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Mesa Boogie claim their amps don't need biasing.
Only true if you use Mesa screened tubes or tubes that
have been selected to match the Boogie biasing specification
or if your tube supplier has tested and selected the
tubes for you. The way Boogie get away with not fitting
bias adjustments to their amps is they recommend you
fit only Mesa tubes. These tubes are screened and tested
to within a narrow specification so that the biasing
adjustment is not necessary. Most good tube suppliers
can offer you any tube screened to the Boogie spec if
you mention you have a Boogie amp. The tubes used and
badged by Mesa are manufactured from one of the 3 main
plants remaining today. The only difference is that
they are tested and screened to match their amplifiers
and they also have a Boogie logo printed on them. Otherwise
the tubes are the same as any other produced by Russia,
Europe or China.
I am not technical and want a quick way of telling if
my amp is going to melt due to the bias being set too
low (tubes too hot).
OK a very simple test to ensure your bias is not set
too hot. If you have replaced your output tubes switch
the amp on and standby switch on if you have one as
if you were about to play the amp. Wait for a few minutes
for the tubes to warm. Darken the room and take a look
at the output tubes. You should see small red hot pin
prick glows from the heater possibly at the bottom and
top of the tube. This is normal. Also if you look through
the holes of the Grey box plates you may see a faint
blue glow. Again this is normal and indicates your standby
switch has applied the High voltage to the tubes.
You SHOULD NOT see the long grey plate area glowing
red or red blotches appearing on them. If the large
grey plate box area glows red the tubes are under biased
and running far too hot. In this situation damage can
occur to the tubes and the amplifier within a short
time. If you see no red hot plates the chances are you
will be safe. However if the tubes are overbiased (running
cold) you may not be getting the best tone from your
amp.
How do I bias my amp
If you have adjustments available on the back of the
amp like on Fender Twin and the like then biasing is
straight forward and requires very little technical
knowledge. You will need a digital voltmeter capable
of reading 30mV (usually a 200mV scale on the meter
will do) and a screwdriver.
Locate the bias adjustment screw on the back of the
amp. Usually there will be instructions indicating the
voltage that needs to be set at the test points (again
can be sockets on the back of the amp). Put your meter
on the test points. Switch the amp both standby and
mains switch on. Wait for a few minutes and then measure
the voltage.
IMPORTANT: LOUD SPEAKER MUST BE CONNECTED AT ALL TIMES DURING BIASING AND WHEN THE AMP IS TURNED ON. Adjust the screw provided until the meter reads the
voltage specified on the instructions (this is usually
around 30mV since most amp manufacturers take 2 wires
off an inline 1 ohm cathode resistor. The 30mV you are
reading translates directly to 30mA through one tube).
This voltage can be 30, 60 or 120mV depending on how
the manufacturer has wired the test points. See their
instructions for the amp in question.
I will not go into this in great detail in this guide
if you do not have a bias adjustment easily accessible.
I will however give an excellent link for you to follow
if you are technically minded and want to have a go
yourself. Duncanamps.com
is a great place to find information about biasing and
all kinds of Tube info.
Please remember there are lethal voltages present in
valve amps. If you are not familiar with working with
high voltages take your amp to a tech.
Well hopefully that has given you some insight into
what Bias is on a Valve amplifier. Hope you found it
useful. Feel free to browse our shop if you need to
replace your output tubes. KEEP MAKING GREAT MUSIC.
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