Acoustic archtop modification - adding a pickup

In this post I'm documenting my addition of an electric pickup to an old (50's era) acoustic archtop guitar. You don't see a lot of acoustic archtops these days. Flat tops are much more popular. Archtops have some characteristics that make them unsuitable for some kinds of acoustic music but they have one advantage: the only difference between an acoustic archtop (often referred to as a "jazz" guitar) and a hollowbody electric guitar is the pickup.

The Guitar

My project guitar is a 1959 Harmony H950 "Monterey Leader" archtop guitar. It's a mid-size archtop with the body made entirely from solid birch. It's not a perfect specimen; if it were I wouldn't want to mess with it. But there's enough dings and scratches that putting some holes in it won't lower the value of the guitar much. I paid $150 for this guitar on ebay.

The Pickup

As much of a beater as this instrument is I still don't want to do anything major to it. So cutting a big slot in the top to mount a pickup isn't going to happen. This rules out standard pickups because of the tight clearance between the strings and the top. But a couple companies make humbucker pickups meant to be attached to the fingerboard of an archtop. I found one on ebay for $15.

One problem with this pickup is that even though the manufacturer lists it as 3/8" thick the cabling comes out of the back of the pickup (towards the body). This adds to the clearance needed between the strings and the body considerably. Initially the treble strings contacted the pickup when fretted past the 15th fret. Also the pickup was angled such that the treble strings made a much stronger signal than the bass strings, which sounded unbalanced. I was able to cut some of the insulation off of the cable enough to take care of the fretting problem and most of the balancing problem.

The pickup brings out 4 conductors which allow for coil splitting but I found the regular humbucking wiring configuration sounded best.

The Pickguard

Next a pickguard was needed to hide the wiring and output jack. My guitar didn't have the original pickguard when I bought it. This is common because the guards were often made from celluloid which deteriorates over time. So I bought some 3-ply plastic from Stew-Mac and drew up a pattern.



My buddy routed that out on his router table and you can see it turned out well.

Tone & volume controls

My initial plan was to install tone and volume pots on the pickguard. The only spot with enough clearance for the pots is on the large bulgy area. It seemed like that would get in the way of strumming. I hooked up an external .02uF/500k tone control just to see what kind of difference it would make and it didn't seem to make that much difference. In the end I decided to leave it without controls.

Pictures

With the pickup and pickguard mounted.



And after the wiring is finished. Note the jack tip on the lower right corner of pickguard.

Sound Samples

These were all played into an Epiphone Valve Jr combo amp and recorded with an inexpensive vocal mic.

• clean
• dirty
• slide

DIY guitar amp attenuator

I got a new Valve Jr. combo amp for Christmas and it turns out that 5W is really way too loud for my ears and probably won't make me popular in the neighboring apartments. I saw mention of a "radio shack attenuator" here and there and eventually went and picked up one of these:

That's an in-wall volume control from Radio Shack, part number 40-993. They go for $18. Inside is an autotransformer with a primary coil that measures 5.3 Ohms and a secondary that is variable from 5.3 Ohms down to 0. It's rated at 25 watts so it should be fine for a 5 watt amp.

Disclaimer

I am not an expert with guitar amps or analog electronics. I have only basic knowledge of electrical concepts. Using the below design with different impedance or higher wattage amps may damage your amp or start a fire. There's a finite possibility that I have made a mistake and this will even damage the amp it is designed for (the Epiphone Valve Jr. combo w/ 4 Ohm speaker). Use this information at your own risk.

Analysis

Ok, so the first thing I did was just insert this between the speaker and the amp's output. This works and sounds fine. But 5.3 Ohms is a bit higher than the 4 Ohm load the amp is expecting. Adding a 20 Ohm resistor in parallel with the primary winding brings that down to close to 4 Ohms. Here's a schematic.

R4 should be a big power resistor. I chose two 10 Ohm, 10 watt resistors connected in series. You may be able to get away with a 20 Ohm, 10 watt resistor but be careful.

I ran a SPICE sim to see what the input impedance for this circuit is.



Dividing those two we see it's right about 4.3 Ohms. Close enough. This is really a guess since the inductance of this transformer isn't known to me.

Construction

These transformers drop right into the wall breakout boxes you would put a light switch in. There's enough room inside for a couple jacks and the power resistors. When you're done you have the attractive white and gray enclosure shown below.

Ok, it's ugly but it works. And for $25 it sounds pretty good.

Sound Sample

I recorded this with a computer headset at very low volumes in my bedroom. I didn't alter the volumes of the samples with anything other than the attenuator.

To hear the sound clip click here. Apologies for my amateur playing.

Final Warnings

1. It's unlikely this would be safe to use with anything but a 5 watt, 4 Ohm amplifier. My understanding is tube amps aren't forgiving when it comes to driving lower-than-expected loads. You could blow the amp's tranformer or anything else in there.


2. Careful with the jacks. If you screw up and plug the speaker in where the amp should be and vice versa bad things could happen. The output side of this circuit has very little impedance when the attenuation is at its maximum. Make sure you label the input (from amp) and output (to speaker) clearly.


3. Keep an eye on it. If it gets hot or you smell melting plastic you probably let the smoke out of something.