An emissions tube tester only tests for shorts & cathode emission. The test provides a relative good-bad reading by testing all the tube elements connected together, but not the cathode & heater. As such, the emissions tester tests for cathode emission measuring the tube as a diode or rectifier, that is just two elements. The emission tester also checks for shorts. The emission tester simply tells you whether a tube is usable or not. The tester gives you an indication how strongly the cathode emits.
So the 5998's are "good" because they not shorted and cathode emissions are 50-60%. That emission rating is only accurate to the particular tube testers voltage, amperage and resistance on that tube. Emissions can be roughly equated to remaining tube life. So put them in the Crack and listen. In circuit is always the best test, once you know that the tube does not have any shorts.
More sophisticated tube testers measure transconductance (Gm). All of the tube elements are tested. But, although a better overall test of tube health than emissions, it does not test the most important criteria in preamp/driver circuits. In preamp and driver circuits, usually triodes, voltage gain (Mu) is important for matching tubes because it is a measure of the tube's gain. Plate resistance, (Rp) determines the output impedance of the preamp/driver circuit. No readily available commercial tube testers measure Mu or Rp. So, again, the in circuit test is the best, once you know the tube is not shorted.
All of that being said, the 5998, in the Crack, is the power tube. Give yours a try!!
Cheers,
Geary
