Instruction Manual

BIASING YOUR AMPLIFIERS
8
DON'T WORRY! IT IS SUPER-EASY!
Convenient test points for checking the standing current of each EL84 output tube is available on the top of the
chassis, near the output tubes. The test points are connected to each tube’s 10-ohm cathode resistor (inside).
Everything is clearly labelled on the chassis because we know you guys are prone to throwing away informative
Owner's Manuals.
Tube bias is a frequent source of confusion and misinformation, especially as it pertains to output tubes. This is
partially due to the practice of using the term ‘bias’ for many situations throughout the electronics industry in
general, along with the nature of the requirements needed to provide a proper electrical environment in which to
efficiently operate a given audio output tube. Here, therefore, is a concise (oh really, Mitch?) picture of what is going
on in the EL84 output tubes when the “bias” control is adjusted…
When the amplifier power is switched on, several voltages are quickly developed by the amplifier power supply.
Around +400 volts DC for the plate, and about –25 volts DC, low current and low power, for the control grids. Six
and 12 volts are also applied to the appropriate tube filaments. All of these voltages are referred to the output tube
cathode voltage, which is usually within a few volts of circuit ground or chassis potential
The heater (orange glowing filament) of the tube inside the cathode sleeve rises to about 1200 degrees C. This heats
the special oxide coated nickel cathode sleeve and, after 10 or 20 seconds, the oxide produces a generous cloud of
electrons, which are negatively charged. The electrons are strongly attracted to the large positive plate, which is
beckoning them from across the vacuum with an irresistible +400 volt potential. Notice here that, as usual, the
differing charges, plus-to-minus, attract, and like charges plus-to-plus or minus-to-minus repel. Without some
control, the tube is in danger of running at full cathode current, which due to prodigious input power from the 400V
power supply, would lead to red-hot plates and quick tube destruction. Now the control grid is used to throttle back
this hazardous situation, by the careful application of a negative voltage to the control grid. This applied negative
grid voltage, Mr. Bias, retards the flow of electrons from the cathode to the plate by repelling the cloud of electrons
around the cathode with a like charge, minus-to-minus, and confining most (or all) of them to the space between the
cathode and control grid. Much like a Venetian window blind is used to control sunlight entering a room.
When you adjust the “bias” control, the control grid is being made slightly less or slightly more negative. How much
this electronic venetian blind has been opened or closed may be observed by watching the cathode current with a
voltmeter at the tube’s test point. The higher the current, the more open the “blind” or valve setting, hence the higher
the voltmeter reading. Notice that contaminants in the vacuum and other factors can sometimes conspire to reduce or
eliminate the effects of the negative voltage impressed on the control grid, which can lead to runaway over-current
conditions and ultimate tube failure.
OK, Mitch, that's enough for now. My eyes are glazing over. HOW DO I CHECK THE BIAS?
Checking the standing current may be performed by connecting a DC millivolt meter between a test point and chassis
ground, with the positive red meter lead to the test point, amplifier on, and zero audio signal present. Just leaving
your preamplifier turned off is not not a bad idea. Adjust the corresponding bias trim control until a meter reading of
250 millivolts is attained. This corresponds to 25 milliamps of standing current because Ohm's Law tells us that
0.250VDC divided by 10 ohms (the cathode resistor) equals 25mA. Changing the standing current through one
output tube may slightly alter the amount of current flowing through the neighboring tubes. Therefore, check the
remaining output tubes and readjust as necessary. Turn off the amplifier and replace any tube that cannot be brought
to the correct standing current when varying its bias adjustment control. Do not let the standing current exceed 50
milliamps (500 millivolts) except briefly as necessary during the adjustment procedure or you might be in for a
surprise. (And we're not going to tell you what that surprise might be or it will spoil it for everybody.)
You should expect extended life from the tubes in your MANLEY MAHI amplifiers if you adhere to the procedures
described above, and check your EL84 standing current at least once every 2-3 months. Generally, the longer the
tubes operate, the less they should need standing current level inspection. And when that sad day comes, and the
tubes have reached their lifetime service limits, be of good cheer, for should you ever need replacements we stock all
the tubes used in these amplifiers, pre-screened and tested, and at very reasonable prices too! Oh yes!