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Question about RF coil

dimkasmir

May 19, 2012
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I'm very knew to circuitry and electronics and am trying to build an AM radio transmitter by this guide: http://antiqueradio.org/transmitter.htm

So far I'm at the acquiring materials stage and am not sure about the RF coil. The specific one that is referred to in the guide is this one: http://www.tubesandmore.com/scripts...evclients/cemirror/ELEVATOR.FXP?item=P-C70-RF
Would something like this - http://www.frys.com/product/2313269?site=sr:SEARCH:MAIN_RSLT_PG - perform the same function? What is the difference between them?
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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More than likely no. Unfortunately, the P-C70-RF link doesn't list the inductance of the primary and secondary windings. The P-C70-RF is a classic design that brought a smile to my face when I saw it. At one time, it was quite common..... Ah, memories. :)

Also, I don't know what the author of that design was thinking when he excluded the tube from his schematic and drew only the tube socket??? That's really lame!
 

davenn

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Sep 5, 2009
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More than likely no. Unfortunately, the P-C70-RF link doesn't list the inductance of the primary and secondary windings. The P-C70-RF is a classic design that brought a smile to my face when I saw it. At one time, it was quite common..... Ah, memories. :)

Also, I don't know what the author of that design was thinking when he excluded the tube from his schematic and drew only the tube socket??? That's really lame!

yeah weird....
tho he did show the tube internals and pin connections further down the page

it should have been all together

Dave
 

Wabajig

Apr 14, 2012
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You could possibly make your own. Based on the size of the wire would be the hard part, posssibly 28 gauge. Wind a primary and secondary based on you ohm readings over a steel nail and use coated wire for insulation. Cut the nail to length when done. Would need to replace C6 with a variable cap for fine tuning. I would ask CDDRIVE and/or the distributor for more of the specifics like size of wire and shaft size. Good luck and hope that helps.
 

duke37

Jan 9, 2011
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The thickness of the wire is not critical. The thicker the wire, the lower the resistive loss, the resistance should be very low.

You must not use a nail for a former, this is conductive and the currents in it will give massive losses at radio frequencies. Use a non conductive former, a piece of plastic pipe or a cardboard tube.
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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I noticed in the photo that the Ferrite slug that usually screwed in and out from the chassis mount clip was absent in that photo. They were basically a long threaded rod about #6-32TPI with the Ferrite slug on the end and inside the tube form. It leads me to believe that the transformer is intended to not concentrate the EM flux but instead radiate as much as possible. The fact that the author mentions a 4 ft. antenna wire further confirms this. The AM band wave length is many times this length. Even @ 1/4 wave. Also, keep in mind that the winding configuration on those models followed choke winding techniques.

Even if I'm wrong (CYA :D) about this there should be a plethora of old time radio information on the net.
 

Wabajig

Apr 14, 2012
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I think CDDRIVE and I timed our answers at the same time. duke 37 might be right about the core, but I would ask CDDRIVE if remembers a ferrite core. The thinkness of the wire is critical though because it determines how many rotations around the core you will have, which in turn determines the inductance of the primary and secondary to be used in your tuned tank circuit. Looking at the transformer, it doesn't seem that the transformer would work well based on its proximity alone.
 
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CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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I think CDDRIVE and I timed our answers at the same time. duke 37 might be right about the core, but I would ask CDDRIVE if remembers a ferrite core.

I remember many RF, IF coils and transformers being available with multiple slug composites. It wasn't this particular transformer but I once worked for a company that manufactured mil spec linear and log RF amps. We used to keep a variety of slug material on the bench to adjust the transformers for that textbook thee pole bandpass (sweep trace) on the the scope. It was always a delicate balancing act between attaining the log curve spec and the picture perfect (fl-fh) -3dB bandpass while maintaining a nice flat top.

