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 to not.
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 proce-
dure 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. You now need to
“check” the fuses located in the BIAS
section. Often you cannot verify if a fuse
is blown visually. Learn how to use your
meter to check for continuity. A good fuse
will indicate at or near ‘0’ ohms. An open
fuse will read many meg ohms. If you do
find a blown (open) fuse, this is a pretty
sure bet one of the power tubes in that pair
have failed. Note the left side of the BIAS
section is for power tubes 1&4, the outside
pair. The other fuse is for power tubes 2 &
3, the inside pair. Remove the suspect pair
of tubes and replace the fuse with a 750mA
(3/4amp) fast blo type fuse. Install a new,
matched pair of tubes, plug the power cord
back in and turn the power on and standby
to 120W. 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? In a pinch you may
continue to use your Vengeance 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 actually the sum of
the currents for each tube in the pair, we
highly recommend replacing 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.
ADVANCED THEORY
(FOR THOSE WHO CARE):
Those of you with electronic knowledge
may notice we are referring to current draw
but are making measurements in millivolts.
Ohms law states that I=E/R or current (I)
equals voltage (E) divided by resistance
(R). Inside the amp are one ohm resistors
in the cathodes of the output tube pairs.
The external test points allow access to
those resistors. When you measure across
those resistors at the rear panel test points,
you are reading the DC voltage drop across
a one ohm resistor. Referring to ohms
law, if R=1 in the formula, then I = E or
current equals voltage. So when you read
for example, 60mV you are also seeing the
equivalent numerical value of current or
60mA. You may have also figured out by
now that since you have separate adjust-
ments for each tube pair, it may be possible
to use alternate tube types at the same time.
Your suspicion is correct. You “tweakers”
can combine different types by installing
one type in the outside (1 & 4) sockets and
another type in the inside (2 & 3) sockets.
Be sure to adjust each pair for the proper
range. This way you can combine the char-
acteristics of the different types. The range
of the bias adjustments are such that you
should have not problem adjusting for just
about any type of compatible tubes.
WARNING: DO NOT be tempted to run
your tubes hotter than the maximum values
in the chart. You may find it sounds really
cool as you destroy your expensive tubes
and possibly damage your amp, of course
voiding your warranty! Also, in case you
haven’t found out the hard way yet, power
tubes get extremely hot (as high as 800
degrees)!!!! NEVER touch the tubes while
the amp is on. Always allow at least 5 min-
utes for the tubes to cool before touching
them after turning the amp off.
6L6/5881 ....................................60mV to 70mV
EL34/6CA7 ...............................65mV to 75mV
E34L ..........................................70mV to 80mV
6550............................................70mV to 80mV
6V6 (JJ ONLY !!!!!!!) ...............30mV to 40mV
KT66 ..........................................60mV to 70mV
KT77 ..........................................65mV to 75mV
RECOMMENDED BIAS SETTINGS
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