I have owned for more than thirty years an AVO Mark IV Transconductance Tester. It has always worked reliably and has been the best tester I have seen to accurately determine if the tube is good. No tester is perfect. But the AVO Mark IV is the closest I have found.
You must remember to differentiate between a simple emission checker and a unit which measures transconductance. Very often you will find adequate emission but the tube will not amplify!
The AVO Mark IV is accurately calibrated to display not only whether the tube is bad but also displays the transconductance which can compare with the manufacturers data.
The AVO Mark IV is British and they were exported to North America. I bought mine for 30 Pounds. I found this on on Ebay for somewhat more:
http://www.ebay.co.uk/itm/AVO-MARK-I...item20f2c163d5