Bias Probe Instructions for the Eurotubes $25.00 Bias Probe Head.
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 (6V6, 6L6, EL34, E34L, KT77, KT66 or KT88) 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.
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 (25 watts for 6L6’s, EL34’s, E34L’s or KT77’s or for 6V6’s use 14 watts and for KT88’s use 35 watts) divided by the plate voltage (lets use 480 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!
25 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.