RF Generator, AKA Low Power Transmitter

One other point about service-grade generators using tubes - they are almost all incapable of modulation above about 30%. That is a limitation of the directly-modulated oscillator. Above that point envelope distortion and incidental FM modulation both increase markedly. IMHO, as one with more than 50 years experience in broadcasting and more than 40 years as a Chief Engineer, only lab-grade signal generators make truly decent low power transmitters.

Most service-grade generators, even if capable of external modulation, may have excessive envelope distortion (audio distortion in the modulating waveform). You might need a better grade of generator. My General Radio Type 805-C Standard Signal Generator has specifications comparable to an AM broadcast transmitter, but at 117.5 ponds it is not portable.

Dale H. Cook, Retired Chief Engineer, Roanoke/Lynchburg, VA

I needed a low power transmitter to show a customer that an old AM/SW radio worked.
 (Most are not impressed with just a tone to prove it works, and the shop I worked at had horrible AM & SW reception).

 Well, look at your old RF Signal Generator. Many have a pair of jacks on the front labelled "Modulation".

Just run a tape recorder line level output into there, turn the generator on, and VIOLA! instant low power transmitter. (Just keep the output level below 100 MW. )

 I found that having tapes of the old time radio programs, swing music, serials, etc. would show the customer that it works just fine.
  Most were so fascinated by the old programs on "Gramps" radio, they never quibbled about the price, and it is cool to set up at home to listen to the old programs on your antique radio. 

The best thing is you can tune the output of the signal generator to any frequency of AM that you want that is "clear", including the SW bands.