Dave Richards AA7EE

April 17, 2013

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!

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