New products under proposal...

Tubes are very nice devices when they work. The trouble is that the tubes available today have very poor quality. That there are any tubes at all available today is probably because of the alleged superiority of tubes in audio amplifiers. There are audiphiles who will pay a premium price for an audio amplifier made with tubes. Actually, tube amplifiers have a lot more distortion than a decent solid-state design because they have a lot less surplus gain that can be used for negative feedback to correct for distortion. However, since tubes are approximately square-law devices, they produce largely even-order distortion products, which are pleasing to the ear.

I think that tubes can be used to design more efficient output stages for Part 15 AM transmitters than is possible with solid-state devices. MOSFETs will not make highly efficient RF output stages because of an internal diode that absorbs power when it is forward-biased during a portion of the cycle. JFETs are much better, but all of them have very high interelectrode capacitances. This makes them difficult to use for efficiently driving high-Q, low radiation resistance, antennas of the kind used for Part 15 AM. Tubes, on the other hand, have very low interelectrode capacitances.

The characteristic curves in tube specs actually mean something. They describe how a tube will perform over a wide temperature range. Early transistor specs also had charecteristic curves, but they were unlikely to accurately represent the actual device. So, eventually, transistor manufacturers stopped supplying characteristic curves. Characteristic curves that accurately describe the device can be used for designing for exacting applications.

Unfortunately, the tubes that are available today do not perform as well as tubes performed years ago. I have had fairly recent experience with tubes. A company I worked for used a marginal HF oscillator using a 6C4 or a 12AU7 as a detector for differentiating between different species of white blood cells. This was a critical application in the company's flagship product. Everything was fine as long as good tubes were available. A small company in the U.S. manufactured good-quality tubes using equipment purchased from large companies that had stopped making tubes. Eventually, this small company stopped making tubes, and nothing but trouble followed. Tubes purchased from Russia and China were terrible. Serbian tubes were a little better, but then there was a trade embargo against Serbia at the time. The tubes that worked well were stocks of old unused tubes manufactured in the U.S., England, Germany, and Holland. There was no choice but to embark on a project to develop a solid-state replacement for the tube oscillator. The tube oscillator had characteristics, some of which were not well-understood, which were difficult to duplicate in a solid-state version. It took six years before an adequate solid-state design was ready for production. The company would have ordinarily shut down a project that took so long, but this was something that had to be done. The JFET that was used was expensive because it had to be a selected device. The selection criteria had to be as wide as possible so that there would be any yield at all in the JFET manufacturer's process. Temperature changes were a problem and had to be compensated for. In the meantime, there were supply problems with the tubes that increased as time went on.

My opinion is that tubes should not be used unless there is a compelling technical reason for using them. Using tubes simply because they are quaint is not a good enough reason. Using a tube to get superior performance presents the difficulty that good tubes are now hard to obtain.

www.part15.us/node/63
"Here is a Updated list of ideas for a Part 15 AM Transmitter kit Design. the fact that people pay $1k for a crystal controlled transmitter, makes me think a kit like this would sell great... "

Experimental broadcasting for a better tomorrow!

I would like to point out that in audio applications like quitar amps and mic preamps used to add "warmth" to voice, that even order distortion products that "are more pleasing to the ear" the superiority is not "alleged". Since "more pleasing to the ear" is a desirable trait in those cases, the superiority would be actual rather than alleged if the even order distortion products are what is being intentionally sought.

In audio applications, spectral purity is not always what is desired. The judicious use of intentional distortion of certain types has been a part of the popular sounds of music for quite a few decades. To that end, some people try many tubes/valves, whether new or salvaged from older pieces of equipment to find ones that are "sweet" (ie, have the characteristics that person desires).

In other audiophile applications, it is not impossible that a given combination of tube amplifier or stages into a specific speaker may yield a sound that particular audiophile finds more pleasant to the ear, even though the total harmonic distortion may be higher than the clean signal that some solid-state devices could achieve. But the name of the game there is "pleasing to the ear" and is a subjective quality.

Now, if a tube circuit can theoretically make a more efficient output stage for part15 AM, for a small niche market it might actually be worth the bother to try assorted individual tubes of a specific until one is found that performs well, even though it might require a bit more filtering or fine-tuning of filters to surpress the harmonics.

