You can't measure hiss on a tube tester- the best way to test tubes for noise is to listen to them. Or get known quiet tubes from a source (the manufacturer of the unit?) to send you a set.
eBay is a good source of tube testers. There are some modern testers too- like the Sophia, that do allow you to listen to the tubes. But your equipment is always the final arbiter.
eBay is a good source of tube testers. There are some modern testers too- like the Sophia, that do allow you to listen to the tubes. But your equipment is always the final arbiter.

