[SwizzyMan]

Everything you need to know about the 6X5. In contrast to ordinary filament-type full-wave rectifiers, the 6X5 is one of a family of heater-cathode full-wave rectifiers. Unlike some large members of the family, there is no connection between heater and cathode in the 6X5 family. Apart from filament/heater burnout, any vacuum rectifier tube has two major vulnerabilities. One is excessive peak inverse voltage, and the other, excessive peak plate current. Both may cause arcs and shorts in any vacuum rectifier.Heater-cathode rectifiers like the 6X5, with no connection between heater and cathode, have a third vulnerability, which is heater-cathode voltage at or near redline. No heater-cathode potential can appear unless the heater is referenced to ground, or worse yet, operated at ground potential (the cathode carries the full B+ potential above ground). Unfortunately, this is the case in almost every application of the 6X5, as it is operated on the heater circuit carrying all the other tubes, a circuit almost invariably tied to ground in one way or another.
The alternative, of course, is a separate rectifier filament winding that could be "floated" without reference to ground, but that separate winding is what designers were trying to get away from in the first place... It was common, unfortunately, for designers to redline many parts (especially tubes) to get the most "bang" out of their products for their invested buck. With the 6X5 being operated at redline (or beyond) in certain radios, there were a lot of bangs. When you redline any tube, you are simply asking for trouble. Redline output and rectifier tubes, and you're asking for BIG trouble. The 6X5 was apparently pushed to redline on heater-cathode potential and/or peak plate current in sets where it is notorious for shorting out, but this was a designer's fault, not the fault of the tube itself (it may also be possible that the tube was originally rated beyond its actual ability to stand up to punishment). The 6X5G and GT are at equal risk of developing a short, it's really how the tube is treated in circuit that affects its chance to short out. 6X5's will last a long time if run comfortably within their ratings, and especially on a floating heater winding. If the heater has no reference to ground, the major cause of a heater-cathode short is removed. The only caveat is that the floating heater winding should be able to withstand full B+ potential to ground in case there is such a short.