Something interesting to try out

The MW-250 is a lot like the AES K-488 "Phono Oscillator", except it is crystal controlled and uses the 12W6 for better audio modulation on the 12SA7.

Cool little circuit, but the price is too high.

I am only interested in Tube designs, becasue tubes more tolerant to SWR mismatches and because Tubes are Classic. If your going AM, you might as well go classic... 😉

My problem is the FM band is packed around here, so AM is a much better choice to look at, and things sound cooler on AM through tubes...

Bow

I should probably expand on my question:

Is it possible to legally get the same range? As in - can we stick them in a weatherprrof box and mount them up high in the blistering sun?

If someone looks at this, and thinks "Wow - thats cool, I gotta do that" PLEASE let us know..

I don't see why not.

The Box may or may not need some ventillation do to the heat created by the tube heaters, but I do not see why it wouldn't work.

Tubes are pretty tolerant to heat, probably more so than Solid state components.

I'm betting it would work just fine.

I am hitch-hiking on this thread since it talks about electron tubes, and I am one of those with fond memories of the tube transmitter days.

Then I got to looking at a an empty honey bottle on its way to the recycling bin, and I wonder...

What if, instead of building a ship-in-a-bottle, we actually built our own electron tubes inside of bottles from various products.

To build a Part 15 transmitter you'd need about four tubes...

The oscillator, a middle-stage buffer, a final RF stage, a modulator stage and a rectifier tube. That's five tubes.

How do tube makers make them?

Can't be that difficult.

Home brew vacuum tubes can be made as illustrated here. The "pros" make them as shown here.

A web search on "how to make vacuum tubes" yields videos on both home made and factory made tubes such as those linked above.

There are some technical difficulties though, including the use of purified metals and heat treating to achieve long operating life. It is actually easier for the hobbyist to make a transistor but the performance of these is rather poor.

Neil

yeah.. i imagine we would not get the purity level you get in the factory.. maybe not close.. but how cool would it be to see your own fab'd tubes glowing and producing radiated audio?

...or are we talking about *gasp* experimenting again? 🙂

Go for it!

Neil

i may try it.. but dude.. if there is anyone on this forum likely to electrocute themself.. its me.

sooo... if the power goes out in sunnyvale, and they cannot id the crispy critter in the garage on fairwood.. tell em its me.

The bad news is that mir is a pile of ashes.

The good news is that his home-made electron tube works like a champ.

The guy making the tubes obviously has more time, money, resources and patience available than I could ever hope for.

One of these days I'm gonna build that 1 tube regen I've wanted to build since I was 10.

Several recent posts on other threads about tubes, but I can't find them... so this older thread is good... especially the linked videos on building vacuum tubes.

PhilB recently made a strong and rational case against the ordeal of building part 15 tube transmitters, but some of us do not listen to reason and still have tube notions.

Then, in Low Power Hour No. 97, the Skeptoid show did an excellent presentation about tubes, and they mentioned that today serious guitarists demand tube amplifiers, preferring the "warmer" sound.

Today's MCM catalog, Page 41, lists tubes available as Guitar Accessories... including 12AX7, 12AU7, 6L6, EL84 and EL34.

Those might be the right tubes for Part 15, and I think Radio8Z might have already mentioned some of those types.

The transmitter I plan will be switchable between 100 mW mode and 10 Watt mode for use with a 3-meter antenna or carrier current.

The blower fans will need to be somewhat noisy for proper realism.

Meters must be extra-large.

Here's a thought. My solid state 100 mW transmitter produces a (calculated) voltage of about 100 V at the junction of the loading coil and the 3 m aerial. A tube circuit can do this without a loading coil thus so as far as the aerial is concerned it will radiate the same with 100 V applied regardless of whether it comes from a tube or a transistor or a brick.

This rather simplistic view avoids questions of efficiency.

Neil

But isn't that 100V of r-f the result of the transmitter power flowing into the impedance of the load, along with the distribution of the load voltage between the radiation resistance of the 3-meter whip and the resistance in the system connection to r-f ground?

The spreadsheet below shows this for one setup that may be close to yours, Neil.  The voltage at the base of the whip is 100V.

A tube-type transmitter will still need to provide a match to the impedance existing at the base of the whip.  That isn't done by providing 100V, (alone) -- it would take some kind of a matching network.

If the matching network for the tube transmitter had lower ESR than the ferrite coil in a transmitter, that could result in somewhat better system performance (other things equal).

For example if the matching network ESR was only 5 ohms and that transmitter was providing the same 86 mW at the output element of the final stage as it did with the 18-ohm coil and the ground R remained at 57 ohms, then the r-f voltage at the base of the whip would rise to 110V, while the field at 1609 m (1 mile) would rise to 0.120 mV/m.

[In Firefox, an enlarged view of this graphic is possible by right-clicking on it and selecting View Image.]