Fixing intonation issues on a fixed bridge

Nyj

Hi all, hope someone can help with this:

I have a variax acoustic with a fixed bridge block and a separate bridge piece which fits in a slot in the block.    The intonation is very slightly out so that the 12th fret is a couple of cents sharper than the harmonic.    So I need to move the bridge piece back a little.    I figure this probably means routing out a bit more of the slot and filling it at the front with veneer.     But I wanted to ask the experienced luthiers out there if there's a better way.   If not, does anyone have a formula to figure out exactly how much to move it?

Winny_Pooh

You could cut a saddle with an overhang like I did on my GS mini

ICBM

Before you do, have you considered ‘piano wound’ strings? Where the winding stops short of the bridge saddle so only the core rests on it. I know that would restrict your string choice a lot - they’re not common, and I haven’t actually seen any for a few years - but they were popular about 20 years ago, and I always used to find they intonated flat compared to other strings... which could be useful here.

Funkfingers

Which of the strings is intonating sharp? 

Is the Variax bridge saddle dead straight with an evenly shaped top edge all the way across or is it intentionally shaved to provide some intonation compensation?

BenSirAmos

And how old are the strings you are trying to intonate?

Nyj

@Winny_Pooh - that's genius but unworkable in my case, since the variax has piezo pickups in the bridge.
@ICBM - I guess that might do it, I'll investigate
@funkfingers - they're all sharp to the same degree.   I'll post photos of the bridge later
@bensiramos - good point, the strings are not brand new

Thanks to all for your input!    I'll post photos (if I can get the photo-posting dialogue to be a good doggie)

Nyj

Here's a photo of the bridge

Nyj

Nope - that isn't a photo of the bridge ... for some reason the photo link doesn't work for me.    The photo is at this link:

https://www.dropbox.com/s/baht4flntblwaw5/P1010167.JPG?dl=0

BenSirAmos

There is really no point messing around with the bridge until you've checked the intonation with good quality new strings. My beater Variax, with the electrics removed and the piezos replaced with odd bits of bone intonates just fine. Why look at more complicated options first? 

(BTW the pic works on my phone but not on my computer - well at least the link to the dropbox pic worked)

Nyj

I'm sure the intonation is out because the bridge needs to be a fraction further from the nut.    Even with new strings it's the same - each string is out by the same fraction of a semitone.    It's not a deal-breaker and almost no one would notice, specially in the mix, but it's annoying me.

Funkfingers

Having now seen third party photographs of a Variax 700 piezo bridge, the obvious question is how vertically does your piezo module sit in the bridge slot? If there is excessive slop in the fit or the guitar top is arching, this may be sufficient to shift the individual saddles out of position. 

ICBM

Nyj said:

Nope - that isn't a photo of the bridge ... for some reason the photo link doesn't work for me.    The photo is at this link:

I had feared it would be one of those! Moving the bridge back if it proves necessary is going to be quite a job.

It's also worth checking the set-up overall - if the action is too high, that will cause the intonation to be sharp.

Nyj

Excellent point @Funkfingers ... it is sloping a bit toward the nut ... maybe if I put a shim on the nut side at the top and the other side at the bottom, it'll straighten up a bit.   @ICBM - the action is almost too low on this guitar!  This is the same guitar I mentioned a couple of weeks ago in the thread about the damaged truss rod.

Funkfingers

Nyj said:

it is sloping a bit toward the nut ... maybe if I put a shim on the nut side at the top and the other side at the bottom, it'll straighten up a bit.

A conventional one-piece bone saddle should lean very slightly towards the nut. 

Adding a shim in the Variax saddle slot might correct for excessive lean. The screws on either end of the piezo module may permit some height adjustment. 

I do not know whether the guitar body is hollow, like a regular acoustic, or chambered solid, like a Gibson Chet Atkins CE. If the guitar has a flexible soundboard, high string tension can deform it. Lighter strings might reduce that deformation.

Nyj said:

the action is almost too low on this guitar!  This is the same guitar I mentioned a couple of weeks ago in the thread about the damaged truss rod.

Seems as if somebody had been attempting to achieve a playing action similar to an electric guitar. 

Nyj

The top isn't arching - a Variax acoustic is some kind of synthetic block with veneer on it.    I'll try out the shim idea

Andyjr1515

ICBM said:

Before you do, have you considered ‘piano wound’ strings? Where the winding stops short of the bridge saddle so only the core rests on it. I know that would restrict your string choice a lot - they’re not common, and I haven’t actually seen any for a few years - but they were popular about 20 years ago, and I always used to find they intonated flat compared to other strings... which could be useful here.

This ^  
Newtone Strings do custom winds - and they are not exorbitant.

But do put a new set on anyway.  On an acoustic, I once measured an intonation 6mm out!!!!   Put a new set of strings on, intonation was instantly perfect!

Nyj

Well, once again, thanks to everyone for the insight and help ... I got a set of 'piano wound' strings and put them on, but the new strings are all out by the same amount ... about 7 cents at the 12th fret and maybe 15 at the 19th ... so I really think I'm gonna have to rout out the slot in the bridge block so the saddle piece can move back a fraction

ICBM

Nyj said:

Well, once again, thanks to everyone for the insight and help ... I got a set of 'piano wound' strings and put them on, but the new strings are all out by the same amount ... about 7 cents at the 12th fret and maybe 15 at the 19th ... so I really think I'm gonna have to rout out the slot in the bridge block so the saddle piece can move back a fraction

It was worth a try, but you're right. If all the strings are out by the same amount it's pretty certain that the bridge is simply in the wrong place.

Nyj

@ICBM is there a formula to determine how much the bridge must move?  

ICBM

You can calculate it from the number of cents and the spacing between the 11th and 12th frets, if it needs to be moved flat. (12th and 13th if it needs to go sharp.)

Nyj

Oh ... ok, thanks for the advice, but I'm not sure I understand ... if the fretted note is 5 cents sharp at the 12th fret, are you saying I must move the bridge back 5% of the distance between the 11th and 12th frets?