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Tube or not Tube: That is the Question

from Eric P. Nichols, KL7AJ on March 22, 2014
View comments about this article!

"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 03/10/2008


Tube or not Tube: That is the Question

No book about Amateur Radio, much less a book about Amateur Radio lore would be complete without a discussion of electron tubes. These glowing globes were the workhorses of all electronics equipment for most of the last century, and still have a secure position in modern Amateur Radio, as well as industry.

The electron tube, alternately known as the Valve in Great Britain, the Lamp in France, and just plain tube in a lot of other places, represents a surprising level of mechanical, chemical and electrical engineering. It was the most sophisticated piece of hardware produced in the early part of the twentieth century, except for, perhaps, the precision pocket watch.

For most of this chapter, we're going to be talking mainly about one particular variety of electron tube, the vacuum tube, a term which has often been used interchangeably but somewhat inaccurately, with electron tube. Be that as it may, it is a fascinating device in any form.

Recall that in our introduction to the electron, we dealt with electrons in conductors. We demonstrated that, though electrical energy in a conductor traveled at nearly the speed of light, the actual electron movement, or electron drift, was painfully slow.

Not so in the electron tube. The electron tube is full of free electrons, which can, indeed travel near the speed of light. You might be asking yourself how a "vacuum" tube could be full of anything, which is probably a good question. Vacuum is a relative term, in this case. But, be assured, there is indeed very little inside a vacuum tube.

Now, although the free electrons in a tube can move very fast, in most applications we keep them somewhat below the speed of light. They are non-relativistic electrons, meaning they behave pretty much as mere particles, and, for the most part, follow the standard mechanical laws of Newton. This is a good thing, because vacuum tubes are complicated enough as it is. There are "tubes" of sorts that indeed use relativistic electrons, such as the free electron laser, but you won't see many of these around the average ham shack.

So, with that background, let's get started. The first thing we need is a bottle of free electrons. A light bulb is actually such a device. An incandescent filament in a glass bottle will "boil" off a certain number of electrons. The problem with a light bulb, however, is that there's a lot of other stuff that gets in the way of our electrons. Our free electrons need a clear path to work in, and in a typical light bulb, you don't have that. You have a lot of Argon atoms clogging up the works. They put Argon in light bulbs to prevent oxidation of the filament, which is supposed to make the bulbs last a long time. Obviously, that doesn't work, so it must be a conspiracy of the Argon companies to sell a lot of Argon. Be that as it may, we have to work with our light bulb a bit to make them suitable free electron bottles.

The first thing we need to do is create a hard vacuum. It's called a hard vacuum, because it's very hard to do. We need to get EVERY atom (or ion) we can out of the thing, which you can't do by just sucking on a straw. There's a three-step process. The first is to use a mechanical vacuum pump to get the pressure down just as far as possible, then you use a diffusion pump to get the pressure down even farther. Don't even ask how a diffusion pump works; I don't even think the guy who invented it knows. They're weird looking things that sort of resemble a percolator wrapped with a tornado-shaped coil of stainless steel tubing. Anyway, with your diffusion pump, you can get the pressure down to something like what you have in intergalactic space. The last step is to seal off the tube where you put the sucker-doodle in, and then you fire off the getter. The getter is a little ring of a very reactive metallic compound, which reacts with any remaining ions inside the tube, and deposits the results on the glass envelope, in a little silvery spot. The getter is “lit” by means of a radio frequency induction heater placed outside the tube near the getter. This process itself is actually very advanced; it was probably the first industrial application of anything resembling the microwave oven. The getter will continue to pick up stray ions for the life of the tube (in theory).

So, now we have a light bulb with a hard vacuum, with lots of elbowroom for those electrons to strut their stuff. Ahh, but how do we get electrons in there in the first place, since the tube is hermetically sealed?

Well, the best source of electrons is in the metal of the filament. When the filament gets white hot (or actually just sort of reddish-orange hot in most tubes), you actually boil electrons out of the metal. Well, perhaps boil isn't the best word, but it's pretty descriptive. If you recall our chapter "Bent Radio," we learned that in the ionosphere gas atoms are ionized by getting hit with ultraviolet radiation from the Sun, which slaps the electrons out of orbit around those atoms. In a vacuum tube, it's heat that slaps the electrons out of orbit. This process is called thermionic emission because the metal the filament is made out of is ionized by heat. It's actually a rather complicated process, (and a somewhat noisy one, we might add). The main difference between a vacuum tube and the ionosphere however, is that in the tube, the ions are held firmly in place, because they are in solid form. The ions can't wander off like they can in the ionosphere.

