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. Congratulations again! You have just biased your amp! The following info is for octal power tubes. Wait about 5 to 10 minutes with the amp in the play mode and do a final bias adjustment after the tubes are hot. Turn the trim pot to adjust the plate current to the desired setting. Your meter will read out the plate current in milliamps. 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. If you had to remove the amp from the chassis then make sure that you have plugged the speaker back in. Place the tube into the top of the probe. Remove one of your power tubes and plug the probe in. 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. Insert the black plug from the probe into the “Common” input and the red plug into the positive input which is usually marked mA. If you have purchased one of our “Cheap Multimeters” then you will find this setting right between 3 to 4 O’clock. 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. Ohm’s law states that “one volt equals one amp” so the measurements are the same. 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. 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. Below is a copy of the instruction sheet we include with all of our $25.00 bias probes.
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