Where's a good source for Russian and Chinese 6AS7?
We stock both and both are available on ebay.
Usually when we ship a set of amps we precondition the tubes first and then play them. In this case since the amp was used it came with its own tubes. So we tested them and played them. Its not quite the same...
At any rate, over the years we've been researching how the tubes fail. Most of them fail immediately after shipment or else when they reach the end of their service life (which can often be 10,000 hours or more). The 'after shipment' part is what concerns us most.
What we've been finding is that most of the tubes fail due to arc-over and subsequent damage to the internal elements. It appears that the mechanism is the cathode coating, which is very much like a paint applied to the cathode (but in this case improves the cathode's ability to emit electrons; this is true of all tubes). The cathode coating can flake apart just like old house paint and if the tube is conducting the tube can arc as the flakes are conductive. Since many of these tiny flakes can be loosened during shipment, if the tube is subsequently subjected to a prolonged heating cycle most of the flakes that are occurring can fall harmlessly to the bottom of the tube and then the tube can provide normal service after that.
This applies to any in-directly heated power tube and is nothing unique to the tube we use. IOW running the amp in Standby for a few hours is a good practice anytime its been shipped, assuming the amp has a Standby function.