So, now we have all these electrons free of their metallic atoms. Since they are all negatively charged, they all repel one another. Ordinarily, because of the mutual repulsion, the electrons would quickly fill the whole "bottle." But they don't.

The reason is, now that the metal filament is ionized, it has a net POSITIVE charge, which tends to pull the electrons back toward itself. So just as the ions in the ionosphere keep a sort of rein on the free electrons, the presence of the ionized filament keeps rein on the free electrons. Actually it keeps a TIGHTER rein on the free electrons, because, as mentioned above, the ions are in a solid state. As in the ionosphere, some of these free electrons recombine with their metallic mother ships; others are continually boiling off. A state of thermionic equilibrium sets in, where the average number of free electrons stays about the same. Hey, doesn't this sound an awful lot like the electron density profile of the ionosphere?

So, we now have this mob of semi-free electrons hovering around the filament. (Let's rename our filament the CATHODE, because that's a better electrical description of the element, while FILAMENT is more a physical description). This electron mob is called an electron cloud, and it has a net space charge. Sometimes space charge and electron cloud are used interchangeably, but to be really persnickety, the space charge is a property of the electron cloud.

If you were able to poke a probe into the electron cloud, you could measure a minuscule contact potential. Some tube circuits actually use this contact potential in a useful manner, but for the most part, it's a mere curiosity.

All righty then. Our one-element vacuum tube isn't particularly useful, except as a really lame light bulb. However, if we stick a metallic plate in the tube, a certain distance from the cathode, and bring an electrical lead from the plate through the glass to the outside world, we can now actually do something with our electron cloud.

Remember our ionized cathode has a net positive charge. Well, if we put an even BIGGER positive charge on our newly installed PLATE, by means of a battery between the plate and the cathode, we can pull electrons away from the cathode to the plate. (In old-fashioned radioese, this battery is called the “B” battery, and the voltage it supplies is called “B+” voltage). These electrons are actually absorbed INTO the plate and conducted out of the tube to the positive terminal of the battery. They are then forced out the negative terminal of the battery into the cathode to replenish the electron cloud. We now have a one-way electric valve, known as a DIODE. (Oh, yes, we should rename the PLATE the ANODE, because all our tube elements should end with an "ODE"). Electrons cannot flow from the Anode to the Cathode inside the tube, because the cold anode has no spare electrons of its own.

Now, there's an awful lot you can do with just a diode. It can be a power rectifier, a radio detector, a logic gate, a current limiter, a timer, even an X-RAY generator!

Before we get too carried away with our enthusiasm, however, we need to know a couple more things about the diode.

Remember we said that our electrons are non-relativistic, meaning they follow the rules of Newtonian Mechanics? Let's look into this a bit, because we have to understand this fact before we can move onto the next level.

As with ANY particle, electrons have a finite mass. A very SMALL mass, granted, but very real, nonetheless. They take TIME to accelerate from the electron cloud to the anode. You may remember from high school physics that F=MA, Force equals Mass times Acceleration. The force in this case, is electromotive force, which comes in the form of Plate Voltage. This is the voltage we supply with our external “B” battery. When a voltage is applied to the plate, the electrons slowly drift out of the electron cloud and accelerate toward the plate. But, unlike gravity, the acceleration is not constant. There is a double-whammy here. The attractive force of the plate, for a given voltage, increases drastically as you get closer to the plate. So not only do electrons accelerate on their way to the plate, but their rate of acceleration also increases during the trip! This actually turns out to be a very useful property; it just makes the math involved pretty hairy.

Let's talk about a new term, plate resistance. In the process of electrons hitting the plate, a certain amount of "friction" is involved. In fact, with enough current, the plate of a vacuum tube of any type can get very hot due to the electrons striking the surface. Actually, the plate can, under certain circumstances, get hot enough to become a cathode, which is generally not a good thing.

