OM/Mandocello Guitar conversions

[Bernie Daniel]

Nope. Computer code I understand...

There is not to much to it really I just have an Excel spread sheet with all the physical data on strings (unit density) from D'Addario, the pitches that I need for each string (i.e., from C2 to E4) on the instrument, and the scale length.

The routine then calculates the expected tension on each string using the formulas that can be found in the D'Addario string date document. The equations are at the bottom of the previous post too.

Adding the tension for all 10 strings gives the total tension (i.e., from the tail piece to the tuners) on the instrument. So it is easy to see how much the tension changes if I decided to change one course from say 0.030" to 0.035" strings.

But keep in mind only about 13 - 15% of that total tension is directed downward onto the top of the sound board! Thankfully! To calculate that you multiple the sine of the string break angle by the total tension.

I like the plan that I have now because the total tension is lowered and the tension on each of the five courses is more or less the same -- I think that provides good even distribution of the tension over the neck?

[Pete Braccio]

Hi Bernie,

Here is what I got:

C 0.064 0.0008278 24.75 65.4 22.45195685
G 0.045 0.00041751 24.75 98 25.42683551
D 0.028 0.00016 24.75 146.8 21.86479319
A 0.016 0.00005671 24.75 220 17.40517078
E 0.011 0.0000268 24.75 329.6 18.46215872

Columns are: Note, diameter, weight, frequency, and tension per string (lbs)
That gives a total of 211.2218301 lbs.

I've put the String_Tension_Calc.xls up on my web site for all to download. In addition to the cittern info, it has calculations set up for bouzouki, octave mandolin, mandolin, mandolin, guitar,, and 12 string guitar.

Pete

[Bernie Daniel]

Hi Pete!

We are getting essentially the same results.

My earlier numbers were higher because I was using an all around heavier string set and I was using the 326 value for the constant. It is the number that D'Addario recommended for wound strings.

I talked to one of their customer service staff today and he said just ignore that and use 386.4 for all strings. That string .pdf needs to be revised and they are working on it.

Here are my calculations for the same string set you suggest on a 24.75" scale:

A= "Desired Pitch"
B= "Associated Frequency (Hz)"
C= "Best String Match (D'Adddario notation)"
D= "String Unit Weight (from D'Addario string .pdf)"
E= "Actual Tension Predicted (lbs)"

A........B..............C................D........ ............E
C2....65.41.........PB064........ 0.0008278........22.9148285

G2....98..............PB045........0.00041751..... ..25.94310326

D3....146.83.........PB028........0.00016......... ...22.317856

A3....220..............PB016 .......00005671.........17.75856625

E4....329.63..........PL011.........0.0000268..... ..18.84044455

Total tension = 215.5495971

So you can see our numbers are within a few pounds of each other.

One new thing I did learn today. When I actually measured it the string break angle on my mandocello it is about 164 degrees -- the sine of that angle is about 0.28. This means for my mandocello about 28% of the total string tension is directed down on the top of the instrument not only 15% like I had suggested earlier.

[Ken Olmstead]

So Bernie, can you estimate the total pounds pushing on the top for the mandocello conversion and the guitar before conversion? I am interested to know the difference in pressure between the guitar set up and the cello.

[Bernie Daniel]

Yes Ken it is pretty quick to estimate with the spreadsheet.

One just has to select the appropriate frequencies and string gauges for the guitar; pitches = E2 A2 D3 G3 B3 E4. Using a D'Addario EJ17 string set for the guitar (i.e., medium gauge, phospor bronze strings at 0.056", 0.045", 0.035", 0.026", 0.017", 0.013") you get 182.1 lbs for the guitar at pitch.

On the same box & neck (i.e., a The Loar LH-300 with a 24.75" scale) set up as a mandocello with a D'Addario J78 string set (0.074", 0.048", 0.034" and 0.022") set to the standard mandocello pitch (C2,G2, D3, A3) you get 282.7 lbs.

Thus, an 8-string mandocello generates about 100 lbs more total tension than a 6-string guitar on the same neck and box.

I suppose the string break angle will not change much so about 25 -28% of the total string tension (tuner to tail-piece) will be felt as down pressure on the top. So using 26% you have 47 lbs down tension as guitar and 73 lbs as a mandocello.

The set up as a cittern will be only 215 lbs total and 56 lbs downward because I am going to use lighter gauge string on it (compared to the mandocello), like Pete did, in order to keep the total tension from going past 300 lbs and 78 lbs down.

All of the mandocellos I have made from vintage arch tops seem to be standing up just fine to the pressure of a J78 string set.

Likewise, the original Gibson K-5 was based on the L-5 arch top guitar so I think there is plenty of reason to believe that a well made arch top guitar will work as a mandocello box as well!

