Owner`s manual

BIASING YOUR AMPLIFIERS

DON’T WORRY IT IS SUPER-EASY!

Convenient test points for checking the standing current of each EL34 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.

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 EL34 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 +575 volts DC for the plate, and about —75 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 +575 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 575V

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 300 millivolts is attained. This corresponds to 30 milliamps of standing current because Ohm’s Law tells

us that 0.300VDC divided by 10 ohms (the cathode resistor) equals 30mA. 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 SNAPPER amplifiers if you adhere to the

procedures described above, and check your EL34 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!