VSL, Tuning and Breaking Strings
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 Ok, now let's see what string gauges you might need  on a 26" vsl dulcimer , for a  1-5-5 tuning for playing in the keys of A and G . (btw a 26" scale is very versatile length for various tunings, but we're sticking with 1-5-5 here)
 TrailDad, we know you want a 1-5-5 tuning for the key of A, which would be Aee. On the  piano chart  I linked to in the last post, you'll see that those two high E notes are one step higher than the standard melody string (d) of the most common dulcimer tuning, DAd. On the piano chart, that high d is the very next key higher than middle C4, and it's shown as the 4th octave on the piano chart, D4. So, for your AEE tuning (or Aee if you want to be perfectly correct), those two strings would be tuned to E4. You always go LOWER for the bass string, so the A note lower than E4 is A3. Thus, your 1-5-5 tuning for key of A will be A3-E4-E4.
 Let's see what the  Strothers calculator  suggests for those notes on a 26" scale.
 It suggests a .014 string for the Bass/A string, and a .009 string for the two high e strings. 
 However, you also know that you'll be wanting to retune all those strings DOWN ONE STEP to play in the key of G, 1-5-5, which is Gdd (G3-D4-D4). For GDD on a 26" dulcimer, the calculator recommends strings of .015, .010 and .010.
 I would recommend the more robust set of .015, .010, and .010 for retuning between AEE and GDD in this situation. I've used .009 for high melody strings and have not liked them- I find them so thin they are both unpleasant to play on and they kink and break too easily. Using .010 gives a good feel and the strings will last longer. As to the .015 bass string for the tonic notes A or G, yes that seems pretty thin and indeed it may be a plain unwound steel string, but remember you are tuning to A3 not A2. You are tuning your  bass string  to the A3 note that is  the SAME note as the middle string  in the standard DAd tuning. So yes, it's going to need to be thinner than the average bass string.
 Now if you wanted to experiment and use the A2 in the octave BELOW the usual D3 bass string in DAd, then you'd need quite a heavy bass string to avoid floppiness on a 26" dulcimer. The calculator calls for a .027 wound string for that A2 note (which they label as A'). That would make it so your two e strings were a 12th up from the tonic A rather than the usual 5th up. Could sound cool, but a bit less traditional. You might also need to widen your slots for a .027 wound string.
 Incidentally here's where a "false nut" capo would enable you to put a more normal gauge bass string and work around this. Avoiding odd gauge strings is why people use various tunings and also capos.
  So, on a 26" dulcimer vsl, using tunings of 1-5-5 only:  
  Key of A:  A3-E4-E4
  Key of G: G3-D4-D4
  Both tunings could be achieved by using strings: .015, .010, and .010 
                                        
                                        
                                            updated by @strumelia: 01/27/21 08:52:49PM