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Need help with building a Keppinger Theremin

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Posted: 6/18/2011 9:07:40 PM
this_guy
From: United States
Joined: 6/16/2011

threads posts

I've recently decided to construct a Vacuum tube theremin as designed by Mark Keppinger, and I have a few questions...

What sort of capacitors should I use? I decided to use 'orange drop' caps for the 400V 'high quality'ones. Would mica caps work just as well, and are there other options? Also, what type of cap would you suggest for the polarized caps (I assume aluminum electrolytic caps are the first option, I just want to know if anyone tried alternatives)?

What is the voltage rating for the trimmer caps? Is it ok to use slightly different values for them (instead of 9-50p, 4-50p or 8-45p)?, and can anyone tell me an approximate value for Variable capacitor C11?

Any suggestions will be appreciated.

Thanks.
Posted: 6/18/2011 11:14:31 PM
Jeff S
From: N.E. Ohio
Joined: 2/14/2005

threads posts

The best place to get information on the Keppinger tube theremin is on the Yahoo! Group kepptheremins (http://groups.yahoo.com/search?query=Keppinger)

There you can get advice from a few fellows that have already built their own and the man himself (Mark Keppinger).

Per Mark Keppinger in regards to C11...

Suggested ranges are 5-15 pF and 10-20 pF - may be obtained by removing all but one rotor plate from an air capacitor.

Sorry about that. I had revised the component list to include this information since it was the source of so many questions. It seems the fine folks at Theremin World have never gotten around to posting the update.
Posted: 6/19/2011 11:53:42 AM
Chobbs
From: Brooklyn,NY
Joined: 12/1/2009

threads posts

Good choice. I just finished building a Keppinger and I am very pleased.
Use Mica caps for the smaller pf values- Id recommend getting a range of values:22pf, 47pf, 100pf, 200pf, 430pf etc to experiment with the tuning.

Film caps- any 400v 5% cap will work. Maybe use the orange drops in the audio path, if that your kind of thing.



tuning caps- I think that mine are all @250v. No problem yet. I used the tiny PC mount mica compression caps.-9-50pf-
like this (http://www.surplussales.com/Variables/Ceramic/CerCap1.html). They are cheap and work fine but need to be mounted somehow. I used little squares of breadboard bolted to the chassis.

for the front panel pitch adjustment I used : Johnson machined plate (CAV) 501185-8
air variable (http://www.surplussales.com/Variables/AirVariables/AirVar1.html)
Works great and the price is right- has a 3/16" shaft though. I made a brass tube bushing to adapt for 1/4" knob.
Posted: 6/19/2011 1:11:23 PM
Touchless
From: Tucson, AZ USA
Joined: 2/26/2011

threads posts

Don't under estimate the advantage of plates that break away, it is very nice in the fine tuning area when you need less response.

On that same capacitor surplus sales webpage
CAV) 160-107-18
Air variable capacitor. 2.5 - 15 pF @ 1250v, 1/4" shaft, break away plates, bushing mount.

Chobbs, I would enjoy hearing a raw sound byte of your theremin, this is an area that my own construction is humbling me at the moment.

I am hoping to better shape and enhance my sound this week.
-

Posted: 6/19/2011 6:36:54 PM
this_guy
From: United States
Joined: 6/16/2011

threads posts

Thanks for all the advice everyone.

Chobbs - I didn't see any 9-50pf caps on that page but after looking around I found 10-60 pf ones. I guess I'll get those and hope for the best.

any advice on the polarized caps(C24, C28, and C30-33)?

also, does anyone have any info on where to get (or how to make) RCA style antennas?
Thanks
Posted: 6/20/2011 12:11:58 AM
Jeff S
From: N.E. Ohio
Joined: 2/14/2005

threads posts

"also, does anyone have any info on where to get (or how to make) RCA style antennas?" - this_guy

I don't know of anyone currently making (and selling) RCA style antennas. Obviously, there is an EXTREMELY limited market for them. They are also expensive to make.

There were drawings of the RCA cabinet and hardware on this website, but the links appear to be broken now for some reason.

Since I drew them, I can send them to you if you like. Plus, we could discuss other matters.

jaygeeess [-A-T-] yahoo.com
Posted: 6/20/2011 12:23:10 AM
Touchless
From: Tucson, AZ USA
Joined: 2/26/2011

threads posts


Theremin Pipe Bending (http://mikebuffington.net/?p=524)

To keep the tube from collapsing while bending, cork off one end, fill it with "water" and freeze the pipe before bending.
Posted: 6/20/2011 10:53:59 AM
Chobbs
From: Brooklyn,NY
Joined: 12/1/2009

threads posts

"Freezing", really? I never heard of that one. Wouldnt the solid ice then make it more difficult to bend?
I've always used sand, salt, sugar, works well. Use thicker walled tube (.065") to avoid kinks.

touchless- (are you on the kep yahoo group?) Does your kep sound better when the vol bias is turned way down( beyond where full mute is lost?) The biggest improvement on my keppinger's tone came from setting the volume and bias to just where it started to clip (with no hand near loop)- I had the vol osc freq too low and was getting constant clipping of the pitch wave which resulted in a duller,more sine-like & less harmonically rich sound.
To get both minimal clipping at max volume and retain muting, I needed to compress the volume field a bit more by raising the volume freq to over 500 khz- I have it at 517khz now- this give about vol range of about 10-12"
Ill try to get some sound clips up soon.

this_guy- nothing special about the electrolytic caps, just use the listed voltage ratings.


Posted: 6/20/2011 12:13:18 PM
Touchless
From: Tucson, AZ USA
Joined: 2/26/2011

threads posts

I was watching Discovery Channel and this commercial manufacturer achieved outstanding results with [b]trombone[/b] brass tube bending using frozen water in the pipe. I have not tried it.

I will be posting another sound byte by the end of the week, to have had someone point out that my sound was “muddy” was a revelation. I seem to be improving; otherwise I will pull my hair out and maybe yours!

PS: The design is simple with excellent fundamentals; just the sound is not quite there yet.

Posted: 6/20/2011 12:43:33 PM
Chobbs
From: Brooklyn,NY
Joined: 12/1/2009

threads posts

oops, sorry Touchless...I didnt read your post correctly. Ah...your's is an original design.
With tubes?
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