In sci.electronics.repair, on Fri, 2 Dec 2016 20:25:14 -0800, Bill
Martin wrote:

On 11/30/2016 03:16 PM, Jon Elson wrote:
micky wrote:

How long does it take a microwave oven to warm up?

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.

Jon

Heh, you could put a flashbulb in there, no missing that indicator...one
of the "urban legends" of the day, when I was working on military radar
systems was that you could take out all the flashbulbs in the base
exchange store if you forgot to blank the output as it rotated by that
direction...no idea if it's true or not, but 5MW could fry most anything
if you wanted to bad enough.

30 years ago I saved a couple flashbulbs for the expected shortage, but
I don't know where they are now.