I really think these should have been provided with the Bias Master. These are an essential accessory for the Bias Master BM4 to enable testing of EL84's and similar valves. See my review of the Bias Master BM4. I believe there are several versions of the Bias Master available, one for EL34, 6L6, 6v6 etc. with octal probes and another with just these for EL84's. I've read the menu - I'll take everything ! (Oh alright, not quite everything, because some amps have 4 EL84's and I only have 2 of these probes).