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 Post subject: Ceramic coil forms
PostPosted: Mar Wed 29, 2006 8:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1204
Location: Watsonville, CA, US
As an extension of the coil form material the other day, I have a question about ceramic forms. Would they be as good or better thn ABS. <P>I am planning another Regenerodyne receiver variation. I have pile of test gear ready to be ravaged. Some of it includes late tube era signal generators. Some of them have wonderfull vernear drives and those polygonal ceramic forms. The RC tank is canned in an aluminum can inside the unit I am thinking of. Further I was going to use shielded cable to come out of the can to the detector. I was thinking of using the litz wire and one of those forms for the detector. The detector would be either a dual triode in cascode or a pentode. Shouldn't that be about the optimal setup? <P>------------------<BR>


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 Post subject: Ceramic coil forms
PostPosted: Mar Wed 29, 2006 8:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1863
Location: Houston Texas USA
Scot...<P>As the radio experimenter and pioneers determined nearly a hundred years ago, ceramic is an excellent material for coil forms...Not only because of its reasonably good dielectric properties (k-value), but also equally important as coil form material, its excellent physical stability in environments of potential mechanical (vibrations and other potential movements) and temperature changes...But to answer your question, and knowing that the k-value of ceramic is right there with ABS and other desirable coil form plastics, I would concur that a coil properly wound with litz wire having a polygonal ceramic form of suitable size is an excellent choice for your intended application...I'm not so keen on the aluminum enclosure insofar as that does affect the coil's unloaded Q and other operating parameters...Here some trial and error may be in order as your project takes shape and certainly the learned advice of others who have wandered down this path is beneficial.<P>Good luck with your project and keep us informed as to its progress and its ultimate performance.<P>Bruce<BR>WC5CW<P>------------------<BR>


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 Post subject: Ceramic coil forms
PostPosted: Mar Wed 29, 2006 9:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11100
Location: Vieques, PR, USA
exray wrote:
<font>quote:</font><HR>Originally posted by Scot Armstrong:<BR><B> The RC tank is canned in an aluminum can inside the unit I am thinking of. Further I was going to use shielded cable to come out of the can to the detector.<BR></B><HR>
<P>Bruce covered most everything in his typical excellent manner but this too needs addressing.<P>If you try to use a shielded cable between the tank and detector two things will happen. One, the capacitance of this cable will become a factor - maybe not critical but the extra capacitance may not be desirable. Two, from an efficiency standpoint, you will have effectively placed the dielectric of the short piece of cable into the equation and that might negate some of the advantages gained with the ceramic/litz.<P>Remember, anything you put in parallel with a tank coil will combine Q-wise just like parallel resistors. The net result will be lower than the lowest of the parallel components. This is one of the factors in "loaded Q".<P>The 'best' way to do this would be with a large, stiff conductor for stability - short as possible and spaced as far away from other components as possible for minimum capacitance to ground.<P>-Bill<BR><P>------------------<BR><A HREF="http://www.sparkbench.com/homebrew/homebrew.html" TARGET=_blank>The Sparkbench</A>


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 Post subject: Ceramic coil forms
PostPosted: Mar Wed 29, 2006 9:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1204
Location: Watsonville, CA, US
How about I mount the tube to the shield can, thus elminating the cable coming out of the LC? Basically emulating the way they used to do it on homebrew UHF super regens. I could hook the grid lead to the tube socket and the other end of the grid leak to the coil or cap. I could then eliminate the leads all together. Maybe even set it up with the tube inside the can and make a second shield inside the can that goes around the tube. I could end up with a what looks like a can with a piece of aluminum tubing poked through the lid. <P> <P>------------------<BR>


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 Post subject: Ceramic coil forms
PostPosted: Mar Wed 29, 2006 9:57 pm 
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Location: Vieques, PR, USA
That sounds pretty good.<P>-Bill<P>------------------<BR><A HREF="http://www.sparkbench.com/homebrew/homebrew.html" TARGET=_blank>The Sparkbench</A>


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 Post subject: Ceramic coil forms
PostPosted: Mar Thu 30, 2006 1:48 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1863
Location: Houston Texas USA
Scot, et al...<P>Bill has brought forward some very important considerations regarding loaded Q. To extrapolate a bit, I like your first choice of mounting the tube exterior to a sub-chassis assembly...For a couple of reasons: (1) Better thermal stability of components, in this case your coil and (2) remember, you must have adequate distance between any shielding or other substantial component and the circumference of the coil (for C de-coupling) if you want to maintain reasonable in-circuit Q and hence, desirable bandwidth characteristics...Populating a given enclosure complicates this consideration.<P>If you examine tube-type military or high-end commercial<BR>communications or test gear you will find such a construction technique widely employed in most oscillator or other low level, frequency sensitive circuits...Tubes are mounted exterior in their ceramic sockets and most often fitted with appropriate shields.<P>From my experience, one of the most troubling issues of homebrew construction projects where oscillator and frequency stability is important is thermal stability...Remedies, including adding or substituting NP0 capacitors of appropriate characteristics in LC circuits can be a long, frustrating experience...IMHO, and along with the electrical properties of this component or that, it's always best to design the physical layout of your project with mechanical and thermal stability given equal importance.<P>FWIW<P>Bruce<BR>WC5CW<P> <P>------------------<BR>


