Eurotubes Pro One Bias Probe for Octal Base Power Tubes. Reads both plate current and plate voltage simultaneously! Self-contained, no knobs, no switches and works without batteries! Compatible with 6V6's, 5881s, 6L6's, KT66's, EL34's, E34L's, KT77's, 6CA7's, 6550's and KT88's Dial in your bias using our Test & Play technology! 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!