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I have an Epiphone SG400, and today when I played I heard that the B string is out of tune when I press on higher frets (8-12 frets). I tried to change the action and intonation, but now the all of the strings are out of tune when I press on any fret. How can I fix it?

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    With a bridge like on the SG, where you change the height of the whole bridge, you need to adjust the intonation of each string after changing the action. Look on YouTube for a tutorial about adjusting the intonation that uses a similar guitar (i.e. not one where every string has a separate height-adjustable saddle), e.g. youtube.com/watch?v=tLtqcwKwzTI – Your Uncle Bob Aug 8 at 21:29
  • Yeah I know, I tried to change the intonation of each strings but still the guitar is out of tune – user1569766 Aug 8 at 21:50
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    You may need to take it into a store that offers set up. It sounds like you tried but it may need tweaking you are not familiar with. Perhaps the truss rod also needs adjustment. – ggcg Aug 9 at 2:22
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Changing the action is more than merely adjusting the height of the bridge - which on SG is by the two larger screws - one each end of the bridge. The neck relief is part of that adjustment.

That aside, assuming the action is to your liking, then the intonation gets adjusted via the six saddles, one for each string. They change the open speaking lengths of the strings, which will generally all be slightly different. Usually,(but not always) there will be a roughly straight line, diagonally, between top (shortest) and bottom. Except very often, the B string is out of line, being slightly longer than 1st and 3rd. However, using a ruler to determine this isn't the way! It's done by comparing the harmonics with the fretted notes. I usually use 12th fret and 19th fret as datum points.

When strings are replaced, particularly with different gauges, the intonation will need checking, and often changing.

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    Might be worth expanding the explanation of harmonics: that you want the harmonic pitch to be the same as the fingered pitch (for octaves -- obviously for 5ths it's same note but different octave) – Carl Witthoft Aug 9 at 13:16
  • @CarlWitthoft - did consider, but it's been aired many times in other answers on this site. – Tim Aug 10 at 8:15
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Firstly, this is an Epiphone which means it doesn't have the same tolerances of a Gibson.

For example I have an Epiphone SG400 Custom and when I added a vibrola tremelo I found the bridge studs weren't even straight - I had to make bigger holes in the bridge by quite a lot!

Secondly, as a result of firstly, the bridge saddles might not be long enough to accommodate the intonation requirements of all six strings.

This is also an issue on thru-body strat vibratos and you'll sometimes see the little spring removed to give it more room to move - sometimes even covering the string hole!

That said best option is to let a shop judge if you need a bridge with wide saddles such as a GOTOH® WIDE TUNEOMATIC ... or if you think the guitar plays fine in every other install get the bridge yourself.

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  • 'Studs weren't even straight' - you mean perpendicular to the strings? No real need for them to be, the lower strings are longer than the higher ones. I have the same guitar, and it's designed to be like that. Epi Les Pauls are the same, not sure about real ones, but the logic is there. – Tim Aug 11 at 7:45
  • I have occasionally had to swap the saddles through 180 degrees to sort out intonation. Looking at real Les Pauls, it appeard that bridges are slanted anyway. – Tim Aug 11 at 8:51
  • @Tim No, I don't mean perpendicular to the strings. I mean the stock bridge must have been hammered onto the posts it was so tight and the 'standard' Gibson replacement bridge (with roller saddles) wouldn't go on those posts without drilling 1/8" larger holes on both sides... and even then it was tight. – Randy Zeitman Aug 11 at 19:02

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