 minutes for the power tubes to cool.
Remove and check the suspect blown fuse
located either left or right of the COM-
MON test point in the BIAS section on
the rear panel. If you do find a blown fuse,
this is a pretty sure bet one of the power
tubes on that side has failed. Remove the
suspect pair and replace the fuse with a
800mA (3/4amp) fast blo type fuse. Install
a new, matched pair of tubes, plug the
power cord back in and turn the power on.
Check the bias reading. If you get
a reading that is reasonably close to the
proper setting, you have just repaired your
own amp. Simply readjust the bias control
according to the chart and you are on
your way. You can thank us later for
saving you a bunch of money and a trip to
the repair shop.
SPECIAL NOTE:
Obviously for this to work, you MUST
carry spare power tubes, fuses, a flat blade
screwdriver and your voltmeter with you.
If a tube fails at a gig, you could be back
up and running in a matter of minutes.
You wouldn't drive your car without a
spare tire, right? Try that with any other
amp. In a pinch you may continue to use
your Renegade with somewhat reduced
performance, even with shorted tubes. No
additional damage to the amp will occur.
Do service the amp as soon as possible, of
course. Because the BIAS readings are ac-
tually the sum of the currents for each pair
of tubes, we highly recommend replac-
ing power tubes in matched pairs. If your
tubes are not matched, one tube could be
running very hot while the other is barely
on but you would still read the sum of the
pair. This is not a good thing.