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Below is a copy of the instruction sheet we include with all of our $25.00 bias probes. Congratulations! By owning a bias probe you’re not at the mercy of a tech anymore and you are now in control of your tone. There are two different kinds of bias probes, one type that uses a resister in the circuit and measures in millivolts and the second type that does not use a resister and measures directly in milliamps. Ohm’s law states that “one volt equals one amp” so the measurements are the same. Our probes measure directly in DC Milliamps so you will need to set your multimeter to the DCA mode (DC Amps, NOT VOLTS!) and if the meter is not auto sensing then select the 200mA scale. If you have purchased one of our “Cheap Multimeters” then you will find this setting right between 3 to 4 O’clock. Insert the black plug from the probe into the “Common” input and the red plug into the positive input which is usually marked mA. DO NOT CHANGE THE FUNCTION SWITCH ON YOUR MULTIMETER WHILE IT IS HOOKED UP TO THE AMP! This will blow the fuse in the meter and you will need to replace it! Make sure the meter is set properly and the probe is plugged into the meter before you plug the probe into your amp. Remove one of your power tubes and plug the probe in. Place the tube into the top of the probe. If you had to remove the amp from the chassis then make sure that you have plugged the speaker back in. You must have a speaker load on the amp when you are adjusting the bias! Turn on the meter and then turn on the amp leaving it in the standby mode for about a minute, then switch to the play mode. Your meter will read out the plate current in milliamps. Turn the trim pot to adjust the plate current to the desired setting. Wait about 5 to 10 minutes with the amp in the play mode and do a final bias adjustment after the tubes are hot. Congratulations again! You have just biased your amp! The following info is for octal power tubes. If you measure the DC plate voltage from pin #3 of any power tube to ground then you can use the formula below to calculate the current draw. The formula for biasing is the plate dissipation of the tube divided by the plate voltage (let’s use 480 for this example), multiplied by 0.7 or 70%. This will get you out of crossover distortion and is a good setting for most applications. You can go as high as 85% percent but you will wear out the tubes quicker! Plate Dissipation: 25 watts for 6L6’s, EL34’s, EL34II’s, E34L’s, 6CA7’s or KT77’s. 23 Watts For the JJ 5881’s 14 watts for 6V6’s 35 watts for KT88’s and 6550’s Example: 25 watts divided by 480 = .0520 or 52 milliamps X .7 = .036 or 36 milliamps Most 50 and 100 watt amps using these tubes will have between 450 to 480 plate volts so even if you do not measure your plate voltage you will find that a setting between 35 to 40mA will be acceptable. The following info is for EL84 and EL844 power tubes. If you measure the DC plate voltage from pin #7 of any power tube to ground then you can use the formula below to calculate the current draw. The formula for biasing is the plate dissipation of the tube (12 watts for standard EL84’s or 9 watts for the JJ EL844’s) divided by the plate voltage (let’s use 350 for this example) times .7 will just get you out of crossover distortion and you can go as high as .85 percent but you will wear out the tubes quicker! 12 watts divided by 350 = .034 or 34 milliamps X .7 = .024 or 24 milliamps Or if you’re biasing the JJ EL844’s the formula would be as follows 9 watts divided by 350 = .0257 or 26 milliamps X .7 = .0182 or 18 milliamps If you’re verifying the bias on a Class A or Cathode biased amp you’ll find that most manufacturers run the tubes at 100% of the tubes maximum dissipation. This is standard/normal.
Bias Probe instructions
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