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 Post subject: Home brew 2.5 mH Choke (and Other Considerations) For Regen
PostPosted: Jun Thu 17, 2021 7:17 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 556
Location: Boston,Ma
Hey All,

I am exploring the idea of maybe trying to home brew a 2.5 mH choke for a simple AM Broadcast Band type 30 triode Regen based on Dave Schmarder's "Single Triode Regen".

Aside from the "single triode audio tube amp simply slapped onto a crystal set idea " (AKA Gernsbach's "Interflex" scheme), the Schmarder version of the basic Regen design is about the most fundamental tube receiver one could ever build.

I was thinking of working with #28 AWG because - 1: I have a lot of it. 2: It's about the smallest gauge I would dare to work with in any winding project (yes, I have snapped #48 - not fun)

In application for this basic circuit, would it be simply be enough to layer wind #28 on a wooden dowel to desired value, maybe break it up into 3 or 4 pi and then call it a day ?

Are there any other "complications" (self resonant frequency) , capacitance, etc, etc that could cause any SIGNIFICANT issues at broadcast band frequencies ?

I have No GDO or any "serious" RF test equipment other than an LCR meter and an ancient tube RF Signal generator.

I have seen home brew chokes done on other home brews (ie Doerle for HF) .

Also , for just giggles, I may include both a feedback vario coupler AND throttle var cap , as well as a variable resistance grid leak -simply to have more bells just whistles and intentionally over complicate such a basic design :D .

Also re my trying to "Limbo LOW " .What do folks think about how low you could run plate/and /or filament and maybe still have it "sort of" work ? Supposedly 45 VDC a good "starting point" good for such Regens (allegedly NOT the 90 volts the tube is rated for) and obviously, I 'd start at 2 VDC heater and go down from there. I think I'd like the "challenge" of how much I can rob from the tube and still "get away with it".

Also , Re alternative ideas to running a true primary/ secondary wound audio transformer (NOT an audio auto transformer) to keep B voltage off the phones.I really don't want B in any of my audio reproducers .I was thinking of the Lance Borden idea of running a fixed audio load resistor on the tube and then cap couple one leg to phones. I can't quite find the "desired" resistor value for the type 30 triode at 45 VDC (or less) / 2VDC (or less). Should it be a "2000" ohm resistor to "match" the phones or a possibly a higher value ( 10, 000 ohms ???) to "match" the tube. I am a bit fuzzy on this. Probably lots of variables here.

FWIW, my understanding of RF design is "moderate"(at best), just to let folks know.

Just throwing it all out there for any comments.

K


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 1:15 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10768
Location: Santa Rosa, CA
Krystallo... haven't seen any posts from you in a long time.

A real pi-wound choke is not only divided into sections, each section is wound in a criss-cross pattern.

Image

That's hard to do if you don't have the proper coil winder (I don't). Self Resonant Frequency is always an issue and I am not sure how important it may be. I aim for an SRF above the frequency range of my receiver. Pi winding and separate sections tend to reduce capacitance and increase the SRF.

https://www.rfcafe.com/references/elect ... -world.htm

Rich


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 1:43 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1174
Location: Tokyo
Have you considered using a resistor instead of a 2.5mH RFC?

A 2.5mH RFC will have an impedance around 8K at the low end of the AM band. Say you use a 10K resistor instead of the RFC. Regen detectors usually draw less than 1mA, often below 500uA. 1mA X 10K = 10V drop. Can your design afford a 5~10 voltage drop?

What you need following the regenerative detector is a low pass filter to shunt RF to ground. This can be either LC or RC. RC is cheaper, and avoids the 'complications' of self-resonance. What value C? Something around 100pF should work fine. It can be larger but larger values will shunt more high frequency audio to ground. Ie, a treble cut.

Rob


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 4:45 am 
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Location: S. Dartmouth MA 02748-1225 USA
The choke winding is a "Universal" wind. The 1/8" cam on the Morris winder will do that. Waxing the wire prior to winding will keep it in place as the the stylus traverses the coil form..

I don't understand the desire to "roll yer own" when it comes to 2.5mh chokes. Unless they have all been spirited away by some dark force. Most Ham flea markets will have junque boxes full of odd-n-ends, very often RF chokes.

FWIW the chokes are more effective for a regenerative at HF than BC. Though, A-K used them in many of their early AC TRF's, might be the high gain of the 24's needed them?

