Tube Final for Part 15 FM transmitter
Can A 6BQ7 or 6U8A vacumm tube be used as a final for providing approx. 50-100 mw RF output?
Anybody have an idea where I could get information to put together such a device?
For many years, the subject of low power tube transmitters has come up periodically on the Usenet newsgroup rec.antiques.radio+phono. You can access the archived posts through Google Groups. There are many, many circuits available for low power tube transmitters. The circuits range from single-tube "phono oscillators" included in some phonos from as far back as the 1930s to multi-tube "real" transmitters that have been published over the years.
Beware that virtually all of these designs overlook the most important point: matching the tube impedance to a short antenna. Typically, the power input to these transmitters may be as much as several watts, but the designs fall far short when a short wire is connected as an antenna. You can pump several watts through the tube and get a 50' range because of the gross mismatch to the short antenna.
A good transmitter design for Part 15, whether tube or solid state, must focus on the best possible transfer of power to a 3 meter antenna, not just power into a 50 ohm load (this is not even close to the impedance of a 3 meter wire or whip or whatever!!!).
Phil B
And... of course, I jumped the gun by not reading your subject carefully. I was referring to AM transmitters, not FM. Sorry.
I have never seen anything relative to low power tube FM transmitters. Quality FM modulation is hard to achieve with simple circuits, but matching to a quarter wave antenna in the FM band should be a piece of cake (50 ohms).
Phil B
And... of course, I jumped the gun by not reading your subject carefully. I was referring to AM transmitters, not FM. Sorry.
I have never seen anything relative to low power tube FM transmitters. Quality FM modulation is hard to achieve with simple circuits, but matching to a quarter wave antenna in the FM band should be a piece of cake (50 ohms).
Phil B
What I had in mind was comming up with a stand alone low power (5mw-50mw) tube linear amp that wouldn't be subject to damage from high voltage transients induced in the antenna by nearby lightning strikes.
I have already had to replace U8 in my Ramsey FM-30 from this and I sent it back to Ramsey because that was a surface mount chip and I didn't want to damage the board or lift the traces.
So what I wanted to do basicly is use the tube linear for a voltage transient buffer/arrester between the FM-30 and the antenna/coax, as tubes are very forgiving compared to the fragile p-n junctions in solid state devices.
Surely someone out there has some ideas on this?