Several design features of tubes, however, prevent the anode from acting TOO much like a cathode, at least relative to the true cathode. First, cathodes are generally made of materials with high thermionic efficiency—that is; they emit lots of electrons at relatively low temperatures. Materials that do this are things like thoriated tungsten and barium oxide. On the other hand, anodes are made (or coated with) materials that are really lousy thermionic emitters, such as graphite. They are also made much more MASSIVE than the cathode, so they are unlikely to reach the same TEMPERATURE for a given amount of heat dissipated.

Because of the inefficiency (and subsequent heating) of electrons being absorbed by the anode in any tube, there is an equivalent resistance through a diode, just as if it were a real resistor. This is pretty straightforward "ohmic" heating. The major contributor of "plate resistance" however, is strictly due to the finite number and availability of free electrons AND the fact that they have real mass that needs to be accelerated. It is this acceleration aspect of the electrons that makes PLATE RESISTANCE very non-linear. In other words, the plate resistance not only depends on the plate voltage, but depends on it in a rather complex manner.

But let's forget the complexity of the plate resistance for a moment. We can ignore the entire hairy math aspect and STILL grasp a very important thing about this electron acceleration.

All we need to know is that the electrons start out very slow, and end up very fast. If we can grasp that, we can easily explain the next topic: amplification.

Leveraging Electrons: The Triode

Imagine for a moment that you're a lumberjack standing on a plateau above a valley. Your plateau starts to gradually roll off in front of you, almost imperceptibly, perhaps dropping only an inch every ten feet. Beyond that, it slopes down a little more steeply, two inches every ten feet. Beyond that, three inches every ten feet. Beyond that, four inches every ten feet. Five hundred feet ahead, it's sloping down at a 45 degree angle. Half a mile ahead, it makes a vertical drop to the valley floor.

You have a log you want to roll down the plateau. You give it a kick, and it starts rolling down the hill, barely accelerating, because it's only dropping an inch every ten feet. It hits a one-inch pebble after rolling ten feet and comes to a dead stop. You remove the pebble. The log starts rolling again, this time a little faster. It rolls right over another one-inch pebble, but comes to a dead stop once again, after it hits a two-inch pebble. You remove the two-inch pebble. The log starts rolling again, even faster. It rolls right over another two-inch pebble, but comes to a dead stop when it encounters a three-inch stone. You remove the three-inch stone, and the log starts rolling again. Until it hits a four-inch rock.

Being the brilliant woodsman you are, you are finally able to conclude that the farther the log rolls, the bigger the rock you need to stop the thing. In fact, once the log reaches the vertical drop off, NO boulder is big enough to stop it.

The plateau you're standing on is your cathode. The valley floor is your plate. The log rolling down the hill is an electron. The gravity that wants to make the log roll down the hill is Plate Voltage. And the pebbles that get in the way of the progress are your third tube element, the control grid.

Actually, we could, in all good conscience, end the discussion right here, as we have described the principle of amplification completely. There is really little more to say on the matter.

But we should probably talk a little about the control grid anyway.

If we insert another electrode between the cathode and the plate, we have what's called a TRIODE. The control grid can be in the form of a screen, a helix, a zig zag, or any other "porous" configuration. Its job is to act as a pebble in the path. The sooner it gets its job done, that is, the closer it is to the cathode, the more effective it is. Once the electron builds up kinetic energy on its way to the plate, the bigger that pebble has to be to slow it down.

The way a grid slows down an electron (actually an electron mob) is that you apply a small negative voltage to it. This voltage is called a bias voltage. This small negative voltage REPELS the electrons back toward the cathode. If the grid is close enough to the cathode, it can completely cut off the flow of electrons to the plate with a small bias voltage. This condition is called, oddly enough, cutoff. If we move the grid farther toward the plate, we need more bias voltage to cut off the electron flow, because the electrons would have already been accelerated a bit. You might have cleverly deduced that the amount of bias voltage is equivalent to the size of the rock. You would have been absolutely correct!

So you see, the more speed the electrons pick up, the bigger the bias voltage necessary to stop them in their tracks.

When the grid is very close to the cathode, however, we can control a very large amount of plate current with just a tiny amount of bias voltage. This is called amplification. A little signal controls a big signal.

