The Attack Delay Unit

Benjy's Adventure Building a Retro Guitar Effect

This article describes my attempt to recapture the past by building an electronic guitar effect like the one I made in 1970. This story includes the description of an initial failure which was turned into success by persistence and a misstep during troubleshooting.

The summer before entering college, I built an Attack Delay Unit (ADU) based on an article in the June 1970 issue of Popular Electronics. The article is "Modify Your Electronic Guitar Sound" (pages 53-61) by John S. Simonton, Jr. I purchased a kit from PAiA for $19.25 (about $137.00 in 2021 USD). An updated construction article with corrections to the original article can be found here.

The Effect

One characteristic of musical instruments is attack: how fast a note swells to peak loudness once a note is initiated. Guitars have fast attack: a played note is immediately loud. Bowed instruments have a slower attack. The volume swells as the note is initiated.

The notes produced by musical instruments also exhibit decay: the amplitude dynamic at the end of note. Guitars have slow decay. A note gets softer and softer over time; however, the ADU also affects the decay of a note so that it ends abruptly instead of decaying.

The attack and decay of a guitar note can be modified in at least three ways: 1. recording a passage on tape and playing it backwards. 2. Using a volume pedal and manually changing the attack. 3. Modifying the attack with an electronic circuit. Using a tape recording was essentially a studio effect. One example of using a volume pedal was on the "I Need You" song by the Beatles. According to a comment at, John Lennon manipulated the attack volume manually while George Harrison played lead because it was difficult to play lead and manipulate the attack volume manually.

At the time of the article, I think that no attack delay effects were commercially available, so the kit was innovative. Nowadays, effects like the Electro-Harmonix Attack Decay effect are available.

A sample of the effect produced by the circuit I reproduced from the 1970's article can be heard here. This is not a streaming site, so if the sample doesn't play, right-click and use "Save As" to download and play a local copy. Note that the accompaniment was produced with Band-in-a-Box.

Recreating the Past

The original version of the attack delay kit I bought was lost and/or damaged years ago, but I would think of it fondly over the years. I recall the effect being very good, so I decided to try to build a new version of the circuit just to see if my memory of the effect was accurate. So in July of 2020, I started purchasing parts for the unit. I found Small Bear Electronics, a company that sells electronics parts particularly related to music effects. They had the obsolete 2N5139 PNP transistors used in the original circuit.

I substituted some of the capacitors for more common standard values, e.g., 4.7 microfarad for 5.0 microfarad, and I used a wall wart 12V A/C transformer instead of the 12V filament transformer used in the original design.

The original kit came with updated information. Several resistor values were changed, and the schematic was corrected to show the proper orientation of electrolytic capacitors. The original schematic had the capacitors oriented as if ground were negative; however, that is not the case with this design.

My plan was to build the circuit on a solderless breadboard to see how it performed. If I liked it, I thought I might have a professional PCB made. Instead of using the PCB design published with the article, I thought I might use a modern PCB CAD tool to produce a new layout.

This is the block diagram from the construction article which I'm using without permission:

I started by building the main amplifier (Q1, Q2) without the FET which is used to control the gain of the amplifier. The main amp worked fine, so I proceeded to build the high-gain amp (Q3, Q4), the rectifier, and the DC amp (Q5, Q6). At that point, I monitored Q6 and played a note. Unfortunately, Q6 remained on. However, Q6 is supposed to be turned off when a loud note is struck. When Q6 is off, some combination of the delay capacitors are allowed to charge, slowly moving the gain of the main amp from 0 to 1.

The Q3, Q4 amp was indeed amplifying the guitar signal, but the output of the rectifier was never enough to turn on Q5 which would turn Q6 off. I tried to troubleshoot the problem off and on for a year without luck. In late 2021, I decided I needed to resolve the problem and reclaim my workbench. I was concerned that the unstable solderless breadboard with long wire leads might have been part of the problem, so I soldered the components to perf board. The results were the same.

As a last effort, I decided to increase the gain of Q4 by changing the bias. The result was I destroyed the transistor. At that point, my plan was to replace the transistor and throw in the towel on the project. The next day I replaced the transistor. I was monitoring the resistor network connected to the Q6 collector. I hit a note on the connected guitar, and I noticed the oscilloscope trace fell to zero and slowly rose to the original level. This is exactly the behavior the circuit should exhibit. I soldered the FET into place, and the circuit worked as designed. Had I not destroyed and replaced the transistor, I would not have found the problem.

My theory is that the gain of the original Q4 transistor was marginal. Replacing it produced enough output to turn on Q5 and turn off Q6. I later noticed that the guitar I was using had low output. Another guitar produced higher output and allowed notes to sustain longer.

The breadboarded circuit looks like this:

There are currently no switches to select the delay capacitors, just jumper straps. If I put the unit in a case, switches could be added easily.

I have no plans at this point to continue the project; however, there's a possibility I'll return to the project later and make a circuit board.

Because the unit uses the charging of one or more capacitors to time the attack delay, the guitar player must insure there is a very small dead time between notes to allow the capacitors to discharge. I haven't found that to be much of a drawback, but it does limit the way the effect is used.

There are two noticeable problems with the circuit. First, it requires a lot of drive to sustain a note for a reasonable amount of time, and the high level of input leads to distortion. Second, there is a small click when a note is turned off. But I'm fairly impressed with the effect, especially from a unit based on a non-commercial construction article.

Copyright 2021. benjy at