I have to disagree with the methods discribed to match tubes. You can not match tubes using a tube tester along " (AVO 160, Hickok 539b & TV DU/7) ". You do need a device to host the tube that's being tested for matching. That device could be an amp or a tube tester. You then need a way of measuring bias current. You can buy a bias probe and meter on ebay, or you can make a bias probe cable and use a multi meter or you can modify your amp to behave like a bias probe by adding resistors which alow the measurements to be made. So lets say you measure your tubes....and you make this simple....lets say you use a tube tester to host the tubes and in adition you use a bias probe. So you run 4 tubes thru the process and you find you have IP (bias current readings from probe) of 24.1, 23.9, 22.9, 23.1 and you find your tube tester says the transconductance values are GM 15000, 14900, 14000, 13900. what does this mean. well it means you have two matched sets. The pair that comprise of IP of 24.1, 23.9 with GM's of 1500, 14900 and the pair that comprise IP 22.9, 23.1 and GM's of 14000, 13900 are the two matched sets that must be used together . In the example I gave, we are testing 6550s' which have an expected transconductance value when new on my tester of 11000 which would make all tubes in this example score 126% or better.