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Author Topic: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build  (Read 201129 times)

G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #60 on: July 23, 2014, 09:16:08 AM »

One thing for your consideration.  Rewire the side-tone osc. so the pot is reversed and the shaft can be accessed through a hole in the bottom plate.

Thank you for the compliments. The people deserving the praise are the team at ARRL that delivered those fantastic projects, in the past and today.

Concerning the sidetone pot, I am actually quite surprised there wasn't one in the original design. First of all, the volume is dependent on the impedance and efficiency of the (high-impedance) headphones; perhaps mine are very sensitive, because the tone is deafening if you build the circuit "as shown" in the book. Secondly, the sidetone volume is surely very much a matter of personal taste (and, how good one's hearing is!) so it seemed essential to have some control. In my case I will probably just set it once, as part of the "initial setup and testing" of the transmitter. It will be a pain because I'll have to power down and get out the chicken stick every time I adjust that pot: if I were intending to use the pot more often, I'd have put it in a better place. BTW one reason I did it this way was to make the modification totally reversible: all you have to do is remove the pot bracket and replace the pot with a 10K resistor, and the TX goes back to a "mint ARRL" version.

You mentioned a bottom plate. I have one but probably won't bother to mount it....

But....... since this isn't your main rig, the hell with it!   ::)

But, mon cher, it is my main rig. I own no other! I've never been on the air, so this TX and its owner will lose their virginity simultaneously.

73 de Martin, KB1WSY

« Last Edit: July 23, 2014, 09:22:26 AM by KB1WSY »
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K8AXW

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #61 on: July 23, 2014, 09:27:33 PM »

Good grief!  What a feeling that is going to be!  Well Martin, I wish you the best and hope you publish your results here.  

I would definitely install a bottom cover and under the circumstances, change the pot so it can be adjusted without removing the bottom cover.  The bottom cover will shield fingers from HV and also give you something on which to stick rubber feet.  Small rubber feet will keep it from sliding as well as scratching your desk, shelf or whatever.

I think perhaps you'll change the volume eventually as well as the tone frequency.
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G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #62 on: July 24, 2014, 04:42:04 AM »

Good grief!  What a feeling that is going to be!  Well Martin, I wish you the best and hope you publish your results here.  

I will start a new thread when I finally get "on the air"; because my station is entirely homebrewed, that thread will be in this Homebrew forum, which is such a wonderful source of Elmering for beginners such as myself. Estimated Date for First Key-Down: sometime in August or September. I have to build the antenna (easy, it's just a wire dipole) but the more tricky issue is getting it "landlord-approved." The owner of my apartment is a former ham but he and his wife do have esthetic criteria (they live in the apartment above mine). Still, it's nice to have a ham-aware landlord. When I first asked for permission to put up "an antenna" he said, "Do you mean just a dipole?" and he gave his tentative OK.

I would definitely install a bottom cover and under the circumstances, change the pot so it can be adjusted without removing the bottom cover.  The bottom cover will shield fingers from HV and also give you something on which to stick rubber feet.  Small rubber feet will keep it from sliding as well as scratching your desk, shelf or whatever.

There are no longer any small children in the house and the door to the shack is lockable. The transmitter weighs 4 pounds (1.8 kilos) so there's no way that you could stick your fingers underneath by accident. For extra safety when I am away, I can always remove the power cord from the IEC socket on the back. I will add the bottom cover eventually: I really should be sticking to 21st century safety habits! The other dangerous item in the shack is the drill press, but it has a removable power key, which is good.

I think perhaps you'll change the volume eventually as well as the tone frequency.

The volume control already exists, so, no problem. The tone frequency is rather high (I would estimate 800 Hz) which is not optimal for *copying* CW but in a way I like the idea of a "higher than usual" tone for outgoing code so that it is clearly differentiated from the incoming copy. I usually copy at around 600 Hz.

73 de Martin, KB1WSY
« Last Edit: July 24, 2014, 05:01:52 AM by KB1WSY »
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W1JKA

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #63 on: July 24, 2014, 07:11:03 AM »

Re: KB1WSY

  I'll repeat what's been said before, for me this is the most interesting topic on the eHam forums as I built a very similar xtmr. when I was a Novice, it was from the Sept. 1964 Electronics Illustrated mag. with picture on cover and could be built for $5.00. I didn't have $5.00 dollars so it lacked the refinement of yours, I had a free junked TV, toilet paper tube coil form, cake pan chassis and donated xtal. I even painted the chassis/coil form blue and transformer red just like the cover pic. I still have and use it after 50 yrs. along with the same screw driver to short out the filter caps after use.
  I had to laugh when you mentioned the drill press, I can handle an occasional AC zap from my tube rigs but what really scares me is the occasional flying chuck key ;). Thanks for posting your progress and keep the pics coming,  
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K8AXW

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #64 on: July 24, 2014, 09:13:55 AM »

Quote
wonderful source of Elmering for beginners

Now THAT'S a hoot!  I have NEVER in almost 6 decades of hamming encountered such quality construction from a BEGINNER! 

