In article ,
Heinz Schmitz wrote:

Michael A. Terrell wrote:

There are a lot of things that are involved in MTBF.

Yes, indeed.
But I guess that we are interested in those which we can influence.
Like e.g. (?)
- heating before applying anode voltage,
- avoiding long periods without anode current,

Useless on low power vacuum tubes that are not used in some bizarre
circuit. Its a different matter on high power transmitter tubes.

Your kind posting shares a property with many usenet postings:
It provokes the most valuable question: "Why?".

So the real question is, is there anyone out there that can give a comprehensive
answer as to why?

Even the belief stated above that "Its a different matter on high power
transmitter tubes" with respect to "heating before applying anode voltage"
appears questionable.

Some 45 years ago I worked at several different stations as a broadcast engineer
and the old-timers there trained us young kids to be sure we thoroughly warmed
up the filaments before applying the plate power. The precise reasons for doing
this were never made completely clear. With the notable exception of one newly
installed RCA BTF-10D,the vast majority of the transmitters I worked with used
mercury vapor rectifiers, one even used two stacked banks of mercury vapor
rectifiers to provide the 17 kV B+ voltage. Obviously in transmitters using
mercury vapor rectifiers it is necessary to warm the rectifier filaments up
before applying power to the plate transformer(s), since dire things would
transpire if the mercury wasn't properly vaporized before the rectifiers where
hit with power from the plate transformer(s). But ignoring the mercury vapor
rectifiers was it really necessary to warm up the power tubes before applying
plate power? I was surprised a few years back to read in the instruction manual
for a tube transmitter the statement that it was not necessary to warm up the
power tube filaments before applying plate voltage! And they weren't just
saying that the automatic sequencer would take care of the correct sequencing of
the filament and plate power, the statement made clear that they were talking
about actually applying filament and plate power to the tube simultaneously. I
found this surprising since I had been trained to warm up the power tubes before
applying the plate voltage, and there are other reasons for warm up, so I asked
about this on a forum or newsgroup, the exact one has escaped me. A current
broadcast engineer replied and said yes that was true, it was generally not
necessary to warm up most power tube filaments before applying the plate
voltage. He did go on to explain that this was not universally the case however
and said there were certain types of tubes that did need to be warmed up first,
but I have forgotten what the reason was that he gave, probably something about
filament construction or material.

So does anyone have a comprehensive explanation of this cathode warm up issue,
for both low power and high power tubes?

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/