Keppinger tube theremin builders - Finding Parts

Posted: 9/1/2010 9:23:25 AM

Joined: 8/22/2010

I've decided to take the plunge into building a Keppinger Theremin - it looks like a very challenging project. Heck, just scrounging the parts it work.

I've ordered many of the parts including the tubes (vacuum and phenolic) and I received a substitution on one of the vacuum tubes. The vendor substituted 6H8C for 6SN7 and assures me that "they are fine for my project" (though I never told them what I'm building). They've offered to exchange them so I feel they are trustworthy at least in that respect, but shipping them back is a hassle. Does anyone know if the 6H8C are a suitable substitution for this project or should I bother with returning the tubes and insisting on the "as advertised" 6SN7?

As well, I'm having a hard time locating C11 - Variable Panel Mount, 1/4" (6.35mm) Shaft. Does anyone know where I can purchase this part, or the part number, or at least the capacitance value of the item?


Posted: 9/1/2010 11:07:59 AM

From: Escondido, CA

Joined: 2/6/2008

I'm guessing you already hit up Antique Electronics for the tubes and transformers, right?

If not, you might check to see what they have for variable capacitors. This late in the game, beggars can't be choosers, so you might have to use a variable capacitor that mounts differently then what Mark used. You can also see if there are any HAM radio swap meets near you. They might have some parts you could use.

Posted: 9/1/2010 2:31:48 PM

Joined: 8/22/2010

I bought all of the tubes from two other sites. I'd looked at Antique Electronics but for some reason made my purchase elsewhere. I do see AE has quite a few of the necessary parts and they seem reasonably priced compared to other vendors. They have a 100 pF trimmer cap that looks like it will fit under the chassis and I suspect it will suffice to tune the oscillator.

I see Mark Keppinger has a facebook page. I'll likely have lots of questions so perhaps he'll endow me with some advice.

Thanks for pointing me back towards AE.
Posted: 9/3/2010 11:08:16 AM

From: Brooklyn,NY

Joined: 12/1/2009

I've been looking at these for my Keps: variables (
I was thinking the :"johnson machined plate (3rd image down)
#(CAV)193-8-52 2 - 26 pF" would probably work well.....
....or maybe "#(CAV) 501185-8 2 - 22 pF" with a 5 pF cap to bring it to 7 - 27pf range.
These appear to be very similar to the variable cap Phillip N. used for his Keppinger, judging by the pics on his build log.

Posted: 9/3/2010 11:25:38 AM

From: Brooklyn,NY

Joined: 12/1/2009

littlefrank, I see you've already ordered phenolic,but if there are any other builders out there who still need the five large hand-wound inductors (L1,L2,L3,L5,L6) for a Keppinger theremin, complete coils sets are available(on my shamelessly pluggged site) (

and see more images info and resources for Keppinger theremin builders at (
Posted: 9/3/2010 10:39:29 PM

From: Hillsborough, NC (USA)

Joined: 2/13/2005

Thanks for the links! I'll add them to the DIY builders resource page.
Posted: 9/4/2010 9:57:46 PM

Joined: 8/22/2010


Thanks for the info.

I found the Surplus Sales of Nebraska site last week and ordered the Hammarlund 5-37 pF. I figure I can adjust the capacitance so the oscillator tunes somewhere near mid-range.

And I did order the phenolic tubing, though Red Arrow omitted the 2.152 from my order. I called them and they said they were having trouble getting stock from their vendor and to try them again in three weeks. Oh, well. Before finding Red Arrow I was well on my way to making my own coil forms from nylon window screen and epoxy. I came up with a method that works though I was a bit worried about the electrical properties of the various epoxies that are readily (cheaply) available. You can purchase suitable epoxy through a web vendor that is made for transformer cores but it sells by the gallon and at 2-parts, $90 per gallon, I found the $200 price a bit scary.

And I was aware of the coils you have for sale on your site and it's tempting. Your coils look great. But geez, it's those cool coils that caught my eye when I first came upon the Keppinger project and heck; the challenge of making my own coils is half the fun (we’ll see about that), and most of the bragging rights.

By the way, I got on to this idea after visiting The American Museum of Radio and Electricity in Bellingham, WA. If you ever get up that way don't skip a tour of the museum, you won't regret it.