Now, calling this process amplification bothers lots of folks; it seems to be just the opposite. Our input (bias) signal can only REDUCE the plate current, not increase it.

The point to remember here is that the tube doesn't generate any power in the first place. Any power the tube "puts out" actually comes from our plate battery. The purpose of the triode, then, is to simply "modulate" the plate current, in accordance with our commands. We supply our commands through the control grid.

Incidentally, since the control grid is negative with respect to the cathode, zero current flows through the grid. This means that the grid draws absolutely no power. We theoretically have INFINITE power gain with such a situation!

As you might imagine, it's not so rosy in the real world, but you can still have incredible power gains with a triode.

At this time we should mention that acceleration of electrons in a vacuum tube is not really the end goal, except in some exotic applications. There is no real advantage to super high speed electrons. Our goal is controlling CURRENT, that is, the number of electrons, not their speed. However because of plate resistance we NEED to have rather high voltage in order to get usable currents. (Yes, as convoluted and contrived as plate resistance is, it still follows Ohm's Law). And since we can't have high voltage without accelerating electrons, we end up with high speed electrons anyway.

Actually, we don't want our electrons to move too slowly, or we end up running into transit time problems. If an electron takes too much time shilly-shallying on the way to the plate, it may not work very well at radio frequencies. We can reduce transit time by making the space between ALL the elements very small, while keeping the ratio of grid spacing between the cathode and the plate the same. In other words, we can scale the tube for higher frequency. But we can't go too far in this scaling business, or we end up risking arc-over, that is electric sparks jumping between elements inside the tube. Not a good thing. So we can reduce plate voltage to prevent arc- over, but guess what? Lower plate voltage means longer transit times. we even begin to grasp the problem of the tube designer? We have all these conflicting requirements that need to be met. The astonishing thing is that the early tube engineers were indeed able to pull this off, using no more number-crunching power than a slide rule!

The fact of the matter is that you had a lot of the smartest people on the planet working on making tubes that ran faster, longer, and with more power. Tubes represented the culmination of all human knowledge. Until the discovery of the transistor, tubes were all we had to work with. We HAD to do it well.

Now, a tube by itself is of limited value. It has to actually work in a circuit. In the next chapter, we will talk about how tubes interact with other components. We will also introduce a few more tube elements, creating TETRODES and PENTODES.

In the meantime, next time you encounter a tube, you really should salute.

Member Comments:
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Tube or not Tube: That is the Question  
by KB0XR on March 22, 2014 Mail this to a friend!
Whew! Takes me back to "A" school for Aviation Electronics Technician in 1961 in the Navy. I had forgotten almost everything until I read this article. Thanks for posting it.
Tube or not Tube: That is the Question  
by K1KP on March 22, 2014 Mail this to a friend!
An excelent article! Even though I cut my teeth on vacuum tubes, it was fascinating to read and a very good visualization of what goes on inside a glowing tube. I just hope this article gets archived somewhere on the internet, so in 30 years, when boat anchors have outlived all the folks that know how tubes work, there's something to teach a new crowd of enthusiasts what goes on between filament and plate...
Tube or not Tube: That is the Question  
by N1NGV on March 22, 2014 Mail this to a friend!
Great article!! I have to kinda echo what the first poster said but in my case it wasn't military electronics school but rather info gleaned from books I'd checked out of my local library. Hopefully there is another one to follow this.
Tube or not Tube: That is the Question  
by W4BD on March 22, 2014 Mail this to a friend!
Well I learned this also in Class "A" Naval Aviation Electronics Tech School at NATTC Memphis(Millington)in 1959. I guess I can still work on old tube stuff better than the solid state even now. Reading Eric's articles brings back a lot old theory that I have long forgotten.


Bill W4BD
Donalsonville, Ga.
Tube or not Tube: That is the Question  
by K1FPV on March 22, 2014 Mail this to a friend!
Nice article! I broke into electronics in the mid 60's learning about tubes. For years most everything I worked on had the "Hollow-State" devices in them. I loved the warm feeling emitted by my tube receiver and transmitter on a cold and snowy winter's night! :-)

As time went on and devices were using the new Solid-State technology, things got smaller and had more features. That is progress and it will continue I presume!