I have a couple of new Millen 2.5mh chokes I use as a standard in the comparator circuit of a Heathkit IT28 Capacitance Meter to verify unmarked chokes... chas

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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 5:16 am 
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Joined: Mar Sun 10, 2013 10:59 pm
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Here's a pretty good You-Toob video about winding some simple chokes that may fulfill your needs:

Choking on Radio Projects?

https://www.youtube.com/watch?v=6LlcRENlnrY

Have fun,
Ron

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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 7:20 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1174
Location: Tokyo
There is a way to avoid the B+ voltage drop when using a resistor-cap low pass filter. This is how National did it in the NC-105/121. It's a regenerative detector used only for CW/SSB. The 47K resistor and the 68pF cap are the low pass filter. The 330pF cap is the coupling to the audio section. The 100K resistor is the plate load.

The 2.5mH RFC is a curious phenomenon in regen radio building. That value appeared sometime in the 1920s or 30s, and seems to have been universally used since then. 2.5mH is somewhat meaningless without knowing the C value of the LHP. It's a sort of regen tradition: there has to be a RFC and it has to be 2.5mH.


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 1:19 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 556
Location: Boston,Ma
Hey All,

Thanks for all of the replies. LOTS of "food for thought".

Rich : The mysterious Mr. Doerle' s treatise , as published in Gernsback's "Radio Publications " (1938), give (what NOW seems to me ) a rather "laissez faire" treatment of the RFC choke , at least as it is employed in the "2 Tube 12,000 Mile Receiver". It is correct to compare the choke in THAT circuit to the "Schmarder" regen ? I would think so, but who knows.

First of all there NOW seems to be a "one size fits all" / "traditional" assignment of a 2.5 mH value in circuit(s) were (apparently) it may not necessarily be "optimal" (IF it really "matters"- will you actually HEAR a difference ??? "Maybe").

It kind of reminds me of another "traditional" , the .001 uF "Telephone Stop" capacitor across the headphone terminals of a crystal set.FWIW, I had "HEARD"(right or wrong) it may or may not be of any real bypass value at Broadcast or above and was just a "hold over" from the early long wave days . Again, I am not sure whether such a cap really is beneficial or not ( we'll save that "debate" for another time :D ). "Maybe" it's just another case of an arbitrary value / traditional component being slapped in , regardless of any beneficial effect or not, just like the old "2.5 mH routine" .

Doerle calls for #36 DSC, 300 T "close wound" (all as one single layer winding at around 2 " long ?? ) on a 1/2" dowel. From what I have just learned here, frankly it sounds kind of crumby.

Also, the schematic shows the choke, but the pictorial completely leaves it out, apparently a bit of a slip up at "Radio Publications".

I know regens are notorious for being "fussy". Just a bit of wind sway in the antenna can cause her to go into a roller coaster ride. Never mind what you do the with GUTS of the rig ! Most "theoretical" bets are probably off and it will probably come down to some trial and error on what actually works on various antennas /grounds/audio loads , etc, etc, etc.There are probably a LOT of variables involved.

The only things that will be hard mounted, at least in the begining, will be a nice 20's brass var cap w/ vernier and the tube socket. Everything else becomes trial and error at first (using modern hookup wire) ,then over to tinned rod and /or 20's cloth wire for "final" version (if there ever IS one :roll: .).

Shinkuukan : I could certainly try a 10K R in place of the RFC, voltage drop shouldn't be a big issue

Still learning after 55 years in the radio hobby ,

de N1NQC


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 2:46 pm 
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Quote:
(IF it really "matters"- will you actually HEAR a difference ??? "Maybe").
When the regenerative output parameters are correct there will be a smoothness to the regeneration control, what ever (control type) that is, no audio growl just the advent of a rushing noise as the detector spills over.. Definitely something that can be heard, more so experienced as there is control manipulation involved.

Quote:
I know regens are notorious for being "fussy". Just a bit of wind sway in the antenna can cause her to go into a roller coaster ride. Never mind what you do the with GUTS of the rig!
A regenerative set that experiences antenna sensitivity will be one of the simpler circuits. Often a two circuit tuner, less so using a three circuit tuner using loose aperiodic coupling. The amount of capacity in the antenna will have a small factor. There is no antenna sway effect when there is an RF amp, though there could be volume effect from tuning shifts. Hand capacity effects can be an issue, in all cases use of a shield behind the panel would remedy the problem.

