Harley Benton GA 15 Ultimate Mod Kit User Manual

- Cosmic Ampworks
®
- The Ultimate Harley Benton GA15 Mod Kit- Version 1.4 - 200127
© 2020 - www.cosmicampworks.com 1 Dedicated to the Advancement of Stellar Tone!
Dear Customer,
Thank you for your interest in our Harley Benton GA 15 Ultimate Mod Kit!
Successfully implementing the modifications described in this manual will transform your GA-15 from an ugly
duckling among guitar amplifiers into the metaphorical swan – a great sounding tube amp! Hopefully, it will be
an enjoyable process too – the journey is the destination :)
Please be aware that some of the mods may be quite challenging to perform. Improper implementation of the
described changes may render your GA15 unusable and can even be dangerous! Opening up and changing
your GA15 is entirely for your own risk and should only be done by properly trained personnel. The
modifications described in this manual do not necessarily comply with any electronic safety standards.
Disclaimer: Cosmic Ampworks
®
assumes or undertakes NO LIABILITY for any damage suffered as a result of
the use, misuse or reliance on the information and content in this manual.
Figure 1. Modified Harley Benton GA15.
1. About the mods
The mods are described on our webpage (https://cosmicampworks.com/harley-benton-ga15-guitar-
amplifier-the-ultimate-mod/). In the current manual, they have been re-arranged in order of increasing
complexity. So, before diving into the deep, there is still some room for learning by doing. It is advisable to
implement the mods one-by-one, and checking carefully whether your amp is still functioning properly after
each mod, before proceeding. Don’t forget to listen to the sonic changes along the way! The manual starts out
with some pretty straightforward tweaks:

Harley Benton GA 15 Ultimate Mod Kit Specifications

General

Brand	Harley Benton
Model	GA 15 Ultimate Mod Kit
Category	Musical Instrument Amplifier
Language	English

Summary

1. About the Mods

1. Adding a grid leak resistor to V2A

Add a grid leak resistor to V2A for correct bias maintenance.

2. Replacing the cathode bias resistors

Replace cathode bias resistors for higher gain and warmer sound.

3. Replacing the cathode bypass capacitor

Replace cathode bypass capacitor for fuller, less boxy sound.

4. Installing a gain control

Install a gain control to replace the stock voltage divider.

5. Assembling a James tonestack and volume control

Install James tonestack and volume control for tube path.

6. Wiring a DC power supply for the preamp heater filaments

Wire DC power for preamp heaters to reduce hum.

7. Install a presence and negative feedback control

Add presence and NFB controls for tonal shaping.

8. Rerouting the wiring for the power tube heater supply

Reroute power tube heater supply wiring to reduce hum.

2. Safety

2.1 Safety warning

Warns about lethal high voltages and need for trained personnel.

2.2 How to - safely - measure voltages

Provides instructions on safely measuring voltages in the amplifier.

3. Required materials and tools

3.1 Contents of The Ultimate Harley Benton GA 15 Mod Kit:

Lists the components included in the mod kit.

3.2 Additional requirements

Lists additional tools and items needed for the mod.

4. Preparations

5. Removal of parts that are to be replaced or become superfluous after the mod

6. Installing new components (wear safety goggles!)

5. Assembling a James tonestack and volume control

6. Wire a DC power supply for the preamp heater filaments

7. Optional: install a presence and negative feedback control

8. Optional: Rewire the power tube heater supply