It was ironic in that one day prior to my retirement, one of the techs working with me came over and asked me to help him. He had a piece of equipment that was all tubes, and since he had just recently went to tech school, he only learned solid state. I gave him a learning experience!

I have as many operating positions in my shack with old boat anchors (using tubes) as I do newer rigs with solid state technology. I still keep that warm spot in my heart for those warm glowing filaments!

Tube or not Tube: That is the Question  
by K7PE on March 22, 2014 Mail this to a friend!
Wow this is why I enjoy so much.
Excellent article.
Bob Hopkins K7PE
RE: Tube or not Tube: That is the Question  
by K9MHZ on March 22, 2014 Mail this to a friend!
Terrific writing as always, Eric!

Tube or not Tube: That is the Question  
by ONAIR on March 22, 2014 Mail this to a friend!
Great article! Brings me back to winter nights sitting in the shack during a blizzard, with those wonderful glowing tubes keeping me warm.
Tube or not Tube: That is the Question  
by K1OC on March 22, 2014 Mail this to a friend!
Ah, wonderful! Brings back memories of studying for first and second phone with the old Ameco study guides, although they weren't as well-written.

The chapter heading suggests this is from a book. Anybody know which one?
Tube or not Tube: That is the Question  
by N8TI on March 22, 2014 Mail this to a friend!
I always wondered how they could have produced vacuum tubes back in the day when horses were still in common use in farming. It really seems that the electronic industry has always been "cutting edge." The idea that the smartest people on the planet were working on hollow state technology certainly makes sense.
RE: Tube or not Tube: That is the Question  
by KH6DC on March 22, 2014 Mail this to a friend!
B+, wow that brings back memories of electronics school and working at Hughes Aircraft in SoCAL.
RE: Tube or not Tube: That is the Question  
by KB0XR on March 23, 2014 Mail this to a friend!
My A school was at NATTC in Millington, TN also. Also B school there a year later.
Tube or not Tube: That is the Question  
by W1JKA on March 23, 2014 Mail this to a friend!
Ahaaa, the glow, comfort and warmth of a REAL ham radio, now we know the cause there of. Great informative article, we need more of same.
Tube or not Tube: That is the Question  
by GILGSN on March 23, 2014 Mail this to a friend!
K1OC, go to and search his name. The book comes up. It has only one review, very negative, but it did prompt me to put the book in my wishlist right away ;-)

RE: Tube or not Tube: That is the Question  
by W1RKW on March 23, 2014 Mail this to a friend!
I like the play on words. very clever.
RE: Tube or not Tube: That is the Question  
by W8AAZ on March 23, 2014 Mail this to a friend!
They were still teaching tubes in high school trades in the mid 70's. Still have the tube lab workbooks. One of my instructors learned tube theory from Lee DeForest. But by the time I was in the military, we learned pretty much all solid state. Now if I could remember it all. But I can still tell kids I forgot more than they know!
RE: Tube or not Tube: That is the Question  
by K1OC on March 23, 2014 Mail this to a friend!
Gil, that's it! Thanks. You can read a few pages and see the table of contents by clicking on the cover. And there it is, Chapter 30. Odd review, though.

73 de Tony K1OC
Tube or not Tube: That is the Question  
by W8ARR on March 24, 2014 Mail this to a friend!
When I was a novice, I had a friend who had a Collins station and a rig that was solid state. He tuned both radios to the same station on both rigs. The Collins receiver was crystal clear with no electron noise. The newer rig when turned up you could hear the electron noise. My dream is to get a Collins station. Well, I can dream.

73 to all of my ham friends out there
Tube or not Tube: That is the Question  
by K9ZF on March 24, 2014 Mail this to a friend!
Wow, now that is an article.

I haven't taken the time to read all of it yet, just gave it a quick scan, but I am already impressed!


