As part of my fascination with vacuum tubes I am almost constantly laying out different amplifier chassis designs. I usually have between two and four different designs in process at any given time. This generally follows a very predictable pattern: design the amplifier schematic, choose a size and layout which is pleasing to the eye, then try to cram everything into the resulting (usually too small) chassis. It seems I am nothing if not predictable.

Because of this personal failing I am usually looking for component choices which are as small as possible. Which brings me to the point of this post; power filter chokes. I have gotten to the point where I simply will not build a tube amp power supply without one. There is too much benefit to performance to not have a good choke in the main filter. There is only one problem, chokes tend to be big.

So several years ago I stumbled upon a very nice solution for an “under the plate” open frame choke. It was an RDC-100 8H 125mA open frame choke sold by Radio Daze (http://www.radiodaze.com). The best part was it only cost $10.95 back in 2010. In contrast to its low price point, the choke is an excellent performer. It is also relatively small which makes it easy to fit into my (usually too small) amp chassis. At the time, after I bought one to try, I ordered six to keep in my parts stash. So on I went designing and building amps, happy with my new found “economy” choke, ordering more whenever I got low. Then couple of years later when I was down to two, I went to order some more… and they were gone!

Radio Daze had shifted over to selling the Hammond line of chokes and transformers and, in the process, the little RDC-100 fell off their inventory. Now I like Hammond chokes but they are expensive. A suitable Hammond replacement for the little choke cost about $29 each; over two and a half times the price. So I went searching for an alternative that didn’t cost quite so much.

I finally settled on the Triad C-14X 6H 200mA choke. It is available from several sources for less than $20 each. Still not the very inexpensive RDC-100 but a reasonable replacement. Unfortunately, the C-14x has only one drawback. Take a look.

The little RDC-100 is on the right and the Triad C-14X in on the left. The C-14X choke is much larger than my old favorite; about twice as large. It also weighs 33.9oz verses 16.3oz for the RDC-100. The physical size makes it hard to fit in some designs and at more than twice the weight, it is much more difficult to hold secure.

Now one might ask about the Hammond equivalents. They cost more but maybe they’re a little smaller than the big Triad choke. So I went and looked at the design specs for the Hammond 159Q and 159P. These are 7H 150mA and 10H 100mA respectively. Strangely enough, Hammond lists these two chokes with the dimensional reference of “C14H” and, you guessed it, they are exactly the same size as the Triad C-14X.

So my search continues. I have tried to get information about sourcing the RDC-100 from Radio Daze but without results. I have also tried to find the Taiwan manufacturer of the RDC-100 but have been totally unsuccessful. And until I can find a smaller alternative, I’ll be using the larger choke in my new designs.

This is one of those little frustrations that happens when sourcing parts. Just when you find something that works really well in your designs a supplier changes something and your favorite part is no more. And then the search begins again.