Mesa/Boogie Mark Five Owner's Manual download pdf (Page 25)

PAGE 21

And that’s right. Tubes don’t effect the bias setting, but the bias setting does effect how the tubes work. But how it effects the tubes is

difficult to measure.

When you set the bias (whether it’s by selecting the right resistors, as we do, or adjusting a trimmer — which is quicker) what you are

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 tubes’ 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 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 — 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 ARE playing. If the amount of current increases and decreases 440

times 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.

BIAS ADJUSTMENT (Continued)

1 2 ... 20 21 22 23 24 25 26 27 28 29

No comments

Mesa/Boogie Mark Five Owner's Manual Page 25