Amateur Radio Emergency Service, Clark County Indiana. EM78el
former K9ZF /R no budget Rover ***QRP-l #1269
Check out the Rover Resource Page at:
List Administrator for: InHam+grid-loc+ham-books
Ask me how to join the Indiana Ham Mailing list!
Tube or not Tube: That is the Question  
by W4LWZ on March 24, 2014 Mail this to a friend!
Terrific article Eric. This takes me back as well. I learned tube theory back in 1964 at Fixed Station Transmitter School, Ft. Monmouth N.J. We got to see some really big transmitter tubes do their stuff.
Tube or not Tube: That is the Question  
by K2ROC on March 24, 2014 Mail this to a friend!
Very well written article! Please publish additional chapters.
Tube or not Tube: That is the Question  
by KA5KBM on March 25, 2014 Mail this to a friend!
Thank you for a very informative article about electron tubes and how they work. When discussing the functions of the electron tube , another function comes to mind, according to those scientists "Lum and Abner"...the tube lights up so the other radio parts can see what they're doing. (HI HI)

Tube or not Tube: That is the Question  
by RADIOPATEL on March 26, 2014 Mail this to a friend!
Dear Friends and E ham and OM Eric
Amazingly simpler way of teaching !!! I need all chapters of the book. TNX
Dinesh Patel - VU2DCI
RE: Tube or not Tube: That is the Question  
by K4PIH on March 26, 2014 Mail this to a friend!
Real radios glow in the dark! This article made me fire up the Swan and go SSB.
RE: Tube or not Tube: That is the Question  
by K9MHZ on March 26, 2014 Mail this to a friend!
I think the best part is Eric's folksy, easy reading prose. If you think about it, he's explained a lot of particle physics via a device that we've all used but never gave much though beyond how cool it looks once the filament heats up. Think about that teacher/professor whom you most admired while in school, and it was probably that person who could explain such things as Eric does. I mean, of what value is it to a student to cram an equation into his skull without even attempting to describe the actual behaviors of particles, matter, etc?

Again, very nicely done, Eric!
Tube or not Tube: That is the Question  
by W3AVE on March 31, 2014 Mail this to a friend!
Reminds me of the early days of 73 Mag and the tutorials by "staff." Unconventional explanations in plain, direct, conversational language. Easy to do, very hard to do well. Good job!
Tube or not Tube: That is the Question  
by W3TTT on April 4, 2014 Mail this to a friend!
I knew a bit about tube operation, but I did not know that! Thanks for the great article. Give me more.

As for the engineers in the field, I have three letters:

R. C. A.

Tube or not Tube: That is the Question  
by W0RSP on April 7, 2014 Mail this to a friend!
Great title -- echoes no less than William Shakespeare and his perhaps greatest tragedy -- HAMLET. As you all recall from your school days, Hamlet is thinking about suicide and starts:
"To Be, or Not to Be. Aye, that is the question."
A witty scholar -- perhaps Fredson Bowers himself, wrote a spoof on deconstructionist literary theory, and began it with:
"Tube E, or not Tube E, that is the question." The spoof was humorously basing its discussion on chemistry where, as we all know -- again from our school days -- test tubes had to be identified somehow, either by numerals or letters.

72, Ade W0RSP -- former chemistry student who stunk up the whole auditorium while experimenting with various mixtures of sulphuric acid and other chemicals with my mates while we were hanging out in the dressing room (chem lab) awaiting the cue for our entrance onto the stage of the big annual performance. The chem guy figured it out and made my chemical life miserable after that. Every one else thought it was greatest prank of the whole year. That was a honor needless to say! :)
Tube or not Tube: That is the Question  
by W4VFZ on April 8, 2014 Mail this to a friend!
Having learned Morse Code while in the 3rd. grade, and went on to get my ham license in 1952, I'm definitely a vacuum tube man. My first "rig" was a 6L6 METAL envelope tube, operating as a "rock-bound", keyed or CW transmitter.

I'm writting a book on Basic Electronics for the totally novice in the field of electronics, and when I reached the section on "Active Components", I start off with vacuum tubes (including klystrons, magnatrons, TWTs, etc.) as they are much easier for the novice student to understand, as compared to such topics as "depletion zones, holes, doping, etc." as related to semiconductors.

I collect vacuum tubes, and have 3 testers, including the vunerable military TV-7/AU.
Tube or not Tube: That is the Question  
by KF7VXA on April 11, 2014 Mail this to a friend!
Great write up. I missed the days of the warm glowing tubes in the transmitter and receiver, but love to look into the slots of my amplifier and see the warm glow, smell the heated tubes, feel the warm air coming out and tuning it for maximum safe output.
The "Old days" must have really been something special.

73's John KF7VXA
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