GZ34 lying about idle

[R.G.]

I see quite a lot of preamp tubes that flash like that. I don't think it's much of a problem. In fact, I was kind of under the impression that the tube designers made the little bit at the bottom of the heater flash on purpose, since it ended up reducing the inrush current and protecting the rest of the heater overall. Could be wrong though.

I don't think so. Tube designers were solving a different problem with a limited set of tools.

Heaters are a series string of heated wire. If part of it dissipates too much power, that part gets hotter and softer than the rest of the heater wire. It has both the heat and thermal expansion stress concentrated at that point. And that's where it will either burn in two or mechanically break when it goes.

Every electronic part has a lifetime limited by one of its breakdown mechanisms. For tubes there are several major ones: emission surface wear-out; deposition of emission materials on grids and plate; evaporation of heater wire; mechanical breakage of heater wire; mechanical breakage of other structures; outgassing with heat. There are others, but these are biggies.

The breakage things are sensitive to mechanical vibrations as well as impacts and stress, but they also happen due to fatigue from thermal cycling. Each time you heat a tube from cold to hot, and back again, the hot things expand, putting stress on the fixed points. Eventually something breaks.

Each second a tube runs, it emits electrons, boils off molecules of emission material from the cathode, and heats molecules of gas away from the parts and enclosure. This can wear out or poison the cathode, gas the tube, or thin down the heater wire.

When a point on the heater wire gets thin, it has a higher resistance, so it eats more power than other parts of the wire. It gets hotter, emits more, and gets even thinner. At some point, the next heat cycle breaks the heater at that point, an arc burns it open, and the tube cools for the last time.

Making a part of a heater flare is either knuckleheaded, which tube designers were NOT, not easily correctible, or designed obsolescence, which was popular in the late 50's and 60's. Could be, but I don't think so.

For long thermal cycling life, you never want rapid heating. You want gentle, slow heating up to, but never over the operating temperature. The slower the temperature change happens, the more gentle the thermal expanion stresses happen and the later a fatigue failure happens. The lower the peak temperature of each heating cycle, the more cycles you get before failure.

A flaring tube heater is a signal that another significant chunk of your tube's life is now gone. I read some analyses of tube life once that produced the conclusion that by eliminating thermal cycles, one could run a preamp tube within its power dissipation limits for upwards of 50K hours. That 5.7years, 24-7.

If you want tubes to live long and prosper, start them slowly, keep the power and temperatures within ratings, and don't thermal cycle them a lot. Overtemperatures reduce the expected life. The degree of overtemp determines how severe the thermal cycle is, and that can reduce additional available cycles down to one in a hurry.