Workshop: implementing the “Ultimate Harley Benton GA15 Mod”

If you’ve read our earlier story on the Harley Benton GA15, then you know that we’ve developed something we like to call “the Ultimate GA15 Mod Kit” – simply because you can’t go any further in modifying a GA15 and still call it a GA15. So, what’s new then? The 27-page PDF manual that exactly describes how to implement those mods can now be viewed below. Please be aware that, depending on the speed of your internet connection, it may take a while to load because of its file size.

Replacing the active IC-based tonestack with a Baxandall tonestack, soldered on the back of the treble and bass potmeters, results in a purely tube-based signal path.instead of the hydrid signal path of the stock GA15.

In the step-by-step manual, modifications are arranged in order of increasing complexity so that, before diving into the deep, there is some room for learning along the way. You can find a list of the required parts on page 6. We can also supply all the required parts as a “mod kit”, potentially saving you a lot of hassle. The price of the kit is € 50,- and transactions will be completed via eBay, benefiting from its secure shipping and payment options – contact us if you’re interested. A description of the modifications can be found in the previous post on the GA15 or  in the manual below. We hope you’ll obtain great results and have a lot of fun working on your GA15, but before starting out, please refer to the safety advice in the manual and to disclaimer below!
Disclaimer: We strongly recommend you to have a qualified amp technician modify your amp if you have not been trained to work with high voltages. A tube amplifier chassis contains lethal high voltages even when unplugged – sometimes over 500 volts DC! 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. The modifications described in this manual do not necessarily comply with any electronic safety standards.


Dear Customer15