A question we hear a lot. Do I need
to have my Valve amplifier biased?
So What is Bias?
Well to put it simply the bias of a Valve amp is set
by adjusting the grid voltages to set the idle current
of the output tubes. This idle current can usually be
set by adjusting a variable resistor (this can be visible
in the form of a variable screw on the back of the amp
or mounted inside the amp chassis). If the idle current
is set too high (under bias) then the valves will run
too hot and the life of the valves will be reduced significantly.
If the current is set too low (over bias) the tone of
the amp may be thin and lacking volume.
Why do I need to Bias
Valves vary in specification. The chances of your new
tubes being exactly the same specification as your old
ones is quite small. There is a good chance that they
will be within an acceptable similar specification window
in which case there would be no need to bias but you
can't be sure that this will definitely be the case.
So the reason you need to have your bias checked whenever
you replace your output tubes is to obtain the best
possible sound your amp is capable of and to maximize
your tube life.
Do I need to Bias if I change my pre amp tubes (12AX7
ECC83, 12AT7 ECC81 etc)
In a word NO. The pre amp tubes work differently to
output tubes and do not generally require any form of
adjustment. The Bias only needs checking when replacing
output tubes such as 6L6, EL34, EL84, etc.
What about matched tubes do they need biasing
YES. It is very important to have matched output tubes
fitted to your amp. But it is also important that you
get the bias checked when fitted. If your amp is fitted
with say Two EL34 in the output stage. You will often
find that amp designs are such that one tube will be
pushing the signal whilst the other pulling through
the output transformer and ultimately to your speaker.
Now at idle current when there is little or NO signal
the tubes need to be perfectly balanced. If they are
not one will push or pull harder than the other. So
when the signal hits them one side of the output section
will be working much harder than the other and the amp
has a potential of sounding bad. Particularly at high
volumes where the one valve runs out of steam whilst
the other is still ready to push more. This is a simplified
explanation but hopefully gets the point across about
the importance of using matched output tubes. The Bias
would need to be set to ensure the tubes are both idling
at the correct current whether the tubes are matched
or not.
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