Tubes are, as you pointed out, not of consistently good quality these days. They are a more primitive device in some regards than solid-state devices and less convenient to the current idea of drop-in no-fuss replacement. However, for certain audio applications they can be worthwhile if their sound is something one wants, and if they could be more efficient in the rf stage for part15 AM it may be worthwhile to some folks.

Probably the biggest question I'd have is how much more efficient *could* they be, compared to the solid-state devices typically used in currently popular part15 AM circuits. And whether that degree of possible improved efficiency would be large enough to offset the time and expense someone would have in making or maintaining a transmitter made with them.

Which boils down to an individual judgement call in the end as to whether it was worth it, much like it does in things like guitar amps and mic preamps.

Even knowing from years of using a tube guitar amp how finicky tubes can be and how hard it can be to find decent replacements and then readjust the circuit for the differences between even two tubes of the same type, I'd at least consider at it as a possibility. As Rich pointed out in the discussions on grounds as relates to towers and masts, radiated power is linear for a given system in that a 100 milliwatt transmitter can use a 250 foot tower as efficiently as a larger commercial tranmitter can. So if an option like a tube final output stage can deliver a greater percentage of the allowed 100 milliwatts into an antenna, performance should be better as regards range.

Whether the improvement would be worth the money (since tube gear is more expensive and time consuming to build), that would be something people would end up having to make their own judgement calls on.

Daniel

I have always wondered what AM was like back in the day of tube transmitters. I have never experienced the sound of tubes. I would consider buying one, depending on the power output and the price.
Travis

While the distortion produced by a vacuum tube audio amplifier might be useful in producing the sound of an electric guitar, such an application is different than that of a transmitter.

Using vacuum tube circuits in a transmitter because tubes produce greater content of even harmonics is virtually pointless. A radio receiver is tuned only to the fundamental r-f carrier and some of its modulation components, and ignores their r-f harmonics. But regardless of that, those r-f harmonics can interfere with other radio services. And the modulation produced by the higher-order audio harmonics fall outside the r-f bandwidth of the receiver, and are likewise ignored (even if they are generated by the transmitter) -- yet they can interfere with the reception of adjacent channels.

A transmitter/receiver system should add as little distortion as possible to program audio. At a given r-f power output, the most important parameters of a broadcast transmitter are the fidelity (linearity) of its audio amplification and r-f modulation processes. The lowest-distortion transmitters used by licensed commercial AM broadcast stations invariably use solid-state circuits throughout their audio, modulation and r-f sections.

The best place to generate a desired "sound" for a broadcast signal is in the audio/modulation processor used to supply the program input waveform to the transmitter. That is the function of commercial audio processors such as supplied by Orban, Inovonics, Omnia, etc.

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Excuse me, Rich. If I gave the impression somehow that I was speaking of higher distortion (whether even order or not) being something desirable in a *transmitter*, I apologize. I was speaking strictly of the use of tubes in audio frequency devices of certain types (such as electric guitar amplifiers) when arguing that if even order distortion sounds better to the ear subjectively, then it can be desirable.

Obviously harmonics (whether even or odd order) are not desirable in the output of the carrier wave. I even mentioned that it *might* take more filtering to surpress possible harmonics since tubes are somewhat more prone to distortion than modern solid state devices. I don't know if they would be more prone to distortion/harmonics in an RF stage or not, I've never used tubes for RF in any transmitting application.

So if my reply gave the impression I was advocating distortion in an rf final as a good thing, I apologize for any confusion.

Now what I found interesting was Ermi Roos's statement:

"I think that tubes can be used to design more efficient output stages for Part 15 AM transmitters than is possible with solid-state devices. MOSFETs will not make highly efficient RF output stages because of an internal diode that absorbs power when it is forward-biased during a portion of the cycle. JFETs are much better, but all of them have very high interelectrode capacitances. This makes them difficult to use for efficiently driving high-Q, low radiation resistance, antennas of the kind used for Part 15 AM. Tubes, on the other hand, have very low interelectrode capacitances."

I was not familiar with those characteristics and hadn't stopped to think that different devices (such as tube, bipolar transistor, MOSFET, JFET) might have different efficiencies for driving the sort of antennas used for part15 AM.

I certainly agree that licensed commercial stations would use solid state devices, especially considering the difficulty finding tubes of good quality in recent years. However, those commercial stations invariably are not using 3 meter antennas on the AM bcb, either.

