Conn 36K (sousaphone) BBb to CC conversion

I've had several requests over the years to create a CC sousaphone but none have ever reached the point of actually doing it.  Usually these requests come from someone who spends all of their time on CC tubas and make their living playing in orchestras or other large ensembles.  They want a horn to do 'pick-up' gigs and play fun music without having to use their regular axe or engrain a new set of fingerings.  To follow is what I did to modify a Conn 36K (fiberglass sousa) to play in CC:

 

Here is what I'm starting with...  a somewhat 'trashed' Conn 36K.  The valve section has been wrenched off the body and the branch off the main tuning slide has been twisted badly.  Doesn't matter because I won't be using the twisted branch anyway.  To cut a BBb to CC basically involves taking about two feet out of the open bugle and that branch is a little over that long.  The main tuning slide is actually longer than it needs to be but I'll leave it long in case I need to cut a little more to get the pitch up to CC.

  

The 'normal' way to cut a piece of tubing is to remove it from the cluster, chuck it in a lathe, and use a cut-off/facing tool to part it and make the end square.  I've discovered a unique way to cut valve circuits without removing the tubes from the cluster.  I bought a couple of counterbores and several pilots.  The counterbore needs to be somewhat larger than the tube being cut and the pilot needs to be a dab smaller than the tube inside diameter.  There's no need for lots of sizes because a smaller pilot can simply be 'shimmed up' with tape.  In this instance, I've used some duct tape and applied a little grease to the outside.  I've turned the shanks on the counterbores down to fit into a 3/8" chuck on a hand drill.  I've faced off as much as 2" of tubing with no problem.  The tube end is left perfectly square with only a very small burr on the inside and outside that is easily removed with a three-cornered scraper.  The formula for exactly how much to cut is a matter of trial and error.  A good rule of thumb is to leave everything a little long.  I can always cut a little more if the pitch needs to come up more.  It takes about two feet out of the BBb open bugle to raise the pitch to CC.  The BBb bugle is about eighteen feet and the CC is about sixteen feet.  The difference is about twelve percent.  The same applies to the valve circuits.  Measure the existing lengths and take off about twelve percent.

  

Below is an image of the valve section with all three circuits cut.  Note that the 'curly-cue' at the top of the third circuit has been eliminated.  Also...  I've move the first and third tuning slides to the top for easy adjustment 'on the fly'.  As I mentioned before...  the main tuning slide has been left the original length so it can be cut if the pitch is still a bit low.  There are a lot of things involved in how much to cut so the best method is to leave some room for error and don't try to be exact with the mathematics.  I've even added a custom pull-ring to the second slide.

Below is an image showing where the cluster USED TO lay on the body.  I'll use this as a general guideline for mounting the cluster after I scrounge around for some transition pieces to replace what I'm cutting out.

With the valve section finished it's time to move on to making the new transition from the main tuning slide to the body.  I've already eliminated the first branch off the main slide completely so a new transition has to be built to make the connection between the main slide and the body.  In my 'boneyard' I found a crook off an Eb Conn tuba that is perfect.  I also cut a piece of tapered tubing from a French horn stem that matched the Eb transition and where I cut the tube going to the fiberglass body.  In the picture on the right you can get a glimpse of the F horn section just above the bottom of the main slide.  Part of the new connecting ferrule can be seen just below the main slide.

    

Here is another shot of the cluster as it will lay on the body after I add the necessary features to actually mount it.  Note that the cluster has moved away from the centerline of the body about 4"...  something that might be important to the next owner.

 

OK...  The valve section is cut and the connection to the body is done.  Now it's time to take it back apart...  devise how the valve section will be attached to the body...   and make the body presentable.  A quick trip to the 'stripper bench' and the heavy paint finished is rolling off the fiberglass.  The stripper has no affect on the fiberglass but it sure does a nice job on the paint!  Once all the old paint is off...  a light rubbing with 220 grit paper and the body is ready to the spray can.  Satin finish quick dry enamel from Ace Hardware does a nice job and functions as a primer if the the new owner wants to put on a custom color.

  

Below is the finished product.  The valve section is attached to the body using the original band clamps.  They had to be slightly modified but work just fine.  The horn plays as well as one should expect a fiberglass CC sousaphone.  Not perfect but what three valve CC tuba is? 

 

 

 

Page created on June 14, 2014

Last updated on June 22, 2014