> The only thing I'm not sure of is the microwave oven - does the
> microwave-generation-thingy depend on vacuum?
It depends on a cavity magnetron which is a vacuum device.
> Rather, it is the practical knowledge that experience brings,
> the "feel" for how to use them, that is at risk.
As vacuum devices are still in use and still being designed into
new equipment I doubt that this "feel" for how to use them will
be lost for a long time.
> Certainly television and radio do not depend on vacuum tubes today
> (well, certainly not on the receiving end; the technology exists to
> transmit with transistors, but I don't know whether it can handle the
> power levels appropriate to mass broadcasting).
The transmission chain almost invariably uses vacuum devices for power
levels of a few KW or more. The ease of implementation of a single
device high power stage still outweighs the benefits of multi module
solid state outputs.
> Radar - as above: the power transmitting stage may still be vacuum
> tube, but certainly _could_ be transistor;
Solid state devices just can't handle the high power levels needed for
long range radar so magnetrons or klystrons are still used.
> the rest definitely can be.
Not practically, 500KW at HF (4MHz to 26MHz) is easy with one valve and
readily available. There are AFAIK no 500KW solid state HF transmitters
available. Even the modulator is valve, it's a 750KW switch mode power
supply with a nominal 11KV DC out that can swing from 0V to 22KV.
> Comm satellites - aren't they solid state these days?
Low power ones are, but you need a big dish to hear those, broadcast
satellites use TWT output stages. Most ground stations use klystrons
for the uplink, even for low power because they out perform transistor
equivalents.
If you'd like to see somewhere that still relies on vacuum technology
then go here ..
http://tx.mb21.co.uk/gallery/woofferton.asp
.. which is where I am.
Lee.
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Received on Tue Jun 22 2004 - 05:07:04 BST