aion HELIOS User manual

HELIOS CLASSIC DISTORTION 1
PROJECT NAME
HELIOS
BASED ON
EFFECT TYPE
PROJECT SUMMARY
DOCUMENT VERSION
Pro Co RAT Distortion
A distortion pedal with a very distinctive and tight mid-to-high-gain character that made it a staple in the
rigs of 1980s hard rock bands.
Distortion 1.0.1 (2023-06-29)
BUILD DIFFICULTY
Easy
Actual size is 2.3” x 1.86” (main board) and 2.3” x 0.86” (bypass board).

HELIOS CLASSIC DISTORTION 2
TABLE OF CONTENTS
1Project Overview 7Drill Template
2Introduction & Usage 8Enclosure Layout
3-4 Parts List 9Wiring Diagram
5Build Notes 10 Licensing
6Schematic 10 Document Revisions
INTRODUCTION
The Helios Classic Distortion is a recreation of the Pro Co RAT Distortion. First released in 1978, it went
through several iterations over the next few decades and was released under other names like the RAT2
and Turbo RAT. The core circuit remained essentially the same throughout its lifespan.
The RAT can be considered an evolution of the MXR Distortion+. It has the same basic op-amp gain
stage followed by diode-to-ground clipping. However, it also significantly alters the EQ in the op-amp
stage as well as adding a tone control and output buffer.
The Helios adds two modifications: a clipping switch to select between different sets of diodes, and a
Sweep control that changes the EQ and gain structure.
The 125B version of the Helios has been overhauled from the original 1590B project, reducing some
flexibility in favor of an easier and more straightforward build. The previous version of the Helios will
remain available.
USAGE
The Helios has the same control layout as most overdrive or distortion effects:
• Filter is a tone control. It works backwards from the traditional tone control operation, cutting
treble as you turn it clockwise.
• Distortion controls the amount of gain in the op-amp gain stage that is then clipped in a later stage.
• Volume controls the overall output of the effect.
In addition, there are two modifications included for added flexibility:
• Sweep (knob) changes the cutoff frequency of one of the high-pass filters in the op-amp stage,
allowing more low end to pass through and thickening the tone.
• Clipping (toggle switch) selects between different sets of clipping diodes which changes the
character of the drive tone.

HELIOS CLASSIC DISTORTION 3
PARTS LIST
This parts list is also available in a spreadsheet format which can be imported directly into Mouser for
easy parts ordering. Mouser doesn’t carry all the parts—notably potentiometers—so the second tab lists
all the non-Mouser parts as well as sources for each.
View parts list spreadsheet →
PART VALUE TYPE NOTES
R1 1M Metal film resistor, 1/4W
R2 1k Metal film resistor, 1/4W
R3 47R Metal film resistor, 1/4W
R4 560R Metal film resistor, 1/4W
R5 1k Metal film resistor, 1/4W
R6 1k5 Metal film resistor, 1/4W
R7 1M Metal film resistor, 1/4W
R8 10k Metal film resistor, 1/4W
R9 47R Metal film resistor, 1/4W
R10 100k Metal film resistor, 1/4W
R11 100k Metal film resistor, 1/4W
RPD 2M2 Metal film resistor, 1/4W Input pulldown resistor. Can be as low as 1M.
LEDR 4k7 Metal film resistor, 1/4W LED current-limiting resistor. Adjust value to change LED brightness.
C1 22n Film capacitor, 7.2 x 2.5mm
C2 1n Film capacitor, 7.2 x 2.5mm
C3 OMIT MLCC capacitor, NP0/C0G Leave empty for OP07. Use 33pF here if using a LM308N.
C4 100pF MLCC capacitor, NP0/C0G
C5 2.2uF Electrolytic capacitor, 4mm
C6 4.7uF Electrolytic capacitor, 4mm
C7 4.7uF Electrolytic capacitor, 4mm Can instead use CX1 for a 1uF film cap. No audible EQ difference
between 4.7uF and 1uF and film caps have better sound quality.
C8 3n3 Film capacitor, 7.2 x 2.5mm
C9 22n Film capacitor, 7.2 x 2.5mm
C10 1uF Film capacitor, 7.2 x 3.5mm
C11 100n MLCC capacitor, X7R Power supply filter capacitor.
C12 100uF Electrolytic capacitor, 6.3mm Power supply filter capacitor.
C13 47uF Electrolytic capacitor, 5mm Reference voltage filter capacitor.
CX1 1uF Film capacitor, 7.2 x 3.5mm Alternative for C7. No audible EQ difference between 4.7uF and 1uF
and film caps have better sound quality.