So how much of a difference in that regard could factors like "low interlode capacitance" have as regards how efficiently a final RF stage with 100 milliwatts of input power can drive a physically short antenna?

Daniel

I recall seeing class C tube RF amplifiers operate in a manner where the grid to cathode voltage goes positive causing the grid to draw current which allows the input coupling cap. to establish the bias for the tube. Isn't this equivalent to the MOSFET (depletion type) gate diode conducting?

Also, unless one accounts for the gate input power (which would be required by 15.219 and which I suspect is minimal) it seems this would have little or no effect on the efficiency.

Maybe I am missing something.

Neil

The diode in the MOSFET that I referred to is a parasitic diode that is in shunt between the drain and the source. This causes negative excursions in the output voltage to be clipped, and this results in a loss in efficiency. Such a diode is not present in JFETs (or tubes).

Interelectrode capacitance affects efficiency because any capacitance that is shunted in some way across the antenna capacitance of a short antenna decreases efficiency. Another type of capacitance that reduces antenna efficiency is the interwinding capacitance of a loading coil. Circuit board capacitances are also bad. I would not use a circuit board for an output tube stage for a Part 15 AM transmitter. I'd use an "airboard" instead. Airboards are not mechanically rigid, but they work better electrically.

Ermi,

Thanks for the reply. That makes sense. I was looking only at the gate.

I thought it was relevant to the thread to pursue this since the efficiency advantage was cited as a plus for the tube design.

Neil

All good discussions and very informative,thanks for the great response! I am aware of the tube situation and for my small application I can find NOS american made high quality tubes in abundance without any problem. My hope is to increase the efficiency at the tube to antenna interface. Most tube output sections need to transform the tube's high impedance to a much lower one for conventional 50 ohm transmission lines. Our required shortened antenna is high impedance already.By not needing such a drastic transformation,the efficiency will increase. The higher interelectrode capacitance also reduces losses. The internal tank coil will be an air-wound,low capacitance unit. An external loading coil will not be needed. Then to work on a quality modulation scheme,there are many to choose from. Series modulation,screen modulation,plate(xfmr) and more. I will be looking to provide 100% modulation at under 1% thd. The oscillator will be crystal controlled. I may offer a synthesized version later. Remember,this is a rough description as I have not begun to build any prototypes as of yet. You will hear updates here as they happen. DO keep the ideas and discussions going,they really do help! Now where is that box of old tubes and parts??!!! Regards,Lee

In this thread, I mentioned that the lower interelectrode capacitance of vacuum tubes compared to FETs can improve the efficiency of Part 15 transmitter and antenna combinations. In other threads, the effects of ground resistance loss, tuning circuit resistance loss, and power loss in the final stage of the transmitter, were discussed. This gives the impression that the causes of inefficiency can be dealt with separately. For example, first ground loss can be dealt with, then the tuning circuit losses, and so on. This systematic piecemeal approach can be used with systems using full-sized antennas, but not with Part 15 AM.

It happens that, because of the electrically small, high Q, antenna that is used in Part 15 AM, all of the causes of inefficiency are closely interrelated.

The underlying cause of the inefficiency of systems using small antennas is the signal bandwidth. If only an unmodulated carrier is transmitted, the efficiency can theoretically approach 100%. It is the audio signal bandwidth that causes the theoretical efficiency of a Part 15 AM system to be very small.

The lowest audio signal bandwidth that can be used with music is about 5 kHz, and for speech only, about 2.5 kHz. Because there are two AM sidebands, the RF bandwidth is twice the audio bandwidth. The RF banwidth is established by the Q of the antenna.

Q = Fc/2BW = Xc/R ~ Rp/Xc,

where Fc is the carrier frequency, BW is the audio bandwidth, and Xc is the capacitive reactance of the antenna. R = R loss + R radiation. R loss is the total loss resistance in the transmitter and antenna, including ground loss, tuning circuit loss, and transmitter loss. Since R loss << R radiation in Part 15 AM systems, The system Q depends almost entirely on the antenna capacitive reactance and the loss resistance. For maximum efficiency, the loss resistance must be exactly equal to the resistance that gives the Q that gives the desired audio bandwidth. If the loss resistance is higher than what is required to obtain the Q that gives the desired audio bandwidth, the theoretical efficiency of the system cannot be achieved.