PS Pete, I looked at the coding in your Excel spread sheet for string tension and it is essentially identical to mine i.e., T=(UW*POWER((2*24.75*F),2))/386.4 so I am not clear sure why we do not get the exactly same results but tiny difference in the pitch or string unit weight numbers could lead to a small variation in the total pressures I guess.

[Ken Olmstead]

So about 26lbs extra pressure on the top with mandocello strings versus guitar, right?

[Bernie Daniel]

So about 26lbs extra pressure on the top with mandocello strings versus guitar, right?

Yes I think that is going to be pretty close to the real world situation. I think it is a decent bet that most well made arch top guitars with either tone bars or X-bracing will stand up to mandocello strings. I'm not so sure about flat tops though.

[Eddie Sheehy]

I'm not finish fitting the bridge properly but close enough for bangin' about...

[Bernie Daniel]

I'm not finish fitting the bridge properly but close enough for bangin' about...

..

Sounds really great Eddie -- very nice tone indeed-- I love the medley of tunes! You got a pretty good deal I think!

[Ken Olmstead]

Sounds really great Eddie -- very nice tone indeed-- I love the medley of tunes! You got a pretty good deal I think!

Agreed, sounds very nice!!

[Eddie Sheehy]

Bernie, the tailpiece on mine is weird. There's a bent rod with the string lops around it pulled against 4 holes in the tailpiece that each pair goes through. Want to make me one of your 'specials'?

[Bernie Daniel]

Bernie, the tailpiece on mine is weird. There's a bent rod with the string lops around it pulled against 4 holes in the tailpiece that each pair goes through. Want to make me one of your 'specials'?

I am out of silver solder wire at the moment but will getting some this week or early next. Gold plated or nickel?

[Eddie Sheehy]

Nickel is fine Bernie. I'll p.m. you.

Thanks.

[allenhopkins]

Sorry to hear that -- when this happened to a some members a couple of years ago I thought that Eastman had came up with an improved tail piece? It was discussed in detail on the Mandocello enthusiast forum -- on Facebook as I recall.

Continuing a partial hijack: my repair guy contacted Eastman, and they do have a replacement tailpiece, a one-piece design that doesn't have the hinge in it. The Eastman rep said they'd had quite a few problems with the originals.

If it doesn't have the ebony overlay that the old tailpiece did, I will really miss the "look" of the old one. Still, better to have one that works.

[Ken Olmstead]

Continuing a partial hijack: my repair guy contacted Eastman, and they do have a replacement tailpiece, a one-piece design that doesn't have the hinge in it. The Eastman rep said they'd had quite a few problems with the originals.

If it doesn't have the ebony overlay that the old tailpiece did, I will really miss the "look" of the old one. Still, better to have one that works.

I wonder when it changed. The one I have coming is a new 2012 and it has the ebony overlay. I will have to see what design it has under the ebony! Supposed to be her Saturday but I have to work. Hoping Mom will still come to the aid of her 47 year old and hang out at my house and wait for the delivery, otherwise not till Monday after work!

[Eddie Sheehy]

I scored a 12-string trapeze tailpiece on Ebay...

[Eddie Sheehy]

Check out the tailpiece on the Gibson K5-C in the Classifieds - looks like one of your's, Bernie...

[Bernie Daniel]

Check out the tailpiece on the Gibson K5-C in the Classifieds - looks like one of your's, Bernie...

Hey Eddie, I could only wish I was capable of doing that!!

That tail piece was custom made by Don MacRostie of Red Diamond mandolins he is one of those ultimate craftsmen who can make anything I guess. I believe he was commissioned by David Harvey (probably) to make a batch of them for Gibson few years ago. It's my understanding that they are just like the original K-5 tail pieces. Makes sense because David is a perfectionist and would go that route.

I contacted Don about one couple of years ago and I think he will let me know if he ever makes any more but all that he made went to David I guess. So I rolled my own so to speak. I doubt mine hold a candle to what Don made.

Likewise I think the bridge in that K-5C was made by Steve Smith of Cumberland Acoustics . I believe that he patterned it off the 1936 ebony bridge on my K-1. My bridge was down at Steve's shop for some saddle work. David happened to be there one day and saw it and told Steve make me some of those for him I guess.

All that says to me that if some customer with a chunk of change burning a hole in his or her pocket would request it, Gibson (aka Mr. David) could build said person a new K-5!!

[Terry Allan Hall]

On Eastman's mando family page (http://www.eastmanguitars.com/mdo-10/), is the MDO-10 an octave mandolin or a mandocello (or can it be used for either, depending on what string set is used)?

Really like the look of it!

[Pete Braccio]

It's an octave. We've been told about it's coming for awhile, but I don't know if anyone has laid eyes on one yet.

[Eddie Sheehy]

bump