Eurotubes Bias Probe for Octal Base Power Tubes. This probe measures in DC Milliamps and will work with any Multimeter that can read in DC Milliamps More accurate because the screen current is not added to the plate current Save money by adjusting your amps bias quickly and easily Works with 6L6, EL34, E34L, 5881, 6V6, 6CA7, KT77, KT66, KT88, 6550 etc. Please note that the central guide pin is fragile just as they are on a tube base. You can break it if you force it into a socket. Breakage is NOT covered by warranty! We take great care in packaging to insure your probe arrives in tact!