Manley SNAPPER Owner's Manual

Contents

Manley SNAPPER Owner's Manual Manual pdf 8 page image

BIASING YOUR AMPLIFIERS8DON’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 thechassis, 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 ispartially due to the practice of using the term bias for many situations throughout the electronics industry ingeneral, along with the nature of the requirements needed to provide a proper electrical environment in which toefficiently operate a given audio output tube. Here, therefore, is a concise (oh really, Mitch?) picture of what isgoing on in the EL34 output tubes when the bias control is adjustedWhen 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. Sixand 12 volts are also applied to the appropriate tube filaments. All of these voltages are referred to the output tubecathode voltage, which is usually within a few volts of circuit ground or chassis potentialThe heater (orange glowing filament) of the tube inside the cathode sleeve rises to about 1200 degrees C. This heatsthe special oxide coated nickel cathode sleeve and, after 10 or 20 seconds, the oxide produces a generous cloud ofelectrons, which are negatively charged. The electrons are strongly attracted to the large positive plate, which isbeckoning them from across the vacuum with an irresistible +575 volt potential. Notice here that, as usual, thediffering charges, plus-to- minus, attract, and like charges plus-to-plus or minus-to-minus repel. Without somecontrol, the tube is in danger of running at full cathode current, which due to prodigious input power from the 575Vpower supply, would lead to red-hot plates and quick tube destruction. Now the control grid is used to throttle backthis hazardous situation, by the careful application of a negative voltage to the control grid. This applied negativegrid voltage, Mr. Bias, retards the flow of electrons from the cathode to the plate by repelling the cloud of electronsaround the cathode with a like charge, minus-to-minus, and confining most (or all) of them to the space between thecathode 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. Howmuch this electronic venetian blind has been opened or closed may be observed by watching the cathode currentwith a voltmeter at the tube s test point. The higher the current, the more open the blind or valve setting, hence thehigher the voltmeter reading. Notice that contaminants in the vacuum and other factors can sometimes conspire toreduce or eliminate the effects of the negative voltage impressed on the control grid, which can lead to runawayover-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 andchassis ground, with the positive red meter lead to the test point, amplifier on, and zero audio signal present. Justleaving your preamplifier turned off is not not a bad idea. Adjust the corresponding bias trim control until a meterreading of 300 millivolts is attained. This corresponds to 30 milliamps of standing current because Ohm’s Law tellsus that 0.300VDC divided by 10 ohms (the cathode resistor) equals 30mA. Changing the standing current throughone output tube may slightly alter the amount of current flowing through the neighboring tubes. Therefore, checkthe remaining output tubes and readjust as necessary. Turn off the amplifier and replace any tube that cannot bebrought to the correct standing current when varying its bias adjustment control. Do not let the standing currentexceed 50 milliamps (500 millivolts) except briefly as necessary during the adjustment procedure or you might be infor 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 theprocedures described above, and check your EL34 standing current at least once every 2-3 months. Generally, thelonger 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 westock all the tubes used in these amplifiers, pre-screened and tested, and at very reasonable prices too!