Monday, November 25, 2013

Filling fret end holes after a refret

This post is a bit of a follow-up to the post about installing frets in an acoustic guitar (here: You see, in this post, we’ll be covering filling the little holes left at either end of the fret slots after you install new frets. This isn’t an entirely necessary job and a lot of people would probably be happy to stop after installing the new frets. Playability won’t be affected at all, but it certainly makes things look a bit tidier if you do this after a refret.

Firstly, let me show you what I’m talking about. These frets have just been installed. As you can see, there’s a little gap in the wood at each end of the fret slot now.

When we sanded the fretboard down while doing the refret, we made sure to save the rosewood dust. This (and some super glue) is what we’ll use to refill the holes.

After removing the strings, we mask off the neck so that the glue doesn’t go anywhere it shouldn’t. I’ve used painter’s tape here, but I’d recommend using electricians tape usually, as it’s a lot less likely to be affected by super glue (more on that later). You will see that we have masked one side only. This was a time thing, the details of which I won’t go into, but normally you’d want to mask both sides at the same time, just for the sake of efficiency.

With the holes angled up the way, we pick up some of the rosewood dust and pack it into the hole.

After brushing off any excess powder (ideally you can put this excess powder back into your store of rosewood powder), we saturate the remaining powder with super glue. Be careful not to add so much that it starts running down your guitar neck. At the same time, make sure it’s enough to saturate the rosewood dust and not just sit on top.

Now, it’s not the end of the world if you let the super glue dry completely, but I’d highly recommend that you (carefully) remove the masking tape while the super glue is still tacky. Otherwise, you might find yourself having to deal with stuck-down tape in places (as alluded to earlier, electrician’s tape is a bit more forgiving here).

Once the glue has completely dried, it’s time to file off any excess. Make sure you file away from the fret ends when doing this, otherwise you might inadvertently lift a fret.

On this particular guitar I’ve decided to remove any and all lacquer from the rosewood along the sides of the fretboard (which would match the exposed rosewood on the face of the fretboard), although you could always re-lacquer it if you were so inclined.

I’m adding a little bit of lemon oil here to give the rosewood a bit of a shine, but will leave it there.

And here’s the end result:

Tuesday, August 13, 2013

DIY attenuators

The Muddy Basin Ramblers ( / recently recorded their new album at Soundkiss Studios ( / here in Taipei. Part of their gear consists of a vintage ribbon microphone fed into an old tube pre-amp, and unfortunately this was proving to be a bit difficult to control.

According to Alex from Soundkiss, “That OP 6 is a beast of a pre amp- it is always eager to distort and get dirty sounding. Without the attenuation, I have had to ride the gain control between 0 & 1 on the dial to get a usable tone, and a fraction of an adjustment can make a huge difference.”

So today’s post is about how we made some attenuators to go between the mic and the pre-amp in order to calm things down a little bit.

Firstly a little definition may be in order. An attenuator is basically the opposite of an amplifier. Its purpose is to reduce the amplitude of an electronic signal, without distorting it. Since it is not adding any power to the signal, it is possible to make them without any power source. In fact the design we will use requires three resistors only. The trick is working out what values these resistors need to be.

Also note that in this post i will be using "impedance" and "resistance" pretty much interchangeably, even though there is technically a difference.

This is the type of attenuator we’ll go with (known as a U-pad attenuator):

And this is where we sourced most of our information from for making these attenuators:

Before working out the required resistor values, we need to know the output resistance of the microphone and the recommended input resistance of the pre-amp, both of which, in this case, were 250Ω.

The three resistors, incidentally, have names. The two that are listed as R1/2 are known as series resistors and the other one (R2) is known as a shunt resistor. The series resistors’ job is to reduce the signal (since this is a balanced signal, these two resistors should be the same). The shunt resistor’s job is to make sure that the output impedance is returned to something close to the recommended input impedance for the pre-amp (250Ω in this case). Note that simply sticking a 250Ω resistor in as a shunt resistor is an approximation, as the resistances of the series resistors and the original output impedance of the microphone can affect the overall resistance by working in parallel with the shunt resistor. However, as long as the series resistors are of a high enough value, the effect will be small enough that it does not cause a significant problem.

