The AM Forum

I was thinkin'  about the recent thread on simple AM transmitters. 

Anybody ever try to make up a solid state version?? 

Something like a pair of IRF510s series modulated by a pair?

A little simpler than a class E rig, but maybe a good stepping stone?

Any ideas??

I'd be interested too!

-Jason kf6pqt

I started something sometime ago but haven't seen it through.  It's basically a collector modulated rig, about 15 to 20watts.  I stopped working on it when I got frustrated with the various xfmrs I was attempting to use to modulate it.

Why not just series linear modulate it with a FET.

Try this circuit.   http://www.angelfire.com/de/RadioAnarchy/

hopefully it is east to copy.  I am not sure if anyone has built it.
I just have the prototype pictured, using a VFO.
But I have a nice one built, using a crystal.   

mostly pirate stations are interested in it. 
 
7 watts output.   wind your own modulation transformer.

If youre not tied to +12V only,  a push pull audio stage running, say + and - 24V can be coupled to a Final running off +24V with a 1:1 transformer. 
BUT you can replace the 1:1 transformer with a single choke (ie secondary of a power transformer) and couple the audio with a large cap.   

(http://www.smoke.com.au/~ic/tmp/AM_sample.png)

You cant swing all the way to 100% modulation with this, but you can get close. If you run the PA off a lower voltage you can of course get > 100% mod.  This scheme lends itself well to recycling of solid state stereo amplifiers.                                   Ian VK3KRI 

Skip all that who-haw with transformers and chokes and series modulate like Frank suggested. Yea, series modulation is inefficient, but at low power, it's mostly insignificant. Dig up a schematic of a CB from the days when MRF476s were used in the final. All these used a simple series modulation scheme you should be able to copy or scale up to your power level.

Ian, what gets hooked to the bases of those transistors?

Thanks,
Jason kf6pqt

Those two bipolars are just the final stage of an audio amp, hopefully something pulled out of the trash for nix.

Series Mod is easy and has potential for really high quality, I just baulk at using the same heatsink on a modulator for a 25W transmitter that I could use on a 100W PA stage.
- Then again I'm probably never going to use all the lumps of second hand aluminium in the shed, so why not.
                                                          Ian VK3KRI

Hey Ian,
I hope to work a VK3 one of these days. We'll have to try SSB I'm afraid. AM would be challenge unless it was on 20M band suprise conditions.
Fred

Hi!

If you want to build a solid state AM rig that sounds great (based on comments received), is easy to build, and is fun to build and use... check out the following page on my Web site:

http://mysite.verizon.net/sdp2/id11.html

Best regards
Stu
AB2EZ

I recently built a 150 watt Class D transmitter  that is designed by K7DYY. 
You can get info about this rig from http://k7dyy.com.
It uses pulse width modulation and the size of the transmitter is only about 3 x 4 x 7 inches and weighs only 2.5 pounds.
You can buy the pc boards from k7dyy for $50 and get all the parts from Digikey and Mouser. 
You can see some photos of my transmitter at http://www.kc3ol.dynip.com/classd

73, Ted, KC3OL
www.kc3ol.dynip.com