Sorry about the long absense. I've had lots of unexpected complications in starting my new job, most of them good, but complications none the less. Since this thread has had a long time to settle down, it's a good time to make good on some PM good-faith promises I made to Scott which can help to clarify the issue of tube replacement. Before I get started, just so it's plain, Scott and I are good with each other, and this is in no way a further argument, but a good opportunity to help clear up some widespread misconceptions about tubes.
on occassion I have had to use a spare. Once when my guitar player dropped his 100w Marshall one of the 6550s had some loose stuff rattling around in it, replaced with my spare.
Big audio power tubes, especially 6550's and KT88's, have lots of internal mass relative to their support structure, and a large physical shock can move the internal parts around without breaking the glass envelope, rendering an otherwise perfect tube shorted. That's exactly what happened to the KT88 that went in my Trace Elliot VR400 when it fell. What happens is that the large mass of the plate and beam-former are really too heavy for the thin metal rods that support them and due to their higher operating temperatures and the larger amount of space than most other audio power tubes like 6L6's or EL34's means that it is quite possible to knock them around enough to bend the internals to the point that places that shouldn't touch, do touch. It's no so much an issue with old manufactered 6550's as they used stronger support beams, but modern tubes can be knocked into shorts; the flipside is that they can also be knocked again to move apart, but it's not a good idea to use a tube that you know has mobile internals. That's not to say that it is common, but it is possible. NOS US tubes, especially GE 6550A's and 8417's, have their own set of problems. If you heard rattling, it was probably toast. However, had the amp been running at the time, it would have required some pretty serious repairs. My Trace Elliot has internal relays which kick in case of failure that a Marshall does not, and even with the relays, the fuse-holder melted before the fuse blew.
The few times I have replaced a pre amp tube at a gig is not because of some failure but that the amp had a drop in power or that is was breaking up a little, my technique ? trial and error.
That's more a case of the aforementioned "expectation" effect, because preamp tubes have so little relative emission to their output that their aging process is so slow as to be imperceptible, even with Chinese junk. It
is possible for most musicians to identify and change a microphonic preamp tube, but not on a gig. And in that case, the tube was already bad from the factory; it did not "go" bad.