Posted: 9/5/2010 11:05:56 AM

From: Brooklyn,NY

Joined: 12/1/2009

uh oh. Red Arrow was out of RAPT21 (2.1") months ago when I last ordered! I doubt that it's that "hot" of an item and its safe to assume that they are still waiting for the same shipment. (... they wanted to hold my order until it arrived!)

littlefrank, go for winding your own coils, but dude, you do not want to make you own (epoxy)tubes. Ive done a lot of work with epoxy and making nice perfect tubes sounds like a disaster (and expensive.)
You really dont need to use phenolic. Paper or cardboard tube works perfectly. Its not as rigid and durable as phenolic, but for theremin coils, mounted to a chassis in a cabinet, paper should work fine...just dont sit on it. Check out raBt21 at red arrow. its the paper version of the 2.1 (ID)". If you really want something stronger, you can get some the coupling tube(2.1" OD), that fits inside the 2.1"ID perfectly. the two telescoping pieces can be fit and glued together(with PVA or even epoxy) for a stronger tube with thicker walls.
(Trash-picked) heavy-duty tubes used for rolls of paper are super strong and make excellent forms.
3.25" OD is a pretty common size and close enough to be used for the large antenna EQ coils.
Any non-conductive material should work. I've been thinking about doing a set with clear acrylic tube.
Posted: 9/5/2010 11:43:04 AM

From: Brooklyn,NY

Joined: 12/1/2009

Winding the coils isnt really that hard, but making them look perfect takes some skill and experience. Plan on the first several coils you wind being for "practice" while you learn the ins and outs. Good news is the theremin wont care how it looks (unless the windings are really messy)and the windings can (very very carefully) be slightly adjusted and repositioned after its wound. You will definitely need to make a good coil winder with a counter to accurately keep track of the # of winds. Make the windings as tight as possible.
Personally, I prefer to keep a multimeter (which can measure inductance) handy.
Tiny brass hardware is really tough to find in hardware stores and will probably have to be ordered. Check out McMaster-Carr or Grainger (they also carry magnet wire.)

Posted: 9/5/2010 2:10:00 PM

Joined: 8/22/2010

Yeah, I got a feel for making epoxy forms with my first attempt. It’s not that hard, I used a hot-dogged shaped balloon to form the nylon screen (I melted the seam together) into a perfect cylinder before applying the epoxy. But it’s messy and that’s even if the balloon doesn’t pop (which thankfully it did not) while applying the epoxy, or before it sets up. And then you have some sanding to do.
I was more concerned about the electrical properties as well as resistance to the heat that could build up inside the cabinet. Seems to me that plain paper might eventually degrade and disintegrate. I’ve got time so I’ll wait on the 2.1 material. All my other searches for “coil forms” and “Phenolic” lead to nothing. I’ll consider acrylic tubing if Red Arrow doesn’t come through. I asked the guy at Red Arrow about the coupling tubes and he just said “I don’t have anything 2.1”.
Well I’m pretty detail oriented and have the patience to go with it so hopefully I’ll be able to make some decent looking coils. I’ve looked at what I believe was your coil winder as well as Keppinger’s (he used a grinder arbor) and I think I can construct something that will work. I found some wooden cones at the local hardware store, but 12 bucks each for something I may not use again is too much. I have an idea for making my own cones, I don’t have a lathe so they will probably be a bit crude, but as long as they center well, I figure they’ll work. Any ideas on cones?
As for measuring inductance, the best I have for a multi-meter is a Fluke 87V and it doesn’t have an inductance function. I was considering buying an inexpensive function generator and using a comparative voltage drop method to calculate the inductance. I see you listed the measured inductance of your coils in a previous post, I’m assuming those are good numbers to shoot for and thanks for sharing that info. I was planning on winding the coils according to the detail outlined in the parts list and that the trimmers would take care of any variance.
And one more question regarding center taps if I may. I found two parts lists that describe the construction of the coils, one by Keppinger, the other compiled by J. Sparks which seems more compliant with what your coils look like. Keppinger used two 150 turn windings, one on top of the other, separated by shellac soaked kraft paper for L1, L4 and L5. Sparks specified a single 150 turn coil tapped at 50 turns. Now I’m pretty sure Sparks is a modification or improvement and I intend to follow his detail, but the tap itself; I was intending to stop the winding process at 100 turns, scrape back the enamel and solder a tap wire in place, poke the tap wire through the form at that position on the coil and then continue with the last 50 turns. Either that or attempt to pull the wire through a hole in the form at the 100 turn position without breaking it, secure it with nail polish and continue with the last 50 turns. Experimentation is expected, any suggestions.
I perused your artwork and you’ve got an enviable talent. My father was an accomplished artist so I think I have some sense of art appreciation. And I have a nephew who I think it pretty talented; his father is a renowned Belgian sculpture.

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