Gettin' tubular, and something new from R F Source

Quote from the "Zenith Designed" description:
As per the F.C.C rules Part 15 this is a perfectly legal transmitter
And you need no license to use it.
The rules state that the transmitter may only transmit 100mw input into a 15 foot antenna and transmit no farther than 200 feet and this transmitter meets all these rules.

This fellow needs to read part 15.219 again. 15 foot antenna?

I wonder if he realizes or cares that selling this could be a problem since it is not certified.

This being said, tubes are great to use for transmitting and a lot of fun since the glow of the tubes gives the appearance that the device is doing something. Many of us remember the blue glow as stray electrons hit the glass and how this changes with the audio. Neat!

Thanks for the links.

Neil

SCWIS

Interesting batch of transmitter information.

Back THEN, in those warm tube days, I lived across the street from a Zenith distributor. I went over there to get a Transoceanic Radio and a spring-reverb unit, which I learned a local AM station had used to add reverb to their microphone mix. But I did NOT know they had a phono oscillator. Glad to find out.

The RF-Source AM board at first looks like a plug-in card for a computer, but on second look I think it's just waiting for a case and power supply. It does not appear to provide a way to back the output-power down to legal level, but perhaps it could be modified. If my understanding of the rules is accurate, simply attenuating the power at the antenna would NOT be sufficient.

Head spinning.

Hi Carl,

"It does not appear to provide a way to back the output-power down to legal level, but perhaps it could be modified. If my understanding of the rules is accurate, simply attenuating the power at the antenna would NOT be sufficient."

Could you elaborate please ... I've been over the rules a lot ... the ATU, whether external or internal, is the final stage, so if you attenuate the power going into it to meet the regs, it should be legal, no?

But, yep, the other notes about over-powered TX' don't provide much incentive to buy to someone in the U.S. trying to stay legal, and the 15' antenna thing is just plain wrong.

Ken Norris:

Yes, as the ATU is the "final stage," so long as 100mW is the maximum input, it is certified.

But if you attenuated the signal-strength AFTER the final stage, where it goes into the antenna, it might not be allowed. That's what I said.

I would think that modifying the RF-Source unit by reducing the input to the final stage from 400mW to 100mW it would meet specs, just like the ATU.

I must disagree. The ATU is not the final stage. As conventionally used and understood in radio engineering a "stage" is a circuit containing an active device which usually provides gain. Active devices include tubes, transistors, operational amplifiers, and other components which can provide gain.

The ATU does not contain any active devices and hence is not considered a "stage". The 100 mW input limit would apply to the amplifier stage which drives the ATU.

Neil

Neil:

I'm glad you made the distinction about "what a stage is," because in my previous note I based my comment on the fact that Mr. Norris called the ATU "the final stage." I personally have no knowledge at all of the ATU.

But the question changes to asking how the ATU can be authorized to have "ground lead, TRANSMISSION LINE and antenna" in excess of 10-feet? Since that system is purportedly certified, why can't everyone else have lengthy transmission lines?

The "Zenith" seller has changed his description to include the 3 meter rule 🙂 but then he sorta starts getting mixed up with the 100 mW DC power input vs RF output, oh well...

I still worry about him selling a fully assembled unit without certification, however...

That can get pretty expensive - $7,500 - $10,000 for a first offense - ouch!

UPDATE looks like the listing has been terminated...

It sure would be nice to get the LCD display options in the RF Source unit in a 100 mW DC input model...

There is a wonderful (at least I think it is) crystal controlled
tube transmitter schematic on the internet.

All you have to do is Google "Working With Crystal Control,"
and a nice article will come up that is apparently connected
with the Antique Wireless Association.

The transmitter uses a 6AK5 audio stage and a 6BE6
crystal controlled oscillator that is also the final RF
stage. The 6BE6 can run 100 milliwatts input.
The expensive part of the whole thing is the crystal,
which isn't cheap. I suppose it would have to be
a pretty durable crystal, too, because the article
says it will have 10 volts of RF on it. So if I'm
correct, we are not looking at a little dinky crystal
like an HC-18. We will want something bigger,
like an FT-243 or HC-6/U. (I hope I'm correct
about those designations - I'm just thinking off the top
of my head.)

The article is very enjoyable. My only concern is -
I wonder if the transmitter would FM, because
there is only one RF stage. Maybe it wouldn't,
because of the crystal. I don't know.

Anyway, it's worth a look. There is also a picture
of a completed working unit. And apparently it
gets out 300 yards or so with just a 3 meter
antenna - which is a bunch of straightened out
coat hangers soldiered together. And the transmitter
is indoors.

One more thing. There was a ham named Ken Cornell,
who's callsign was W2IMB. He had a wonderful
book. It was called "The Low And Medium Frequency
Experimenter's Scrapbook." Ken is no longer with us,
unfortunately. The book, and it's many editions that
followed, had tube and solid state transmitters and
receivers for the Part 15 AM broadcast band and the
160 -190 kHz Part 15 band. There was also antenna,
ground info, and a section on how to wind gigantic
tuning coils.

