"Walter Harley" wrote in message
...

Modern commercial radio transmitters (including FM band, 88-108MHz) by,
e.g., Harris use MOSFETs. So clearly it is possible to get up to 25kW.
I believe they also use them in 100kW AM band transmitters. A bit of
meandering on Harris' web page will tell you more.

I rather doubt that there are any tube-type 50 kW AM broadcast band
transmitters sold these days. The 50 kW solid state Harris unit seems to be
extremely widely used. Such transmitters use lots of hot-swappable
modules - and can operate quite well with a few modules removed. At least
some of these high-power AM rigs are essentially huge D-to-A converters.
Look in the IEEE Transactions on Broadcasting for some ideas.

The 88-108 MHz units are conventional amplifiers, made of a number of
modules and combiners.

The trick in the near future is adding IBOC digital to these rigs without
non-linearities and unacceptable mixing products. Although "digital", the RF
is of course analog in nature. I gather that solid state rigs accommodating
IBOC were at the NAB show a couple of weeks ago.

When WTOP was running IBOC tests on 1500 kHz I don't know whether they were
using their main solid state 50 kW Harris rig or the tube Continental 50 kW
backup.