BIAS ADJUSTMENT (Continued) doing is establishing the correct amount of idle CURRENT that flows through the power tubes.
But you can’t adjust the current directly, you can only change it by adjusting the amount of bias VOLTAGE that goes onto the tu bes’
control grids.
Voltage and current are NOT the same. Current is the AMOUNT of electricity, the “quantity” — and is measured in amperes. Voltage
is the degree of electric charge — like the “pressure” to use the old water analogy. Let me illustrate how different voltage a nd current
are:
When you scrape your feet across a carpetted floor in dry, wintery conditions, your body can become charged with 50,000 to 100,000
volts of static electricity. And when you reach for the door knob, a spark jumps and you feel it! The voltage is super high but the
current (measured in micro-amps) is tiny - otherwise you would die from electrocution.
Contrast this with your car battery, which puts out a mere 12 volts. You can lay your hands right across the terminals and not feel a
thing. Yet the amount of current available can run to several hundred amperes .. enough to turn over a cold engine and get it started.
So current and voltage are two totally separate electrical parameters — though when you multiply them together, you get POWER,
which is measured in watts.
When you set the bias of an amplifier, you are adjusting the static VOLTAGE at the control grid of the tube in order to produce a
desired amount of idle CURRENT flowing to the tube’s plate. A small change in grid voltage, produces a large change in the amount
of current flowing — and that’s basically how a tube works. Say that again because it’s super important: A small change in voltage at
the grid causes a large change in current flowing to the plate. See, that’s the essence of amplification: A small change causing a large
change. And here it’s a small voltage change causing a large current change.
The bias conditions are what determines how much current flows through the big power tubes when you’re not playing. And what
drives your speakers is flucuations in that current flow when are playing. If the amount of current increases and decreases 440 times
per per second, then you’ll hear an A note. If the fluctions in current flow are large and still at 440 per second, you’ll hear an A that is
loud!
But for purposes of biasing, it’s the amount of “plate current” flowing with no signal applied that’s important. Unfortunately current is
hard to measure because the circuit must be interuppted — as in “cut the wire” — and the meter spliced “in series” with the broken
circuit. But measuring VOLTAGE is easy. It is not necessary to interrupt the circuit because a voltage reading can be taken in parallel
with the circuit intact.
Thus, as a matter of convenience, most bias settings are given in volts at the grid ... even though current through the plate is the
important factor. In fact plate current is so inconvenient (and dangerous) to measure that Fender doesn’t even state what the correct
value should be. They only give the grid voltage that will produce that current. (That’s the minus 52.) But that only happens if the
tubes being used are “in spec.”
As long as the tubes are “in spec”, the right bias voltage will always give the correct plate “current” — but then there’s no need for the
bias voltage to be adjustable! If the tubes are not in spec, then the only proper way to re-set the bias is to cut the circuit and measure
the current while adjusting the bias ... but no manufacturer I know even states the desired current value! Be that as it may, when the
original bias voltage is altered far enough, it will compensate for the tube’s abnormal performance and the correct amount of idle
current flow may then be restored. Clearly this is something most repair techs should not attempt.
Some newer amps have LED indicators connected to the circuit which will turn on when the right threshold of current flow has been
reached. This is an improvement, and almost worthy if you’re willing to except resistors and lights added into your amplifier’s audio
path — which we aren’t. The other “advantage” of this system is that it allows some amp manufacturers to avoid matching their power
tubes. The thinking is that adjusting the bias to each tube separately eradicates the inherent differences between the tubes by
insuring that the same current flows through each one.
Again, this has some merit ... but it’s still not as good as using tubes that are matched in the first place because compensating for the
mis-match causes the push-pull circuit itself to become unbalanced. Two wrongs don’t really make a right.
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