I had this book 30 years ago and it is now gone.
I do remember vividly a section on AM BCB
transmitters that used tubes. (From an early
edition of the book.) These circuits were AM and
CW. Ken was able to reliably talk using Morse
Code (cw) over an eleven mile path to another
similar experimenter. He used split AM frequencies
for that, such as; 1575, 1585, or 1595 kHz.
(Back then, the top AM Part 15 frequency was permitted
was 1600 kHz.)

I also remember a transmitter for the AM band that
used a 6V6 tube for the RF stage. The 6V6 ran with
only 20 volts on the plate, if I remember correctly.
He said that the tube operated very well at that
plate voltage and was quite efficient.

I love tubes. I swear I would build one of these
circuits if I could. And I wish I still had Ken Cornell's
great book.

There are other tube circuits for AM Part 15 on the
net and on YouTube.

Best Wishes,
Bruce, MICRO 1690/1700

I wonder if the transmitter would FM, because
there is only one RF stage. Maybe it wouldn't,
because of the crystal. I don't know.

I'm sure you know that signals generated by crystal oscillators can be phase modulated to produce FM. So the answer to this concern is it could.

Other than finding the filament wiring process boring I always enjoyed building tube stuff. It seemed that they always worked and the tubes were very forgiving of mistakes.

Anyone else remember the "back pocket" rule?

Neil

Yup. I know crystal controlled oscillators
can be phase modulated.

It's a funny thing - my first 2 meter FM ham set-up
was about as awful as you could get. It was 1972.
A lot of us poorer guys were using funny little
homemade transmitters for 2 FM, or in my case,
a 100 mW board I got to transmit. The receiver
was an analog 144 - 174 MHz VHF "public service"
monitor (portable, battery powered, terrible.) But
the transmitter was phase modulated to get the FM.

After that, a friend lent me his handie talkie. It was
a 2 channel Varitronics HT-2. It was the size and
weight of a brick and had one local repeater crystal pair
in it. It sure was fun, though.

So, now that I've rambled on about that -
just what is the "back pocket" rule?

Best Wishes
Bruce, MICRO1690/1700

I was hoping someone would ask. It goes by some other names but essentially it is "when working with tube circuits keep one hand in your back pocket".

The idea is to prevent forming a circuit through your chest which could happen if you used two hands. I always did this when working around 440 V 3 phase. Some thought I was nuts but sometimes being nuts is how you age enough to get gray hair.

My first ham setup was a Drake TR-3 resplendent with lots of glowing tubes. I bought it used in '76 from a radio shop. I later found it was owned by a local ham who had installed a crystal to bootleg on CB. I unmodified it. This, to this day, is my only low band rig. I have used it for AM, SSB, and RTTY and have received very complimentary remarks about the audio quality. At 280 W PEP it is no slouch though the receiver could be a bit better.

Anyway, back to part 15, those interested can site search for KnightKit broadcaster to read my escapades with this. The KK was my start in radio broadcasting technology (with one hand on the volume knob and one you know where).

Neil

Now I remember! Keep one hand in your
back pocket so you don't get blown across
the room by high voltage!

You are extremely fortunate to have a
Drake TR-3. Anything from Drake is
wonderful! I have operated Drake gear
but have never owned anything from
them myself.

Best Wiishes
Bruce, MICRO1690/1700

"I must disagree. The ATU is not the final stage. As conventionally used and understood in radio engineering a "stage" is a circuit containing an active device which usually provides gain. Active devices include tubes, transistors, operational amplifiers, and other components which can provide gain.

The ATU does not contain any active devices and hence is not considered a "stage". The 100 mW input limit would apply to the amplifier stage which drives the ATU."

Yeah, I meant the ATU is at the point of final ... IOW, the INPUT right at the ATU line is 100mw. Now, the ATU for a certified Talking House TX can basically have as long a line as you want ... they sell the ATU with 100' of 75-ohm cable. That's just ordinary TV cable, albeit it comes in outdoor-indoor, heavy jacket weather-resistant varieties, but it's not standard 50-ohm antenna line.

The only thing I can come up with in the compliance issue is that the core signal carrier may be thought of as reflecting the signal back from the shield and cancel itself enough to weaken it to the status of unintentional radiator. But the shield is grounded to the chassis, and is the only ground the ATU has AFAIK, so how will that work?

As to tubes, I've always liked tubes because they are more like living things than solid-state stuff. If you have a transistor that is glowing, it had better be powering a lamp of some kind, or else you need to unplug the unit it's in before it goes up in smoke (although I've seen power transistors get very very hot).

Anyways ... what, other than look and feel, would be an advantage to having a tube-driven AM transmitter? Is fidelity to human hearing better at the receiver? Would a tube transmitter sound better on a tube receiver, or would a solid state receiver be the same?

But the shield is grounded to the chassis, and is the only ground the ATU has AFAIK, so how will that work?

While it is true that the RF from the transmitter is shielded and enclosed in a coaxial transmission line capacitive coupling from the antenna to the shield of the line will cause a current to flow in the shield which is not balanced to zero by the current in the center conductor. Thus this current will cause the shield to act as a counterpoise to the antenna and radiate a signal. This is why the rules restrict the length of the transmission line. This system, certified with the ATU, appears to be an exception to the rule and it would be interesting to know why this was certified.

Neil