On 19 Oct 2012 at 6:58, J. Forster wrote: > If you're worried about the filament burning out, operate the thing at > maybe 10% below design voltage. Lifetime is a power law of applied > voltage. That should lengthen the life a lot, but still maintain > emission. Eimac says no more than 5% low....but that is for power tubes. Ken W7EKB