In the formula for Q above, Rp is the parallel equivalent of R if the equivalent circuit for the transmitter and antenna is converted from a series equivalent circuit to a parallel equivalent circuit using Thevenin's theorem. If this sentence is confusing, don't worry about it. I include it here because it is needed for explaining what is contained in the next paragraph. The symbol "~" in the formula for Q is used here to mean "approximately equal to."

There will always be capacitance in the transmitter, coupling circuit, and loading coil, that is effectively in shunt with the antenna capacitance. We will call the antenna capacitance Ca and the shunt capacitance Cs. With Ca and Cs in parallel, Q increases by the factor (Cs + Ca)/Ca, causing the bandwidth to be reduced by the factor Ca/(Cs + Ca). The only way to restore the bandwidth is to increase R by the factor (Cs + Ca)/Ca. So, because of Cs, the efficiency is reduced by a factor of Ca/(Cs +Ca).

To make what I just said a little clearer, I will give a specific example. Let's suppose that Fc = 1.7 MHz, and BW = 5 kHz. Q = 1700/(2*5) = 170. Xc = 3000 ohms. R = 3000/170 = 17.6 ohms. R radiation = .13 ohms. Without Cs, the efficiency is about 0.13/17.6 = 0.74%. Ca = 1/(2*pi*1700000*3000) = 31.2 pF. Suppose that Cs is 100 pF. This causes the efficiency to decrease by the factor 31.2/(100 + 31.2) = 0.24. The total efficiency is 0.24*O.74 = .18%. This is the maximum theoretical efficiency, produced if the total loss resistance of the system is not greater than 17.6/.24 = 73 ohms.

Ermi Roos wrote The lowest audio signal bandwidth that can be used with music is about 5 kHz....

Unfortunately a new, consumer-level AM receiver having a usable audio output up to 5 kHz is rare these days.

From a system viewpoint, Part 15 AM setups may as well not bother radiating 10 kHz r-f bandwidths, as a large chunk of that is rejected by most modern AM receivers. And permitting a narrower r-f bandwidth for Part 15 AM transmissions could raise the efficiency of their antenna systems.

Here is a link to further comments on the bandwidths of AM receivers that appeared in this month's Radio World.
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First, let me thank you for your interest in bringing new products to market for our community! That is fantastic!

Here are some of my thoughts on priorities. First, many people seem to struggle with hum problems on AM. This is easy enough to fix with add-on units, but having a balanced input in the transmitter would go a long way toward solving these problems. Next, although I love tubes, they run hot and take a relatively greater amount of power. You can't really consider putting a tube transmitter outdoors at the antenna base unless you are prepared to do a LOT of work. So the market for a tube transmitter seems limited to nostalgia buffs who want to broadcast to their old radios around the house. There is a unit available for that purpose already. I don't know how well it sells.

Another problem with tubes is that old bugbear, liability. Most hobbyists today have no knowledge of how to behave around tube circuits! If you were going to make a tube transmitter, you'd really need to consider selling it as an assembled product and getting it type accepted. Otherwise, you would not be able to take advantages of the high impedance matching advantages of the output. I've written about my Knight-Kit wireless broadcaster elsewhere. It will directly drive a 10' wire with 100 V p-p, which works pretty well. The problem is that the input power is more than likely way over 100 mW, but when Allied got it approved, it met the radiation limits with the short wire antenna. A hobbyist building one of your units from a kit couldn't really operate it that way and take advantage of the alternative Part 15 requirements. It would have to be throttled back to 100 mW input, and then it would probably go about 50 feet.

Where I think you could make a contribution is in coming up with a more efficient antenna matching circuit for indoor operation. An indoor antenna is never going to work nearly as well as an outdoor unit, but many people just can't put up an outdoor antenna. If you could package this in a way that is easy to set up indoors, you would have made something useful.

Finally, if you are really into the RF side of things, design your transmitter so that the output stage is at least 60% efficient. The current designs are apparently in the 30's, and that's leaving a lot on the table. With a switching type output, (e.g. Class E), you could achieve efficiencies in the 80-90% range which is way better than what is currently being sold, as far as I know. Finally, full modulation capability with low distortion is a must!

WEAK-AM
Classical Music and More!

The proposed project has been shelved due to health issues.

Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm