Mesa/Boogie Rectifier Stereo owner manual

BIAS ADJUSTMENT: (Continued) Voltage and current are NOT the same. Current is the AMOUNT of electricity, the ÒquantityÓ

Ñand is measured in amperes. Voltage is the degree of electric charge Ñ like the ÒpressureÓ to use the old water analogy. Let me illustrate how different voltage and 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 Ñ 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 tubeÕs plate. A small change in grid voltage, produces a large change in the amount of current flowing Ñ and thatÕs basically how a tube works. Say that again because itÕs super important: A small change in voltage at the grid causes a large change in current flowing to the plate. See, thatÕs the essence of amplification: A small change causing a large change. And here itÕs 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 youÕre not playing. And what drives your speakers is flucuations in that current flow when are ARE playing. If the amount of current increases and decreases 440 times per second, then youÕll hear an A note. If the fluctions in current flow are large and still at 440 per second, youÕll hear an A that is LOUD!

But for purposes of biasing, itÕs the amount of Òplate currentÓ flowing with no signal applied thatÕs important. Unfortunately current is hard to measure because the circuit must be interuppted Ñ as in Òcut the wireÓ Ñ and the meter spliced Òin seriesÓ 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 doesnÕt even state what the correct value should be. They only give the grid voltage that will produce that current. (ThatÕs the minus 52.) But that only happens if the tubes being used are Òin spec.Ó

As long as the tubes ARE Òin specÓ, the right bias voltage will always give the correct plate ÒCURRENTÓ Ñ but then thereÕs 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 tubeÕs 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 youÕre willing to except resistors and lights added into your amplifierÕs audio path Ñ which we arenÕt.

The other ÒadvantageÓ 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.

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