HELIOS CLASSIC DISTORTION 4
PARTS LIST, CONT.
PART VALUE TYPE NOTES
D1 1N5817 Schottky diode, DO-41
D2 1N914 Fast-switching diode, DO-35
D3 1N914 Fast-switching diode, DO-35
D4 1N914 Fast-switching diode, DO-35
D5 1N914 Fast-switching diode, DO-35
D6 1N914 Fast-switching diode, DO-35
D7 1N914 Fast-switching diode, DO-35
D8 3mm LED, 3mm, red diffused
D9 3mm LED, 3mm, red diffused
Q1 2N5457 JFET, N-channel, TO-92 Any general-purpose JFET will work the same here.
IC1 OP07 Operational amplifier, DIP8 Original IC is LM308N. See build notes.
IC1-S DIP-8 socket IC socket, DIP-8
TONE 100kA 16mm right-angle PCB mount pot
DIST 100kA 16mm right-angle PCB mount pot
VOL 100kA 16mm right-angle PCB mount pot
SWEEP 1kB 16mm right-angle PCB mount pot Modification. Turn it all the way down for the stock RAT circuit.
CLIP SPDT cntr off Toggle switch, SPDT on-off-on
LED 5mm LED, 5mm, red diffused
IN 1/4" stereo 1/4" phone jack, closed frame Switchcraft 112BX or equivalent.
OUT 1/4" mono 1/4" phone jack, closed frame Switchcraft 111X or equivalent.
DC 2.1mm DC jack, 2.1mm panel mount Mouser 163-4302-E or equivalent.
BATT Battery snap 9V battery snap Optional. Use the soft plastic type—the hard-shell type will not fit.
FSW 3PDT Stomp switch, 3PDT
ENC 125B Enclosure, die-cast aluminum Can also use a Hammond 1590N1.

HELIOS CLASSIC DISTORTION 5
BUILD NOTES
LM308 and OP07
The original RAT used the LM308, an early op-amp with a very low slew rate compared to modern
alternatives. The slew rate is critical to the unique sound of the RAT.
In the mid-1990s, the LM308 supply began to dry up, so after some research & experimentation, Pro Co
changed over to the OP07. Despite insistence from gear enthusiasts that the LM308 sounds better and
the OP07 is inferior, it’s been shown on more than one occasion (both by the DIY community and more
thoroughly by JHS) that the OP07 performs and sounds identical when used in the same circuit with the
same settings.
Therefore, while it’s very possible that one RAT with the LM308 and another with the OP07 may sound
slightly different in A/B tests, this is due to the differences in component tolerances and manufacturing
changes throughout the years, not due to the IC itself.
The LM308 has been obsolete for a long time, and most of what’s available today on eBay and
component distributors in Asia are relabeled fakes, so it’s not recommended to risk using one. Let the
article from JHS convince you the LM308 is not special, and use the OP07 with a clear conscience!
C3 compensation capacitor
The OP07 op-amp has an internal slew-rate compensation capacitor while the LM308 does not. C3
(33pF) should be omitted when using the OP07, but it’s required for the LM308 to work properly.
Clipping switch
The clipping switch allows you select between different sets of diodes.
The downward position of the toggle switch is the stock RAT clipping arrangement, which has one
silicon diode in each direction. The upward position of the toggle is two diodes in each direction, a higher
clipping threshold with a more open sound. The center position leaves only the LEDs for a more dynamic
and transparent tone.
If you look at the schematic, you’ll see that the LEDs, D8 and D9, are always connected. However, in
either the up or down switch positions, the lower-threshold diodes are engaged. The LEDs have no
effect in this mode since there’s no signal left for them to clip.
Feel free to experiment, For example, you could jumper D7 for asymmetrical clipping. Or, you could omit
the two clipping LEDs for a diode-lift mode in the center position.
Sweep control
The Sweep knob (also called the “Ruetz mod” after Philip Ruetz of Ruetz Amplification who came up
with it) puts some resistance in series with one of the low-end filters in the op-amp gain stage which
changes the EQ and feel of the pedal while reducing some of the gain.
This control is entirely out of the circuit when the knob is turned all the way down (full CCW). However,
if you want to omit it, you can jumper pins 1 and 2 of the pot.

