Coolidge Schematic - vari-loopstick

Posted: 2/17/2005 5:47:16 PM
ketil b

From: edinburgh

Joined: 2/17/2005

Hi all I think I have what you are looking for. It's from a magazine from the 60s. The article is called

"For that different sound, Music a la Theremin
By Louis E. Garner, Jr.
Published November 1967, Popular Electronics"

Is this known to any one on here?

Posted: 3/17/2005 12:23:01 AM

From: Vermont, USA

Joined: 2/17/2005

Ah, The mysterious Loopstick... apparently out-of-date and nonexistant. So true. I have begun a pursuit of the Coolidge Theramin assembly, and came to the same problem--Where to get a loopstick. Well... I surfed the web and asked various sources offline, finally asking a HS Physics teacher. He said it seems to be nothing more than a simple inductor, and to make an easy substitute, one should:
wrap a pencil or similar object in magnet wire (like that which is used on motors) and experiment with different lengths to see what will work. My builder (former IBM and other big-time tech) is going to try this technique. I'll post my results as they happen. I'm also interestend in anyone else's experiences with this loopstick hangup and if you try this shortcut what happens.
Have a nice day.
Posted: 3/17/2005 6:19:16 PM
model citizen

From: Auckland, NZ

Joined: 3/8/2005

You can find variable inductor loops in most any electronics parts store...they're still used in AM radios and the like. Mouser should surely have 'em.

Of all the schematics i've looked at (when I was considering making one rather than buying something that worked correctly!) the Coolidge looked the easiest (build and components). Some others (Minimum and those 14x ones) didn't look too bad either.
Posted: 4/5/2005 4:26:57 PM

From: Vancouver Canada

Joined: 4/4/2005

I have also been having trouble with finding this item. I live in Vancouver (Canada) a relatively large city with a number of speciality electronics stores. None of them carried variable inductor coils of any sort. Also, the Radio Shack scematic does not identify the frequency or paramaters of the 'vari loopstick'.
Incidentaly, a google search lead me to a site selling vintage electronic gear. there was a 'Vari Loopstick' brand variable inductor coil (50-425 uHy) on it.
Posted: 4/5/2005 4:34:50 PM

From: Vancouver Canada

Joined: 4/4/2005

By the way, this my first forey into the world of electronics. The coolidge schematic notes that I should cut off the shield connections on the two transistors. Can anyone explain (in as simple terms as possible) what that means?
Posted: 4/19/2005 2:04:00 AM

Joined: 4/19/2005

The loopstick that Radio Shack sold under part number 274-1430 was a variable inductor with a ferrite core. The value range was approx 150uH to 350uH. It had a tap terminal connected at approx 5uh or roughly about a 10th of the way into the windings. (These are rough figures but close enough). If you don't have a way to measure inductance, a good rough estimate would be to use a ferrite antenna coil from a scrapped AM radio. This will normally be more Uh because of smaller tuning cap value in modern superhet radio (RS coil was made to work with 365pf cap to cover AM broadcast band). To make up for this, just slide the ferrite slug in and out of the coil to adjust value. It will work fine you just won't have the neat threaded rod adjustment for inductance. (You can make one though) The originals can be found on Ebay very rarely in the 15 - 30 dollar price range. Good luck. Any questions, email me. BTW, if you find an original, you can make a very cool little crystal radio by putting a 220pf fixed cap across it and germanium diode on the tap. One side of coil goes to water pipe ground and one side of coil goes to longwire antenna. 47K resistor goes from diode to ground and crystal earphone hooks in paralell with resistor. Tune by screwing the slug in and out. The original versions were very high Q with progressive windings and litz wire and will tune several stations with just this basic setup very niceley. Of course no batteries as it works from the power of the airwaves. Fun little coil. I Wish that Radio Shack/Philmore/Grayburne/JW Miller still sold them!

Posted: 4/19/2005 2:08:12 AM

Joined: 4/19/2005

Sorry, forgot one thing about the basic crystal radio. If you want more selectivity wind another coil over the top of the loopstick. Start with 20-25 turns of fine 30 ga or so wire and hook one side of that to Ant and other side to water pipe ground. Will inductively couple to the main windings and give you more seperation of stations. Sorry know this is theremin forum but these do make Killer crystal sets and would be a shame if you find one and don't try that nice little experiment.

Here is another easy crystal set to have fun with!

Posted: 4/19/2005 3:34:48 PM

From: Hillsborough, NC (USA)

Joined: 2/13/2005

Thanks for the info and links Roger!
Posted: 4/25/2005 10:21:11 PM

From: Vermont, USA

Joined: 2/17/2005

Still at the Coolidge Schematic...
It is taking a long time, due to lack of time...
We have decided to try replacing the vari-loopstick with variable capacitors. Any opinions on this? The variable capacitors can be found inside of AM radios.

Also, if anyone is interested, I have found a site selling the "oldschool" ferrite-core vari-loopstick, as called for in the Coolidge Schematic... This link will (should) bring you there:

Keep us posted on any new developments, opinions, etcetera.
Posted: 5/5/2005 12:56:14 AM

From: Toronto

Joined: 5/5/2005

Holy smokes this is a hard thing to find!
I'm sad that the person who was going to try his home made loopstick didn't follow up. However, from what I can tell, one can make a 10 microhenry loopstick by looping fine wire around a half inch tube 42 times in four inches. The formula is
Inductance = (radius^2 * turns^2) / ((9*radius)+(10*length))
where all lenghs are in inches (sorry to the scientists, but I found it online and I'm too lazy to convert) and the inductance is in microhenrys (that's still a funny unit). In proper units the formula is
H = (4 * Pi * #Turns * #Turns * coil Area * mu) / (coil Length * 10,000,000)
where mu is permeability. Air is 1 and steel is around 2000, I hear - so a steel core (the thing you wrap around) means a much shorter inductor.
I will try this tomorrow, and tell you all about how miserably I fail!

You must be logged in to post a reply. Please log in or register for a new account.