dac90a - to replace or leave alone??

[Yorkie]

A valve tester won't necessary detect issues which may cause hum. I think in the UL41, (and of course other valves), that the hum is induced from the heater to the cathode. A valve tester will test for heater/cathode shorts or leakage and the valve may test good in that respect, but that doesn't mean that it will not hum in use. The other tests a valve tester conducts are of course heater continuity, and emission, which again, may be fine, but the proof is in the hearing, which is the most critical test of all!