Sunhearth dulcimers are indeed special.  They were the direct ancestors of the Bear Meadow dulcimers and are known for their exquisite intonation and warm tone.  Although my personal preference is an instrument with geared tuners and a 6+ fret, I would not suggest making any major changes to that beautiful dulcimer.  It looks to be in great shape.

A Sunhearth dulcimer is a treasure!  I know because I owned one (a different model than yours).  I passed it along to a young dulcimer player because I'm reducing the number of instruments I own and I wanted a particular young person to have it.    Enjoy your Sunhearth!  

It’s has 3 of the 4 fine tuners. I’ll need to whittle another. 

That's a beautiful instrument. Does it have the fine tuners between the bridge and end pins? They are small pieces of ebony the string goes through. I have one of the Sunhearth teardrop dulcimers.

Ken

"The dulcimer sings a sweet song."

Thanks for follow up….I have a few decisions to make based on your expertise.

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Thanks for the addition Strumelia. I was going to mention that, but I guess I got lost somewhere along the way. That's what happens when you reach three quarters of century.

Ken

"The dulcimer sings a sweet song."

Great info from Ken.

I do want to add though- you said this dulcimer has no 6.5 fret. That would make it a little hard to play in DAd using chord style playing. You'd frequently get frustrated not having that note on the melody string. 
However, if you try a 1-5-5 tuning (DAA or CGG) you would have most needed notes on the melody string. You can try out playing mostly on the melody string and leaving the other strings unfretted, sounding as drones. That's a very pretty older way of playing and this dulcimer might be perfect for giving it a try. The 'home' or tonic note of the tune is then located at the 3rd fret of the melody string.
For DAA/CGG tuning, you could use .010 or .012 gauge for the melody AND middle strings, and then as Ken suggested a .020 or a .022 for the bass low string. The thicker the string, the tighter it will be, so you don't want to go too tight/heavy on an instrument with wooden tuning pegs.
If you are used to fretting with your fingers, you can try 'finger dancing' style, where you fret the melody all on the melody string and play the other strings open and unfretted. It's a pretty way to play! You get the ancient sound of the drones.

Does your wonderful Sunhearth have the typical sunburst sound holes? We'd love to see a picture of it, it must be beautiful!

Ken,

thanks for sharing your knowledge on my instrument. Very helpful. I’ll give your string gauge suggestions a try. 
Dennis

Your Sunhearth dulcimer made by Walt Martin in Roaring Spring, PA is a great instrument. These instruments were originally meant to be in a 1 - 5 - 5 tuning (DAA or CGG primarily) and using a noter. You might have to lower the action a little if you choose to play chord/melody style.  You should be able to tune up to DAd using  0.010 for the melody strings, a 0.013 for the middle, and 0.020 for the bass. If those are too floppy, you can increase to 0.11 or 0.012, 0.014, and 0.022. Increasing the size of the strings will increase the tension of the peg and if they are friction pegs you may have trouble keeping them in tune.

Ken

"The dulcimer sings a sweet song."

Thanks for quick reply….the VSL is 27 ¾. I was planning on DAD tuning but if this particular instrument was not designed for DAD I would try something else. You answered my question on keeping it original and will forgo adding 6 ½ fret. Dusted and hanging in humidified room with my guitars.

What is the VSL (vibrating string length)? Measure in inches from the nut to the bridge. Also tell us what tuning you are hoping to tune to.

You should know that your dulcimer by Martin is one that is coveted by many. Don't make changes to it! It is pretty valuable especially since it seems it is in pristine new condition with original packaging. Just wipe off the dust. Let us know about the VSL and your preferred tuning.

I recently picked up a Sunhearth AD3 made by Walter Martin in 1976. It’s his number 266. The date, number and his signature are visible through the sound hole. Condition is near new and still in original cardboard box with sales sheets and the setup paperwork. The “mimeograph” setup paper states it had 3 strings @ .009 and one .12. This is my first dulcimer, but these gauges don’t seem to be what is standard. Any insight on this instrument and advice would be appreciated. 
thanks