Martin, I don't wish to continue with the bottom cover subject but I wasn't thinking of kids.  Based upon my own personal experience, your fingers wrapping around the bottom flange while picking up that transmitter can encounter HV. 

It would be interesting to observe your beautiful transmitter arcing across the room bouncing off the ceiling and a few other things in the room!   ;D

You might also be surprised at the language you never thought you knew!
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G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #65 on: July 24, 2014, 09:33:07 AM »

You might also be surprised at the language you never thought you knew!

I discovered new depths in the English language (and in French; I'm bilingual) while constructing this transmitter. Specifically, when trying to position nuts and (particularly) lock washers in hard-to-reach (and sometimes invisible from my position) places. I do have one of those old Heathkit "nut holders" (a red plastic tube) which was very useful and I improvised with straws, double-sided Scotch tape on the end of a finger, and so forth.

73 de Martin, KB1WSY
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G3RZP

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #66 on: July 24, 2014, 09:35:10 AM »

AL has it right about the bottom plate. When I was an apprentice at Marconi in Chelmsford, I worked in a 'screened room' - a big wooden cage of 2 by 2 timber covered with chicken wire inside and out. Originally it had one door and it was very obvious where after construction, another door had been added. This was because a guy in the room by the one door had picked up a receiver which for some reason had no bottom plate on it and the guy couldn't let go because he had grabbed something connected to the B+ line. Because of where he was, nobody could get past him to kill the power........They did eventually, but he was off work for some weeks.

Fit the bottom plate!!!



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G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #67 on: July 24, 2014, 11:27:09 AM »

Martin, I don't wish to continue with the bottom cover subject but I wasn't thinking of kids.  Based upon my own personal experience, your fingers wrapping around the bottom flange while picking up that transmitter can encounter HV. 

Fit the bottom plate!!!

Alright, alright. Sheesh, you would think you guys are interested in Stayin' Alive, or something.



73 de Martin, KB1WSY
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KB1GMX

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #68 on: July 24, 2014, 11:50:28 AM »

As one with experience with grabbing and having other "help" me HV can really hurt.
I had a 500W RCA VHF-low base grab me when the dispatcher put is hand on my
shoulder and point into the final cage.   When I woke up it was about to beat him
senseless but he was already out for the count. He got the ride with lights and
sirens to the hospital.   When I visited that site again he would not come near
remotely near me.

Myself I'd have suggested a sheet of Lexan, one can then look and admire the work
but it keeps them "pointing fingers" at safe distances.

However, from a different perspective,  RFI, emissions a metal cover and even a
screened top are warranted.  This is one area where open chassis construction
was deficient.

Allison

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G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #69 on: July 24, 2014, 04:37:25 PM »

I am posting this photo specifically to tease G3RZP, W7UUU and so forth.



Don't worry. I'm in the process of building a "proper" dummy load with non-inductive resistors. However, this "lightbulb" load will get one "outing," just to honor Ye Olden Days. Then it will be ceremonially retired -- yes I know, it doesn't present a constant load, and, it radiates RF.

In fact that's quite interesting, in terms of the load. When I connected it to my VTVM, this 15W incandescent lightbulb first presented a DC resistance of 85 ohms. Then, it decayed, with the resistance increasing gradually over time, to reach about 120 ohms a few minutes later. This is with the potential supplied by the 1.5-volt battery inside the VTVM, which is much lower than the 117VAC the bulb is manufactured for. Why am I seeing this decay? (Sorry, not really an RF topic at all, but interesting.)

73 de Martin, KB1WSY
« Last Edit: July 24, 2014, 04:44:31 PM by KB1WSY »
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N7EKU

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #70 on: July 24, 2014, 08:20:17 PM »

In fact that's quite interesting, in terms of the load. When I connected it to my VTVM, this 15W incandescent lightbulb first presented a DC resistance of 85 ohms. Then, it decayed, with the resistance increasing gradually over time, to reach about 120 ohms a few minutes later. This is with the potential supplied by the 1.5-volt battery inside the VTVM, which is much lower than the 117VAC the bulb is manufactured for. Why am I seeing this decay? (Sorry, not really an RF topic at all, but interesting.)

73 de Martin, KB1WSY

Hi Martin,

The resistance of metallic conductors increases with temperature.  The small current running through it (supplied, as you say, by the meter's battery) is heating the tungsten filament.  The resistance increases with temperature due to interactions between the electrons and phonons (the general excited state of the atoms or molecules of the substance).  It's easy to picture, that as the temperature of the substance goes up, so does the excitement of its atoms or molecules, and so there are more interactions between the phonons and the things that carry current.  So -- increased resistance!  The description of phonons gets really deep into quantum physics but the concept isn't too hard I think.

The opposite however happens with semi-conductors -- their resistance decreases with temperature so that's why transistors have to be controlled for thermal runaway.

