Tube testing

[microstrip]

One of the drawbacks of tubes is that they need to be changed after a certain period - going from 1000 hours in some tubes to 10000 hours for others types, sometimes depending a lot on the equipment.

How do you check for the need of tube replacement in your system?

I have a home made dynamic tube tester and check mainly for transconductance - I measure it and record it when I buy new tubes, but sometimes I feel that I am replacing the tubes before it is needed. Modern Audio Research equipment have an hour counter and advise to replace after a typical use of 4000 hours for signal and 2000 hour for power tubes.

Some people advise to check for variance of plate resistance, but I could not find any evidence of correlation between its variation and tube life.

My VTL MB750's tubes last longtime. As they have B+ fuses, time to replace the tubes is easy to establish - the first two time it blows a fuse I replace the faulty tube and fuse, at the third fuse I know it is time to replace the twenty four 6550s ...

 

[mep]

24 6550 tubes. Yikes! And I thought replacing 12 was bad with my Jadis Defy 7 MKII that is now gone and not missed. For output tubes, I always used bias as an indicator of tubes wearing out. For small signal tubes, looking at the getter is a good indication of tube health. However, many small signal tubes go noisey before they have reached end of life. In a perfect world, all tubes would die a natural death of cathode depletion. Some tube testers have a cathode life test which measures the emission of the cathode. This is the best indicator for the health of a tube.