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 Post subject: Ceramic coil forms
PostPosted: Mar Thu 30, 2006 2:26 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Watsonville, CA, US
Great stuff, thanks. <P>I have two oscillators in mind for convestion. Since the piece of gear I end up adapting is already a RF oscillator I will take a close look at how it was done in the first place. They are both pretty high end laboratory grade gear. I think it is HP or Techtronics but, I don't have them at hand as I write this. I bet the engineers knew what they were doing when they were originally designed. <P>I have two possible pieces of gear to ravage. It sure looks like they will make a nice regens, in either case. Everythng is already there. Just the compontent values will need tweaking. As far as the LO and front end there is pleanty of room for that stuff. One has a vertical drum for reading the frequencies of the various LC settings. Both have those neat old vernears with the smooth greasy feel. I am thinking I can add switches next to the drum that will activate the crystals for the various oscillor frequencies. That will make a very neat looking rig. <P>Here is another hint for regen makers. A signal generator is almost a regen as it sits. It is no trick to convert one. <P> <P>------------------<BR>


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 Post subject: Ceramic coil forms
PostPosted: Mar Thu 30, 2006 4:28 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
The design of a good variable frequency oscillator involves a lot more than a lot of people think. Voltage stability is easily achieved nowadays. Mechanical stability is crucial if you want it to operate correctly.<P>Once upon a time, a young ham wanted a VFO after getting his general class license. He tried a friends Heathkit VF-1 (what a joke!), a Knight Kit VFO, slightly better, but still wanting in performance, and a converted BC-696 Command Set transmitter (worked fine, but too much crap on the bench and only 80 meter coverage).<P>So, it was decided that he would build his own. The tube, of course was a 6AG7, and the circuit an ECO, or Electron Coupled Oscillator, which is a Hartley with a pentode tube. He read up on VFO design in all the ARRL and Radio Handbooks that he could get his hands on. One of the biggest problems was obtaining the mechanical stability.<P>He had a large 6 inch reduction drive dial that looked neat and used it with carefully hand printed frequencies on it. What he finally ended up with, he used a variable capacitor made out of Invar from an old BC-375E tuning unit and a ceramic coil form from the same tuning unit. These components were firmly mounted on a 1/4" aluminum plate and a nice flex coupling coupled the capacitor to the dial. The tube was mounted above these components, so the heat it generated would rise above the frequency determining components. It made an awkward looking setup, but the proof is in the pudding.<P>That VFO worked fine and was as stable as any Collins PTO. Lessons learned? Mount everything like it is going to be mounted on a 16" gun on a battleship. Watch where you place the heat generating components. Keep the air circulation constant without any sudden changes of temperature, regulate the voltage, use a good quality coil and capacitor and a coupling that won't bind up over part of its rotation.<P>This story is true. The guy who built it was ME, back in late 1965.<BR>Curt<P>------------------<BR>Curt, N7AH<BR>(Connoisseur of the cold 807)<BR>QCWA# 25085 AMI# 242<BR>CW forever


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 Post subject: Ceramic coil forms
PostPosted: Mar Thu 30, 2006 8:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1204
Location: Watsonville, CA, US
Good advice. That is basically what I am trying to achieve with this one. I have made several of these simple superhets with Crystal LOs. I want to make something that I will not tear down later. Stability is one reason I am attracted to the idea of a crystal contolled LO. Another new factor is that I just got a milling machine. I have been using it quite a bit to cut holes and such in thicker stock. In real thick stuff nibblers and punches are no good. I have a big slab of 1/4" aluminum stock that I will try to incorperate into this build. Now that I have the both a lathe and a mill I can make the RX like a machine instead of the normal flimsey tin bend up, more like a transmission in a car. I have used channel stock and thick plate for the chasis in the past. The channel is JB welded to the deck, drilled, tapped and screwed toghether. When you do that the only thing that moves is the slight bending of the stock and that is not much. I will keep my digital camera close by as I work and post it for others. <P>------------------<BR>


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 Post subject: Ceramic coil forms
PostPosted: Mar Thu 30, 2006 9:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1863
Location: Houston Texas USA
Scott...<P>"The channel is JB welded to the deck"<P>Me thinks not necessary and a one-way procedure foreclosing on the disassembly and potential use for a different project, if so desired later on.<P>Digital image documentation, an excellent idea.<P>Just my two-cents worth.<P>Bruce<BR>WC5CW<P>------------------<BR>


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