Using the link mentioned above (, I came up with the following list of values (we decided to try 10dB, 20dB, 30dB and 40dB only at first). Note that there are some compromises going on here, but since we weren’t sure exactly how much attenuation was required, it really didn’t matter. The main priority was to keep the output impedances about right and also to make sure that the resistors were well matched in order to reduce picking up noise, etc. 

As an example, let’s say we want a 20dB drop. As previously stated, we would like an output impedance of 250Ω, so we’ll just go with a 250Ω resistor as the shunt resistor (R2). Again, this is an approximation, but it’s close enough in this case. Looking at the table at that link, we see that for a 20dB drop, the “k” value is 10. R1 is the shunt resistor value multiplied by the “k” value minus one, so in this case it would be 250*(10-1) = 2250Ω. Of course each series resistor is half of this, so we will require two 1125Ω resistors, or as close to this as we can find. Note that since the series resistors have to be as well-matched to each other as possible, it’s wise to buy 1% tolerance ones (the shunt resistor’s accuracy doesn’t matter as much).

All right, let’s make an attenuator (I think the following photos might be from the 40dB one).

First we locate one of these XLR Tubes:

Mine came already wired inside (I think its original purpose was to swap two of the signal paths):

We remove one of the screws and open one end:

And then we unsolder the wires:

Then we remove the screw at the other end and open that:

We unsolder two of the wires but can leave the third one in place, since we’ll be keeping that:

Then we join the three resistors like so (the shunt resistor is the one in the middle):

We add some solder:

And then we solder them to the two remaining legs of the XLR connector:

Next we reconnect the two wires that we previously unsoldered to the ends of the series resistors (having slightly shortened them first):

Add some heat shrink to minimize the risk of short-circuits:

Twist the two wires from the series resistors together:

Feed the sleeve back over the wires and secure with a screw:

Solder the wires to the connector at the other end of the XLR tube (note that these wires did not go back in the same position that they were originally in, since, as previously mentioned, this XRL tube’s original purpose was to swap two of the signal paths):
Secure the second end of the XLR tube:

Test resistances to make sure everything is connected up as it should be (this one's actually measuring the 10dB attenuator):

And then we make some labels up in Illustrator:

Stick them on, et voila, some attenuators:

Here’s one in place:

And finally some of the band recording (as with all images, click to enlarge):

Remember to buy their album when it comes out soon (possibly in October, according to their blog:!

Sunday, June 23, 2013

Refretting an acoustic guitar

OK folks. Hold on to your hats. Today we're going to refret an acoustic guitar.

Here's the patient:

And here's a close-up of the frets and fretboard. As you can see, some of the frets have worn quite badly. Additionally, there are some pretty serious divots in the rosewood board now too.

This is going to be one of our main tools for pulling the frets. It’s a fairly standard, and very cheap, end cutter. From now on I’ll be referring to it as a “fret puller”.

In order to help it get UNDER the frets, we’re going to grind/file the surface flat.

I’m resting the neck of the guitar on a huge rubber support, which I can easily move up and down so that it’s directly under whichever fret I’m pulling (or later on, replacing).

Before we actually start pulling the frets up, we’re going to heat them up with a soldering iron. The one I’m using is 60W with a flat tip on it. I’ll hold this near one end of the fret for about 30 seconds, moving it around a little, but keeping it towards that end.

Then we get the fret puller and get it under that end of the fret and slowly encourage it to pull away from the wood. Patience is a virtue at this stage; pull too fast and you’ll remove a chunk of wood while you’re at it. I’d generally still have the soldering iron touching the fret this whole time, by the way, but I had to put it down in order to take this photo.

We then start to work our way along the fret with the fret puller, keeping heat on the part we're about to pull up.

At the very end, take great care as this is a likely place for wood to be pulled out by accident.

And there we have it. One down, 19 (in this case) to go:

The same process is repeated for each fret.

Here it is half done:

And all done:

Before putting new frets on, we’re going to fix up the rosewood board itself, removing all those divots in the process. To do this, we’ll use a radiused sanding block (somewhat surprisingly, this guitar has a 12” radius, so we’ll stick with that), with fairly coarse sandpaper (in this case 150 grade, though anything up to 400 or so would probably work fine).

