National Radio Company Dials and Reduction Drives

There are a few things I’ve been wanting to write about here. A couple of fellow builders have regen builds that I’m keen to relate. I have also wound two more coils for The Sproutie and would like to pass the details on, as well as a few thoughts on the subject, and coil-winding tips. However, I am experiencing a certain malaise toward writing at the moment, so will take the easy way out for the time being and simply post pictures of some vintage parts I recently acquired.

Like most builders, I like to keep a stash of parts that my be needed for upcoming projects. I don’t buy indiscriminately though – a part needs to have a good chance of being used for me to consider taking it on board. During my career as a DJ and voiceover guy, I collected CD’s fairly indiscriminately over the course of a couple of decades and ended up with a large collection that has taken a great deal of time and effort to slowly reduce in size. I don’t want to do the same thing with radio parts. However, given that I think I still have a regen or two left to build, there are a certain number of quality variable capacitors, reduction drives, and dials that still need to be given a good home in my shack 🙂

I get many of my vintage parts from eBay. I am quite picky about condition, and also reasonably disciplined when it comes to price.  There’s a Hammarlund MC-100-M variable capacitor on eBay right now. It’s used, in good condition, and it has a “Buy It Now” price of $60. There is no way I would pay that much for such a part. I have bought similar units in very good used, and NOS (new old stock) condition for $15-$20 in the last year from eBay. I consider $15 for a part like this to be a good deal, but 4 times that is exorbitant. I imagine the seller is thinking that he will sit back and keep it listed until someone buys it. Perhaps someone will.

This week, 3 really good parts arrived in my mailbox, all manufactured by the National Radio Company. The first 2 were knobs and skirts, fitted with 5:1 “Velvet Vernier” reduction drives. These planetary friction drives operate smoothly, and look good too. The first one came in a really cool box. It’s in fantastic (unused) condition and was still in it’s box –

A view from the rear –

The big manufacturers such as National, Hammarlund, EF Johnson, must have produced huge numbers of their parts, yet it still boggles my mind that there still seem to be quite a few sitting around in NOS condition, never having been used before. It is a thrill to incorporate these parts into a new project, and is one of the factors that drives me to carefully plan anything before building it. I want these remaining parts to be used in something worthwhile.

The other knob and Velvet Vernier drive was even more impressive. It was so clean, bright and shiny, my friend commented that it could have been manufactured yesterday. I agreed. It really looks that good –

The reduction drive looks like it is made from stainless steel, which contributes to it’s shiny, new look. It even came with 4 original stainless steel mounting screws, in perfect condition, one of which has a pointer head for placing at the very top of the numbered skirt.

Here’s the same part from the back. I was gobsmacked when looking at this for the first time. It could indeed have been made yesterday –

A couple of days later, another knob and drive I have been wanting for a while arrived. As a teenager, a local ham gifted me several large boxes full of tubes, chassis, and assorted parts. Among them was the famous National HRO dial, coupled with an NPW-0 gear drive. The NPW-0 was the one in which the shaft that was coupled to the variable capacitor was perpendicular to the front panel.  I’d spent plenty of time as a teen admiring that dial and gear drive, but I lacked the skills and tenacity to incorporate it into a project and to this day, I don’t really remember what happened to it. My parents probably committed much of my parts stash to the trash after I’d been in the US for a few years and they’d figured that I most likely wasn’t coming back for it.

The unit that arrived in the mail a couple of days ago was the famous HRO dial, this time coupled with a gear drive that has an output shaft parallel to the front panel.  I wanted one of these because the internal mechanism is much simpler, consisting of just a split gear and a worm drive (the other type uses six gears). A nice variable capacitor came with it, which measured at ~15-245pF. It will make a good, accurate band-setting capacitor. I am not yet sure how easy it would be to use this drive with a different variable capacitor, or if I am “stuck” with the one attached to the unit, but using the attached one will simplify the physical construction of a receiver somewhat –

The vague plan right now is for this HRO dial and drive to end up in a regen (what else!) I was even thinking of having two identical ones – one for accurate bandsetting, and one for the bandspread, though that might be overkill. I have been doing some thinking as to what, if anything, my next regen should be. I haven’t built many regens and don’t think I will become one of those enthusiasts who builds many dozens of them. I don’t have a lot of regens to compare The Sproutie with, but I don’t think that regen performance gets much better, with the possible exception of the circuits that are at the very cutting edge of modern regen research (yes, there is such a thing). The Sproutie is fantastic for AM SW listening, being both sensitive and stable, with a regeneration control that slides smoothly into oscillation. SSB reception is a little trickier but from what I’ve been reading, it shares this in common with most (if not all) regens. I’m curious about tube regens but have doubts as to whether even a good tube regen would have better performance than a well-designed and constructed solid state one. NOTE – If anyone has experience or thoughts on this subject, I would very much like to hear them.