The lower resistance of metallic is one of the reasons incandescent bulbs burn out when you turn them on (the other is the shock of the temperature change and all its effects).  Here's one that's 112 years old and still going:

http://www.centennialbulb.org/cam.htm

Wow!

73,


Mark.



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G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #71 on: July 25, 2014, 03:48:28 AM »

The resistance of metallic conductors increases with temperature.  The small current running through it (supplied, as you say, by the meter's battery) is heating the tungsten filament.  The resistance increases with temperature due to interactions between the electrons and phonons (the general excited state of the atoms or molecules of the substance).  It's easy to picture, that as the temperature of the substance goes up, so does the excitement of its atoms or molecules, and so there are more interactions between the phonons and the things that carry current.  So -- increased resistance!  The description of phonons gets really deep into quantum physics but the concept isn't too hard I think.

Mark: Many thanks. So, indeed, a lightbulb is a terrible dummy load! But 1968 Novice Ham wasn't expected to spend non-existent dollars on a high-power, non-inductive resistor. So, as explained in "How to Become a Radio Amateur," my little transmitter should be loaded up as follows:

First of all, remember that antenna socket J2 is a female RCA (phono) socket. "Insert a No. 6 screw about 1 inch long into J2 until it makes firm contact. Connect a 15-watt, 115-volt lamp to the output of the transmitter, with one terminal of the lamp connected to the screw at J2, and the other terminal of the lamp connected to some convenient point on the chassis.... If a socket with short leads isn't available for the lamp, solder wires to the shell and base contacts."

There you have it, the ham homebrewing philosophy in a nutshell. Don't spend a few cents on an RCA plug and a lamp socket. Instead, ram a No. 6 screw into the antenna socket and solder the wires directly to the bulb, unless of course you happen to have a socket "available" in your (non-existent) Novice junkbox!

Proving that no homebrewed project is ever "finished," however basic the project, this morning I fine-tuned my incandescent dummy load. I had noticed that the shell of the bulb was connected to the center wire of the coax, and the base of the bulb was connected to the braid. This theoretically meant that the "hot" side was connected to a larger expanse of exposed metal (the bulb shell or screw). So this morning I reversed the contacts, just in case it slightly lowers the risk of RFI. (OTOH both legs of a half-wave dipole radiate, don't they? So does it matter which way round the bulb is wired?)

Martin's First Law of Homebrewing: If you connect a two-wire lead to something, and do it at random, you will always connect it the "wrong" way round the first time even though the statistical probability of getting it right is actually 50 percent!

73 de Martin, KB1WSY
« Last Edit: July 25, 2014, 03:53:05 AM by KB1WSY »
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G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #72 on: July 25, 2014, 02:26:56 PM »

A productive day. Here's Chapter Eleven, "Building an Ersatz Dummy Load -- and Then a Real One":

http://tinyurl.com/kwz7fc2

Tomorrow, other commitments permitting, I will check all the wiring on the transmitter and begin testing it.
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N7EKU

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #73 on: July 25, 2014, 05:06:57 PM »


Mark: Many thanks. So, indeed, a lightbulb is a terrible dummy load! But 1968 Novice Ham wasn't expected to spend non-existent dollars on a high-power, non-inductive resistor. So, as explained in "How to Become a Radio Amateur," my little transmitter should be loaded up as follows:


Hi Martin,

I wasn't saying that a light bulb is a terrible dummy load -- you just expressed some curiosity about the resistance of the bulb changing so I thought I'd "en-light-in" you a bit.  Hihihihi!  Lol!

I haven't measured them "in action", but I'm sure they were recommended as dummy loads for tube transmitters because their resistance stayed in a reasonable range for tuning up the output network.  Then when you hooked up to your antenna, you were pretty close already and only had some fine tuning to do.

Actually, I don't think a 50ohm load would have been any better in those days anyway, because the antenna didn't necessarily have to be 50ohms since the output network could be set to match a range of values.

For myself, screws pushed in sockets and clip leads are all OK until something unnecessarily lets its smoke out (been there done that.)  I usually eat a few cents and minutes on genuine plugs/socket/wires.  Have to say I try to obey all the old cliches if can (penny wise..., a stitch in time..., etc).  Even then Mr. Murphy kicks my butt sometimes anyway!

Nice looking stuff -- and I'll just add that I'm happy you put a bottom cover on too (from another who's been knocked on his ass and never want it to happen to myself or anyone else ever again!)

73,


Mark.




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G3EDM

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RE: "A Simple Two-Tube Transmitter" ARRL 1968 -- Build
« Reply #74 on: July 25, 2014, 05:12:52 PM »

Nice looking stuff -- and I'll just add that I'm happy you put a bottom cover on too (from another who's been knocked on his ass and never want it to happen to myself or anyone else ever again!)

Once again I would like to thank K8AXW, G3RZP and now N7EKU for reminding me of what really matters: Stayin' Alive!



73 de Martin, KB1WSY
« Last Edit: July 25, 2014, 05:24:38 PM by KB1WSY »
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