As far as this coil. Yes, it came with a slug as I described in my last post. It's a chassis mount model that's inserted from the bottom (circuit side) of the chassis. If you look closely you'll see little ears on the metal cap. When the cap was pushed through the chassis hole it would snap in place. The slug screw protruded from the top (tube side) of the chassis. If you look at the photo you can see the small hole that the screw/slug threads into.
 

TedA

Sep 26, 2011
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dimkasmir,

The short answer to your question is no. The coil from Fry's will not work.

The Fry's coil has far too little inductance, and lacks a primary winding.

Unfortunately, the Tubes and More website has just changed; last night the specified coil could be found, today it cannot! ( And where did the Google Cache feature go? Google still has links to the old pages that are now gone. )

I did look at the information on the specified (AES P-C70-RF) coil last night. This appears to be sold for use as an RF interstage transformer for the MW AM broadcast band. It is made rather like the classic "vari-loopstick", but the slug is probably threaded and screwed directly into the coil form, without the threaded brass rod sticking out the metal mounting clip at the end. There is a separate primary winding on this coil. There was no inductance specified, but as a replacement part, it would presumably work with the typical range of variable tuning cap values of about 140 to 400pF, maximum.

It is possible to determine some of the necessary specifications for the coil by analyzing the transmitter schematic.

On the schematic, the coil winding terminated at 1 and 2 is part of an LC resonant circuit. This circuit determines the transmitting frequency. The coil winding terminated at 3 and 4 is the primary. It couples energy from the tube's plate into the resonant circuit to make the circuit oscillate at the transmitting frequency.

Most of the capacitance for the resonant circuit is 150PF C6. There will be additional capacitance due to the tube and stray capacitance. Perhaps 15pF more, as a guess. This gives us 165pF as the resonating capacitance.

If we assume a 1MHz resonant frequency, about the center of the MW AM broadcast band, we can calculate the necessary inductance to resonate with the 165pF capacitance. This gives us 154uH as the nominal value for our coil. You will want an inductance value reasonably close to this.

So the Fry's coil you asked-about, at only .151 (uH ) to .209 ( uH ), comes-up a bit short. I looked for a suitable coil at Fry's and found nothing. ( Fry's used to be an electronics shop. Now a search for "transformer" turns up mostly plastic children's toys…)

The AES part may still be available, you might check with them. Other vendors of parts for old radios sell similar coils.

There are many possible alternatives, depending on what you want to do. You might buy a new coil, salvage one from an old radio, or wind your own from scratch.

If you want a coil that looks like the one at Fry's, you might try one for ( or from ) a transistor radio. Mouser still sells 455kHz IF transformers that might be adapted. The associated oscillator coils are gone from the Mouser catalog, but would also work.

Tube-type MW radio oscillator coils or IF transformers might be a better option. They would be larger, and easier to work-with.

Some choices for the coil would require some modification of the transmitter circuit to adapt it to the particular coil. Coils with low voltage insulation might be safer with an isolating capacitor on the primary. Or, you might change the circuit to a Hartley oscillator, which can also work with a tapped coil, rather than one with an isolated primary winding.

The value of C6 may have to be adjusted to get the frequency you want with a different coil. You might want to substitute a variable capacitor for C6. This would be particularly useful if the coil you use is not adjustable.

You did not say where you are located; if we know where you are, someone may have better suggestions about parts suppliers.

An old junker AM radio might be your best source for parts. It would have the coil and a tuning capacitor.

If you want to wind your own coil, look for articles on building crystal sets and regenerative receivers; the same coils should do the job in your transmitter.

Be sure to come back here with more questions as they arise.

Good luck finding all of your parts.

Ted
 

john monks

Mar 9, 2012
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Did you consider winding your own coil? When I was a kid I built almost this same circuit using a toilet paper roll with about 75 turns of #22 gauge wire. The other coil was, as I remember, about 20 or 30 turns. This creates a crude tuned transformer. So the phasing should be such that when the plate voltage goes negative the control grid goes positive. If you don't know what I mean try it both ways and you will see. C6 as TedA suggests should be a variable capacitor such as an old tuning condenser out of an old radio. Good luck.
 
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