All these "problems" are exacerbated in an HF receiver and even problematic when the set is used for CW or SSB. Bread-boarding a Shortwave set becomes an exercise... That is why so many of the early 30's commercial regeneratives were on a metal chassis.

My personal preference is for breadboard sets. There is an advantage that the base board can be populated with numerous holes in the wood and still have a very usable receiver during a development phase. Best part transferring parts to a "new" baseboard is easy.

I've been at regens a bit longer. I still much prefer vacuum tubes over a solid state rig. The earliest regen in my collection is from 1919...

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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 3:35 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 1122
Location: Montreal, Quebec
2.5mH chokes became such a standard value. Even in 1971, it was all over the ARRL Handbook. I thiught it was an absolute value.

And then I started noticing 1mH chokes in the very same places. Maybe I just started noticing more, but I think there was a shift, and it took so long to be more promitive. I know elsewhere someone was looking for 2.5 in the past year, and I said something about it not being a magicalvalue.

An RF choke is jhst a inductor. For VHF, you might as easlyy wind one as buy one, but those are low value. At one time, Radio Shack sold a pack of rf chokes, and they were specified in some projects as an inductor, rather than winding one.

The big difference with 2.5mH is that they ate big, big inductance and physically large as a result. More chance that the inductor becomes more a tuned circuit.


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 5:11 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10768
Location: Santa Rosa, CA
Here is my Doerle (2 x #30 triodes) from before our Great Fire of 2017. They were both supposed to be #30, but the rear one is a similar triode. Can't remember the number. The volume control is the filament rheostat.

Image

http://makearadio.com/visitors/v5.php

Dave credited me with building it, but I really re-built it from a radio I bought at a swapmeet in Monterrey around 1992. As you can see, it has a nice big 2.5 mH choke. Was originally built with a "vari-loopstick" coil for BC band only. I changed it to plug-in coils for BC to about 17 MHz. I enjoyed playing with it.

Have since used 1 mH chokes with good success. Still not sure how to make good decisions about chokes. As noted in comments from others, the radios "play" but is there smooth regeneration and stability? There is a lot of "lore" in regen building; some of it not quite rigorous. My favorite is the use of the grid-leak R/C in FET regen circuits.

I'm going to the CHRS sale tomorrow in Alameda. First radio "market" in over 15 months. Hope to find some plug-in coils, chokes, tube sockets, etc. Stuff you can't buy from Mouser!

Rich


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Fri 18, 2021 6:55 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 556
Location: Boston,Ma
Hey All,

After seeing all of this good info, I think I now know how I am going to proceed.

FWIW, I just cut the front panel (Hey, it's a start, right ?).

Thanks for the replies so far,

K


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Tue 22, 2021 12:36 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1174
Location: Tokyo
Perhaps the 2.5mH value was always used because it had always been used, and everybody had 2.5mH RFCs in their parts inventory, so rather than specify, say, 1mH (for SW) or 5mH (for MW), the default 2.5mH value was used because it worked well enough and because it had always been used and everybody had 2.5mH RFCs in their parts inventory...


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Wed 23, 2021 9:40 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 556
Location: Boston,Ma
Shinkuukan,

Hmmmm- 5 mH for b'cast you say.


Will try and see.

K


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 Post subject: Re: Home brew 2.5 mH Choke (and Other Considerations) For R
PostPosted: Jun Thu 24, 2021 1:04 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 1174
Location: Tokyo
K, it's not the absolute value of the RFC that's important, it's the combination of C and L values that matter.

What's needed after the detector is a low pass filter to shunt RF to ground and pass the audio signal on to the next stage for amplification. The filter can be either RC or LC.

Let's say you're using a 2.5mH RFC and a 500pF cap for the low pass filter. These are very common values. You can get identical results by using a 5mH RFC if you cut the cap value in half, to 250pF. Or you could use a 1mH RFC and increase the cap to about 1200pF.

So, why not just use a 1mH RFC (smaller and maybe cheaper) and a larger cap? Larger caps will shunt more audio treble to ground. This may or may not be a good thing. Not good for HiFi reproduction, but useful for cutting hiss on SW.

Better yet, if you can afford a small B+ drop, use a RC LPF. Much smaller, and much, much cheaper.

Rob


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