Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 09:49 AM 3/25/2006, you wrote:
Original poster: "Henry Hurrass" <dr.hankenstein@xxxxxxxxxxxxx> I think the suggestion that a power tubes' plate is supposed to glow a "dull red" during normal operation is ignorant. If the plate is glowing, the plate's maximum dissipation is obviously being exceeded and the life of the tube(s) will suffer due to, in part, the following factors:
This isn't true. Many tubes are specifically designed to run with the plates glowing. Particularly if the tube has to transfer heat by radiation, there's a significant advantage in getting the tube hot, because radiative transfer goes as the fourth power of the temperature. The key is in keeping the glass envelope cool enough that it won't soften/melt.
For higher frequency tubes, it's essential to run them hot, because you need physically small size to reduce transit time to run at high frequencies (as well as to reduce parasitic C and L), but still need to dissipate significant power. The only way to get there is use that T^4.
An interesting example is a radiation cooled traveling wave tube for space applications (like on a ComSat). They run the collectors at several hundred degrees so that they can reject the heat to cold space without needing a big radiator.
There are a LOT of transmitter tubes designed to run pretty bright red in normal operation.
The key is to read the manufacturer's data sheet, and adhere to what's given therein. (I suspect more tubes get killed by incorrect filament power/temperature or by not limiting flashover energy, than from excessive plate dissipation)