In sci.electronics.repair, on Wed, 30 Nov 2016 23:20:17 -0500, Mike
Duffy wrote:

On Wed, 30 Nov 2016 17:16:05 -0600, Jon Elson wrote:

The heater in the magnetron tube takes a couple seconds to warm to the point
where it will emit electrons. You can hear it easily on the old transformer
microwaves, the fan starts and it begins to hum, then a couple seconds later
the hum gets much louder. That louder hum is when the magnetron tube starts
conducting.

Newer microwaves with switching power supplies may delay turning on the HV
until the heater has warmed up, and they may not have that transformer hum,
so it may be harder to tell when the RF comes on.

+1, You are the only one who did not lambast the OP for using the term
'warm up' to mean 'cause the ambient MW intensity to reach its operational
range'.

Just for the record, I consciously used the phrase, because it reminds
me of simpler times, harmed only by the extra time it took to turn on
the radio or tv. (In the movies, sometimes they would turn something
on and it woudl start immediately. I did stay at a hotel once 8 years
ago that was also operating in the 30's and it still had the remote
speaker/channel selector for the central radio it used then. Each room
had one and the patron could swtich between two or three stations, and
adjust the volume, so when you turned it on, it went on immediately.
Unfortunately, the hotel finally closed.)

When I am judging time for extremely small loads (like softening butter
without liquifying it), I allow 4 seconds for my oven.

I'm pretty sure mine is less tnan 4 seconds becuase I really have used
7 seconds and found noticeable heating, more than I think 3 seconds
would have done, based on the prior 10 seconds.

BTW, if you get one of those slabs of chocolate chip cookies, precut,
they say to cook them in a hot oven of course, but 37 seconds per square
does a good job. Not like baked, but like a differen4 recipe.

Another reason to know the startup time is if I make two of them, it
only has to start-up once, so it needs less than twice the time, but the
instructions already say less than twice the time, and their
differential is greater than 4 seconds. I think there is some reason
for that other than start-up, warm-up time.

I find that the hum does not get louder though. Instead, I notice that the
fan speed lowers a bit, presumably because the supply voltage for the fan
is then being loaded down by the power consumption by the magnetron.