So I’m rather stuck, as I think that with The Sproutie, I may have stumbled upon a regen architecture and circuit that works particularly well and is convenient to use. The coils are easy to wind, as there is no separate tickler tap/winding, and the regeneration is smooth. This exact same circuit was used in the WBR. The WBR achieved it’s high oscillator/antenna isolation through the use of a balanced tank circuit. The Sproutie achieves it through the use of an RF stage, so there are no problems with common mode hum, a malady often experienced with designs that don’t employ an RF stage. You won’t experience issues with microphony either, if you build it sturdily (that part is up to you!) I can’t see why I should even experiment with other circuits, unless they are likely to offer a substantial improvement.  Circuit-wise, the only thing I’d quite like is a bit more audio power, so that I can really pump audio into a separate speaker for those occasions when receiving a strong and quality signal.

These are the features I’d like for my next regen (if it ever gets built) –

1) One or two National HRO Dials and drives

2) A bit more audio power for driving a decent external speaker to room-shaking volume

That’s it really. My Sproutie is that good. I am looking for an excuse to use this beautiful HRO dial in another regen, and cannot think of any substantial circuit improvements. Sure, I could build a tube regen but if it’s not going to perform any better, what’s the point? I know they glow, and that alone is cool, but I need a little more of a reason than that.

Thank you for letting me think aloud, and share some gratuitous pictures of my new vintage parts acquisitions.





9 thoughts on “National Radio Company Dials and Reduction Drives

  1. Dave – to make pulling impossible on strong SSB/CW signals and have excellent frequency stability I would suggest building a stand-alone local oscillator (can be a DDS or VXO) alongside a regen receiver. Ideally its coupling into the receiver should be variable to handle a wide range of signals. The receiver is set at the point just before oscillation with the external oscillator used as the BFO. Even on weak AM signals the external oscillator scheme can be beneficial (work on this was done in the 1930s and there’s a QST article somewhere online about this). The separate oscillator can also be slightly offset and used as a CW transmitter if desired.

    1. Peter – what a great idea. I remember seeing a website in which a fellow was receiving SSB and CW on a crystal set by using an outboard oscillator. It hadn’t occurred to me to try this with a regen.

      1. Here’s a 1933 article about using a separate heterodyne oscillator with a regen: . I think there are a couple of other articles from the same time period describing similar schemes.

        A somewhat fun thing to play with is single-signal reception: you can set the regen just below threshold and tune the heterodyne oscillator somewhat off-center from the regen’s peak frequency response. The heterodyne oscillator will, as with any DC receiver, convert both sides of zero-beat to audio, but the below-threshold regenerative amplification (off-center from zero-beat with the heterodyne oscillator) will preferentially amplify only one side of zero-beat, yielding a sort of single-signal effect. I say “sort of” because this type of selectivity will fail in the presence of strong signals (see above article for details), and the selectivity is not so great to start with since it’s based on regenerating a single LC circuit at high RF frequencies. Still, it is fun to play with. As I recall, I was able to measure about 20 dB of opposite sideband “suppression” (it’s actually preferred sideband boosting) with this scheme at 7 MHz.

        You could even go further and use the regeneration to boost a signal say 20 kHz above zero-beat (effectively eliminating the opposite sideband at -20 kHz), then use SDR techniques to mix the 20 KHz signal down to baseband. In other words, a low-IF superhet using regeneration at RF to eliminate the RF image. (A few years ago I called this scheme the “audiogenerodyne”, since the IF is at audio frequency and regeneration is required to remove the image.) Again, this scheme is probably quite impractical to use in practice, but can be somewhat fun to play with. 🙂

  2. “However, I am experiencing a certain malaise toward writing at the moment, so will take the easy way”
    We hope you are okay Dave and like nice photos but they look blue. Maybe you feel betterr?

    1. Thanks Steve! Yes, one of the photos was taken in the late afternoon sun, and had a warm tone but the others were taken in the shade and came out a little blue. I didn’t attempt to adjust the color balance as I rather liked it, but I probably should have reduced the blue just a little.

      I have one or two blog-posts that I want to write, but I have also been winding coils for The Sproutie, and it’s hard for me to shift my focus from one task to the other. That is all – nothing more serious than that. I am feeling fine, but thank you for the concern!


  3. A solid state regen can work just as well as a tube device. And it can also remain simple. The problem with most solid state regens being made has to do with the source r/c being used. They form an audio time constant which causes howl and also greatly reduce recovered audio. There is a better way to do it….use a green LED.

    With a total of 9 parts I have achieved very comfortable audio level into 8 0hm phones and also minimal pulling. Better yet is to supply negative bias to the gate with a separate 9v battery, but the green led keeps the design simple. If I were to use one stage of audio I would blow out my eardrums. The receiver I built is a monobander for 75/80 with a gear reduction drive. What Ive come up with is the best of all worlds, and so simple you wouldnt think it would even work. Id be interested in sending you pictures or even sending it for a test drive to see what you think.

    73 Doug

    1. Doug – I’d be interested in seeing a schematic. If you have pictures of your receiver too, that would be great. My e-mail address is good on QRZ. You have confirmed what I was thinking – that a solid state regen can work as well as a tube one. I very much understand the appeal of tubes but they consume much more power, and are more prone to microphony than solid state devices and as such, its hard for me to justify using them. Thank you for your comment!


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