My Ongoing Pre-Occupation With High Quality Air-Spaced Variable Capacitors

I’ve blogged before about air-spaced variable capacitors.  I’ve always liked ’em, but I think my understanding of what makes a good one is maturing a little more. I was the winning bidder on a really nice-looking specimen on eBay a few days ago.  Ever since placing the winning bid, I had been excitedly looking at the pictures of it posted by the seller. It looked great. How exciting when it arrived in the mail yesterday and I got a chance to see it “in person”, as it were!  I got it for $11.50 and I think I scored –

It is NOS (New Old Stock) meaning that while it is old, it has never been used. Surprisingly, there seems to be quite a few of these high-quality NOS caps still floating around. Here are the specs for this series of variable capacitors from Hammarlund –

The cap that I scored has nickel-plated brass vanes. Brass is good, as it expands and contracts with changes  in temperature less than aluminum does (the other main material from which variable capacitor rotors and stators are made.) Also good are the bearings on each end of the rotor shaft. I can’t see them, but I assume the bearings are hidden away. It gets better. This capacitor has wide-spaced plates, meaning less change in capacitance with temperature changes than a part with closer spacing. Oh – and this is all firmly mounted on a ceramic base. Ceramic is a great insulator and I’m thinking that this must also be good for the physical stability of the component with regards to changes in temperature.

I just noticed something. As you rotate the shaft clockwise, the capacitance increases. It’s normally the other way around. This must have been intended for use with a drive mechanism that translated the rotation of the tuning knob into rotation of the capacitor shaft in the other direction, or perhaps it’s from an era in which users expected to see wavelength increase with clockwise rotation, instead of frequency. I hope that the length of shaft protruding from the other end is enough for me to connect to, otherwise it might end up on the shelf for a few more decades! (EDIT – this wasn’t the case. It was used, very successfully, in The Sproutie Regen Receiver.)

One thing you may not appreciate from these photos is the feeling of solidity. This is a beautifully engineered part. See how the shaft is off-center? This makes for a non-linear relationship between the rotation of the shaft and the change in capacitance.  The change in capacitance occurs in such a way as to make the higher frequencies a little less cramped together, which is what happens with a capacitor where the relationship is strictly linear.

I mean, really – do variable capacitors get much better than this?  I don’t have definite plans yet for this little beauty but if my current interest in regens continues, I can see it paired up with the Jackson Brothers Dual Ratio Ball Drive and Dial I just ordered from the UK and used as the main tuning cap in a general coverage regen receiver – all built on a generously-sized aluminum chassis with front panel. (EDIT – unless I am able to connect the ball drive to the rear end of the shaft, this is not going to happen. Fingers crossed.)

Scroll back up to the top of the page and look at this fabulously engineered piece of American history sitting on top of it’s original box. That’s what it feels like to me – a piece of American history, and I got it for a few bucks. I will feel terribly privileged to be able to incorporate it into my own project at some point, though I’m going to hang onto that box.

Incidentally, while riding around Oakland, I noticed that this commercial space is up for lease.  It would be a good place for a ham-oriented business don’t you think? EDIT – It is now March 2014 and I recently noticed that this space has been turned into a coffee bar – the type that looks like it is part chemistry lab, with much glassware used in the brewing of the coffee.  Aah well – better than being left empty!


10 thoughts on “My Ongoing Pre-Occupation With High Quality Air-Spaced Variable Capacitors

    1. Thanks Bas. I have one or two more capacitors like this on the way and I’m pretty sure that at least one of them will find it’s way into a home-brew project before long.

    1. I know eh? Anything radio-oriented would be cool in that space. Even if the initials aren’t right, the last name works!

  1. I worked for Jackson Brothers Ltd here in the UK for around nine years after I left school in the late seventies and never got tired of working with the variable capacitors and slow motion drives. There is just something really satisfying about using and handling something so well engineered and tactile. Although JB itself now appears to be completely out of business the company has a long history and the product range over the years was pretty wide although some products like the “U”, “O” and “E” type capacitors and the epicyclic drives were manufactured for a very long time. So long that the operators when using the presses to make the stator packs had to hold some of the press tools a particular way due to wear on the tools to allow the finished capacitor to be calibrated on the test bridges.

    The variable you show in the photograph looks rather nice as well and looks similar to the “U” type JB made and I look forwards to seeing it in use. You mentioned coupling the variable to the slow motion drive, one of the small flexible couplers JB produced should work OK although the solid brass ones will work as well although this can make alignment a bit more dificult.

    I sometimes look on Ebay myself but considering the amount of variables, drives etc I have already I have to resist temptation and let some one else buy the items on sale despite the low prices st times.


    Kevin G6UCY

    1. Kevin – how interesting to read your comments, and I couldn’t agree more with you about the enjoyment of these well-engineered parts. Although JB’s product line is now owned by a different company in the UK, it looks as if the product line is smaller. It’s not surprising really, with the reduction in use worldwide of air-spaced variable capacitors. They are still supplying some of them new though. I do hope that will continue. I wonder where they are manufactured?

      I don’t have the space, the budget or the desire to become a “radio part hoarder”. I try to buy things that I will need in the future, but occasionally slip up and get something that looks destined to just sit on the shelf.

      Thank you very much for sharing your experience with Jackson Brothers!

  2. I’d lease that building in a shot even it wasn’t for radio related trade, but you can be sure that I would be sending CW from work with this on the walls 🙂
    A very cool building, I wonder what businesses have been there previously? A spit and polish and it would look mighty grand!

  3. Well guys I have the same love for great engineering and coils and caps should be a song. Can someone give me an example how to mechanically wire a 2-20 pf air var cap to my antenna wire. I want to use it on a slab of wood and run it in series to my antenna connection on the back of my regen.

    1. Hello Andrew,
      When I went to the high school across the street from this building if I remember correctly it was a bank. The name is long forgotten, that was so long ago. Right around the time that fat little man was flying a kite in a storm. Dave K2ZU

  4. Hello, I have this capacitor, which I got from a local hamfest last year. Now I use it for mag loop tuning. It bears differenet markings, probably a military PN. It was dirty, so I disassebled and cleaned it. It has no bearings, just the usual bushings. Otherwise a very good and solid piece of past engineering. Thanks for posting

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