Electric Guitar Output Voltage Levels

I was recently interested in how much output voltage you get from guitar pickups. To find out, I connected a guitar to an oscilloscope and did some measurements. Here are the numbers:

• The values are peak voltages in millivolts (double the values for peak-to-peak)
• The 'A' values represent the maximum transient peak voltage I observed (just after the string leaves the pick)
• The 'B' values are measured after about two seconds into the tone.
• I picked (strummed) hard. That's obviously a very subjective statement. Your mileage will vary.
• The values are averages over three to five repetitive measurements.
• The scope used has an input impedance of 1MOhm and an input capacity of 18pF. Probe attenuation was at 1X.

The guitar I used has a volume control and a tone control. For the measurements, volume was on maximum output and the tone control was on minimum impact.

I used the following pickups:

• Single coil (neck and middle): GFS Pro-Tube lipstick
• Humbucker (bridge): Artec Vintage Humbucker LPC210N

The measured DC series resistance of the pickups are as follows:

• Neck: 4.8K
• Middle: 6.2K
• Neck & middle (parallel): 2.7K
• Neck & middle (series): 10.9K
• Humbucker: 8.3K

The difference between soft, medium-hard and hard picking was in my case about a factor of 2 to 3. Meaning: picking the open A string softly I got about 10mV, medium-hard 20mV and hard 30mV (which you'll find under 'A' in the table, above).

And here the screenshots referred to in the table:

a.bmp - neck pickup, open A string.

b.bmp - neck pickup, open E chord. This is one of the lower samples. Most other measurements came in higher.

c.bmp - bridge humbucker, open A string.

d.bmp - bridge humbucker, open E chord.

Scratchy Pots?

Your pot is...

• either too old (resistive layer worn off; wiper doesn't connect well anymore...),
• is dirty (has dust particles under the wiper),
• has DC on the wiper,
• or... (let me know if you've experienced other sources)

For a lot of people obvious, for me something I learned recently: If you have DC voltage at the pot in your audio circuit, your pot will sound scratchy as you adjust the level. Once adjusted, no problem, but as you move the dial you hear an annoying scratchy noise.

A typical cause for the DC are defective caps that leak DC into the circuit. In my case, however, it came from an error in the schematic.

Here's the problem area in my tube overdrive circuit (see the earlier RT-1 post):

And here's the fix to the circuit:

To fix it, I added an additional cap after the pot wiper (C17, you may have to scroll to the right to see it). This reduces the effective coupling capacitance by 50%. To remedy this, I roughly doubled the input impedance of the following buffer circuit (R26 & R27).

By the way, see this very useful page about pots in general: http://www.geofex.com/article_folders/potsecrets/potscret.htm