SCHEMATIC
HELIOS CLASSIC DISTORTION 6
GND
2M2
100uF
GND
VA
100k
100k
GND
VB
22n
OP07
2N5457
1M
VB
1k
1n
GND
VA
OMIT
GND
100pF
100kA
47R
560R
GNDGND
1k
100kA
1k5
1M
3n3
22n
GND
GND
10k
GND
100kA
GND
VA
GND
1uF
47uF
1kB
2.2uF 4.7uF
4.7uF
GND GND
1uF
RPD
C12
IN
OUT
R10
R11
C1
IC1
1
2
3
8
6
74
Q1
R1
R2
C2
C3
C4
DISTORTION
1
2
3
R3
R4
R5
FILTER
1
2
3R6
R7
C8
C9
R8
VOLUME
1
2
3
C10
C13
SWEEP
1
2
3
C5 C6
C7
CLIP
2
3 1
D2
1N914
1N914
LED
D3
D5D4
D6D7
D8
D9
CX1
1N5817
+9V
1
MLCC
00n
47R
D1
C11
R9
GND

HELIOS CLASSIC DISTORTION 7
DRILL TEMPLATE
Cut out this drill template, fold the edges and tape it to the enclosure. Before drilling, it’s recommended
to first use a center punch for each of the holes to help guide the drill bit.
Ensure that this template is printed at 100% or “Actual Size”. You can double-check this by measuring
the scale on the printed page.
Top jack layout assumes the use of closed-frame jacks like the Switchcraft 111X. If you’d rather use
open-frame jacks, please refer to the Open-Frame Jack Drill Template for the top side.
LED hole drill size assumes the use of a 5mm LED bezel, available from several parts suppliers. Adjust
size accordingly if using something different, such as a 3mm bezel, a plastic bezel, or just a plain LED.
x: 0, y: -1.20
ø15/32”
x: -0.775, y: -1.20
ø5/16”
CENTER (0,0)
ø3/8” ø1/2”
0.385”
0.625” 0.625”
ø3/8”
OUT DC IN
125B
VOLUME DISTORTION
FILTER SWEEP
FOOTSWITCHLED
CLIP
x: 0, y: +1.06
ø1/4”
x: -0.65, y: +1.71 x: 0.65, y: +1.71
ø9/32” ø9/32”
x: -0.65, y: +0.41
ø9/32”
x: 0.65, y: +0.41
ø9/32”
0 1 2
CM
0 1
INCH

HELIOS CLASSIC DISTORTION 8
ENCLOSURE LAYOUT
Enclosure is shown without jacks. See next page for jack layout and wiring.
125B

HELIOS CLASSIC DISTORTION 9
WIRING DIAGRAM
125B
IN +VGND N.C. N.C. OUT
PCB
IN
GND +V +V JACK GND JACK
OUTIN
GND GND PCB
OUT
Shown with optional 9V battery. If battery is omitted, both jacks can be mono rather than one being stereo.
Leave the far-right lug of the DC jack unconnected.

HELIOS CLASSIC DISTORTION 10
LICENSE & USAGE
No direct support is offered for these projects beyond the provided documentation. It’s assumed
that you have at least some experience building pedals before starting one of these. Replacements and
refunds cannot be offered unless it can be shown that the circuit or documentation are in error.
All of these circuits have been tested in good faith in their base configurations. However, not all the
modifications or variations have necessarily been tested. These are offered only as suggestions based
on the experience and opinions of others.
Projects may be used for commercial endeavors in any quantity unless specifically noted. No
attribution is necessary, though a link back is always greatly appreciated. The only usage restrictions
are that (1) you cannot resell the PCB as part of a kit without prior arrangement, and (2) you cannot
“goop” the circuit, scratch off the screenprint, or otherwise obfuscate the circuit to disguise its source.
(In other words: you don’t have to go out of your way to advertise the fact that you use these PCBs, but
please don’t go out of your way to hide it. The guitar effects industry needs more transparency, not less!)
DOCUMENT REVISIONS
1.0.1 (2023-06-29)
Added more information about the LM308 vs. OP07 op-amps.
1.0.0 (2018-08-22)
Initial release.