Bias probe for indirect measurement of quiescent bias current of a tube. The adapter is inserted between the tube and the socket and gets connected to a standard voltmeter (not included). Pay attention to the quality of the measuring device and sufficiently charged batteries!

The quiescent current is measured indirectly via a precision resistor integrated in the adapter. The advantages of this measuring method are harmless voltages at the measuring plugs, which are in the mV range, as well as short ways for the current, since it does not need to be fed through the measuring device (common practice with many measuring adapters of Far Eastern production). As a result, there is no risk of oscillations of the amplifier which can lead to possible damage of the tubes or the amplifier itself.

Note: This adapter cannot be used when additional tube brackets are installed. These must be removed before using the measurement adapter and reattached after the measurement is complete.

Procedure for the measurement

Determine the resistance of the adapter, including the measuring leads. To do this, connect the adapter to the meter, but do not plug it between the tubes and the amplifier, yet. Select the measuring range “Ohm” on the meter and note the measured value, e.g. 1,1 ohms.

For the final measurement insert the adapter between the tube and the currentless amplifier and connect it to the meter, select the measuring range mV and switch on the amplifier. When the amplifier has reached a stable operating condition, read the value in mV from the meter and calculate the quiescent current based on Ohm's law.

Example:
measured voltage = 47 mV, resistance of the adapter = 1,1 ohms.
Formula (I = U / R) or in this case:
bias current (in A) = measured value (voltage in V) / measured value (resistance in ohm)
0.047V / 1.1 ohms = 0.042A or 42mA respectively

For the calculation of the correct bias current as well as further information on the bias measurement, please refer to further documents or the bias setting instructions in the info area, as well as the manual of the amplifier itself.

This measuring adapter is suitable for all EL34, 6L6, 6550, KT88, 6V6, 6CA7 and other tubes with the same pin layout.

Attention high Voltage! Measurements and work on tube amplifiers should only be carried out by qualified persons.