my amp need to
have the bias adjusted?
a single word - yes. All existing amps, with no exception, will
benefit from having the bias set correctly for the power tubes.
is a huge amount of incorrect information traveling the world
about "fixed bias" and "automatic" or "auto" bias.
bias, if the term is being used correctly, refers to a negative
voltage being fed into the control grids of the power tubes via a
negative voltage bias supply.
supply runs at very little current and 25 to 60 negative volts.
current amps have a truly fixed bias - that is, the voltage is
permanently set by a resistor in the bias supply - to provide just one
Amps have an adjustment pot somewhere on the amp chassis.
economy some amps do not have an adjustment pot, possibly because
of cost, or possibly because the manufacturer does not have confidence
in you to adjust it right.
it can not be made to accommodate all tubes - totally
Automatic Bias name is often applied to a cathode biased amp.
this circuit the tube's bias is set by a high wattage resistor
connected to the tube's cathode.
looks pretty permanent!! -
but, the bias can be
measured and can be adjusted by changing the resistor.
FACT. Cathode bias does not, in any way, automatically make an
amp Class A, no more then a fixed bias prevents it from being a Class A
amp. Either one can be either Class A, or Class AB.
THINKING. Overcome the wrong concepts about fixed and automatic
bias by doing a little logical thinking.
do we have "matched tubes" and how are they matched?
are matched by measuring their characteristics, current draw,
transconductance, and finding others that have the same character.
matching process DOES NOT go inside the tube and
change the grids, plates and cathodes to match them to another tube,
the tube is already manufactured and sealed in a vacuum.
very fact that they have to be matched clearly shows you that they
are not able to be manufactured to extremely close tolerances - there
are considerable variations between individual tubes, and even larger
variations between brands.
could not possibly think that the manufactures make the tubes to
all these hundreds of different specifications on purpose - an absurd
- since the tubes are obviously NOT scientifically consistent, how
in the world can ANYONE make an amp that knows about these variations
and adjusts itself automatically?? and
with just a 10 cent
car is a very good example of the same variable science
can buy new tires all day long and NEVER get a set that is
automatically balanced - impossible as the tire manufacturing process
is not that consistent.
must have the wheels balanced or the car will shake itself to
a million dollar race car has to have the wheels balanced.
power tubes in your amp, even if they are a matched set most likely
have very different characteristics from the previous ones - the bias
needs to be adjusted so they run in the correct operating range.
power tubes that came with the amp will all have been different
EVEN IF the company supplied matched sets, each set will have different
specs from some of the others.
is impossible to order 1000 sets of exactly the same match specs -
that would require testing and sorting of maybe 150,000 tubes.
this in mind, a simple look at the bias supply will show all
have the same bias resistor (if it is not adjustable) - how could it be
right all the time? - it can't and it is absurd to think it can be.
you see it has been changed, as it may be with a very high end amp,
there will have been an effort to bias the amp correctly, but this does
not prepare it for the future - unless you have exactly the same
matched set of tubes with exactly the same specs from the same
manufacturing batch you still would benefit by setting the bias.
companies supplying their own brand of tubes change brands over
the years, again resulting in a different set of specs even if their
color or number coded labeling is the same.
summary. there is no magic - everything is subject to the
normal electrical and physical laws. no one can bypass reality.
- set the bias!