The fretboard is sanded down until the divots are removed completely (luckily there’s plenty of material to work with). Of course the same amount of material needs to be taken off all along the fretboard.

I should add that we've taped off the soundhole so that the guitar doesn't end up getting filled with rosewood dust. In fact, this is a good way to collect the dust, which, mixed with super glue, can make a very good filler in the future.

OK, the fretboard is now nicely sanded down:

Let's brush the dust out:

Before installing any frets, the fret slot edges are bevelled with a triangular file. This will help minimise the risk of any wood lifting when we hammer in the new frets (or widen the slots).

Now the fret installation begins. We’re using pre-radiused, pre-cut-to-length frets for this. We used this method before when refretting the Hofner Colorama here:, and it worked out great there. However I think this will be the last time we use this method, as some of the frets at the upper end of this fretboard (farthest from the nut) were cut a bit shorter than I would’ve liked).

As it happens, the fret tang width is just a bit too much for the slots we already have on this guitar, so they're cut a bit wider with a hacksaw. Luckily the blade on this saw cuts the slots at the perfect width for these frets.

I’m lucky enough to be able to fit a ruler into the slots in order to check they are deep enough (in this case we need 2mm depth), but if you find yourself doing this and can’t fit a ruler in there, you can using something like an exacto knife blade marked at 2mm to test the depth instead.

OK, the slots are the right size now, so let’s hammer in the first fret. We’ll use a plastic-faced hammer for this, since it is less likely to damage the frets. First we hammer in one end:

Then we can either hammer in the other end before hammering in the middle, or we can just keep going along from one end to the other. I tend to do both ends first.

OK, that’s one fret done.

Halfway done:

The next few frets will go in the same as the others, but then we have to overcome a new problem, which is how to deal with the frets over the soundboard of the guitar. One solution I've seen is Frank Ford's lead ballast weight, which can be seen about halfway down here:

I figured I'd make a poor man's version, or rather, a man-that-doesn't-have-the-exact-same-resources' version. This dead blow hammer is filled with whatever the safe modern-day equivalent of lead shot is (for all I know, it actually is lead shot):

You may notice that I've cut about an inch off the end of it. That's so that I can get it inside the soundhole of the guitar, like so:

I then lift up the hammer (making sure the head of the hammer is under the fret I'm about to deal with), raising the guitar itself into the air, and hammer the new fret into place:

And finished:

Except of course we’re not finished at all. Now we have to bevel the edges of the frets so we don’t rip our hand to bits when we try to play the guitar.

You can do this one at a time:

Or several at a time:

Ending up with something like this:

After that, we need to level the frets, which is to say that we need to make sure the frets are all the same height.

First, let’s make sure the fretboard is level and, if necessary, adjust the truss rod to make it so:

Once it is, we colour in the frets with a Sharpie pen or similar:

Then we find something straight and flat (in this case, a spirit level that has been confirmed as really being dead straight/flat), and stick sandpaper to the bottom face:

This is then run along (not across) the fretboard, making sure that the same amount of attention is given across the width of the fretboard:

Once the black marks have disappeared, the frets should be level.

The next step isn't STRICTLY necessary, but I like to do it just in case I've inadvertently affected the radius of the frets.

We mark the frets again, and then run the radiused sanding block up and down a few times until all of the black is gone. This should be a very quick process.

Now we need to crown them, which is to say that we need to re-shape them so that they are rounded on the top.

So we mark them with a Sharpie again, and we file them to give them a kind of bevelled/rounded edge. You’ll notice that I am using fret protectors so that I don’t damage any of the wood while doing this.

What we want to end up with is a thin black line still showing along the centre of each fret. This way we know that we have not affected the overall height of any of them. 

Finally we run some steel wool up and down each fret to clean/polish/smooth them. We give each fret the same amount of attention so that if we are removing any material, at least it will be the same for each fret.

And the fret job is done:

The guitar is stringed up and any final adjustments are made:

Note that after doing this, I had to adjust the truss rod and change the height of the nut a little bit. This is to be expected, and in some cases you might even need to install a new nut. For more information on making a new nut, have a look here: