The AX84 Tone Brewery


Homebrewing tube instrument amplifiers is a hobby which has been enjoyed by those with electronics skills for years. As the popularity of the Internet has increased and the worldwide distribution of how-to information by sites like AMPAGE.ORG and AX84.COM has become common, musicians with a burning desire to learn and healthy respect of electricity are now able to build their own circuits and experiment with their ideas. What has been missing until now is the ability for homebrewers to easily audition new ideas.

A typical scenario: You have a nice tube amp that you built, but you have an idea for a new preamp. You can either build an entirely new amp, or you can modify your existing one. The first option can be time consuming and expensive while the second option will make it hard for you to compare the original amp with your new idea. Of course if you want to try a different poweramp, you're in for even more trouble.

The Tone Brewery is intended to provide a standardized modular platform for the construction of experimental tube amp circuits. By providing the major components of a tube amplifier in exchangeable modules, the Brewery allows the hobbyist to easily experiment with new designs by replacing only the modules of interest. The foundation of the platform is a standardized power supply module (PSM) that can be reused over and over again in nearly whatever amp design the builder desires.

The Brewery can provide a foundation for experimentation that allows the builder to test and compare different ideas before committing them to a final form. While the standardized nature of the platform imposes some restrictions on the builder, it does so in an attempt to maximize the ability to reuse and share module designs.

It is important to understand that the Tone Brewery is not an amp switching solution. The platform does not specify or recommend the details of any switching mechanism beyond providing a standardized set of power voltages and physical dimension specifications that can be used. The power supply is robust enough that it can be used to build a variety of modular designs, including those that include channel or tube switching, though this capability is left (along with most other circuit design decisions) to the module designer.

This document describes Release Candidate 2 of "The AX84 Tone Brewery Platform, Version 1.0"


Since the infrastructure features of the Brewery are standardized, these benefits are enjoyed by the hobbyist:

  • A single power supply can be used to power various projects of differing designs.
  • Modules using expensive components such as transformers and output tubes can be used repeatedly, reducing the cost of experimentation and comparison.
  • New preamp or power amp designs can be tested and compared simply by switching modules. No longer must you rip open a functioning design to try a new idea.
  • Building modules designed by other builders is easier since only the components required for the module are necessary.
  • Modules are less expensive to build and can be shipped to other builders for evaluation easier than a complete amp, while leaving the remainder of the modules available for use in the interim.
  • Since the chassis sizes are standardized, the same enclosure can be used again and again. Gigging is even possible since the amp modules can be secured in the enclosure.
  • Virtually any amp configuration that can be conceived can be built (and in less time than any other method) since the same power supply module (which does not include a power amp) can be reused over and over again.
  • Modules

    It is intended that the size of the chassis for each module type be standardized such that the modules can be interchanged in a common enclosure. The size of each module type has been defined as 6x9x3 inches making a full amp using three modules eighteen inches wide.".

    The Power Supply Module

    The function of the Brewery Power Supply Module (PSM) is to provide the necessary power that the other modules require to operate. As the foundation for the Tone Brewery system, the PSM voltages and connector types are mandatory to ensure compliance with other modules. As already mentioned, the PSM is intended to be useful for a nearly limitless array of different amp configurations. The PSM may well be the last power supply you ever build due to its massive flexibility.

    The PSM offers several voltage outputs for various purposes: A 6.3vac dc-referenced filament supply, two semi-variable and regulated b+ supplies for a power amp and preamp and a 12vdc miscellaneous output. The first semi-variable supply runs in six steps from 120v to 320v while the second supply runs in six steps from 200 to 400. The purpose of using a stepped supply instead of a fully-variable design is to allow for positive voltage selection without an attached measurement instrument. Each module shall specify its desired supply voltage which you can select with confidence on the PSM.

    The PSM uses the common connector type shared by all modules, but provides it in a 'female' format to offer a bit of shock protection to the user.

    The Power Amplifier Module

    The Power Amplifier Module (PAM) is responsible for taking a input signal ranging from 100mVAC to 10VAC and amplifying it so that it can drive a loudspeaker.

    A PAM may be of any topology or design, with the following restrictions:

  • Must utilize the standard voltages provided by the PSM
  • Must use the common connector types for power and signal
  • Must provide its own output transformer, tubes, sockets, speaker jacks and impedance selection mechanism (if necessary)
  • A push-pull PAM must incorporate its own phase-splitter and presence control if desired
  • The input of a PAM must incorporate a mechanism to accomodate an input signal ranging from 100mVAC to 10VAC
  • May be constructed without tubes if desired
  • The Input Flavoring Module (Preamp)

    The Input Flavoring Module (IFM) is responsible for taking the input signal from a guitar and shaping it in preparation for output to the loudspeaker(s). While it can certainly take the form of a traditional preamp, it may also take the form of a complete 'mini amplifier' so long as it provides an output signal that can be accepted by a PAM.

    As with the PAM, IFM modules may be of any design, provided that they observe the following restrictions:

  • Must utilize the standard voltages provided by the PSM
  • Must use the common connector types for power and signal
  • Must accept a conventional guitar input signal through a traditional phono jack
  • Must provide its own controls for gain and tone
  • Must include its own tubes and sockets
  • Must output a compatible signal for the input of a phase splitter
  • Should provide an output level control
  • May be constructed without tubes if desired
  • Mut have an output impedance of 10k or less
  • Should provide an output of approximately 1VPp
  • Combination Modules

    It is permissible for combination modules which incorporate an IFM and PAM in one enclosure to be developed for the Brewery platform. In such a module, it must observe all of the restrictions of the seperate modules. Notable among these requirements is that the chassis of the combination module must be either 6x9x3 (single width) or 12x9x3 (double width).


    (Version 1.0, Release Candidate 2)

    Power Connections

    Preliminary: The Tone Brewery shall isolate the user from dangerous high voltages in its external connections. It will do so by presenting high voltages on 'female' socket connectors only. For cost and availability reasons, a 12 conductor Molex socket was selected by the design group.

    Power inputs on the IFM and PAM modules are to be presented in a 'male' plug format. This is so that the cables that approach them will be in a 'female' socket format providing some amount of protection to the user.

    The PSM is to offer the following supplies on a continuous basis:

  • 6.3 AC volts at up to 6 amperes
  • 150ma 'Poweramp' Supply Output: 400, 360, 320, 280, 240 or 200vdc (fuse protected)
  • 50ma 'Preamp' Supply:320, 280, 240, 200, 160, or 120vdc (current limited)
  • Raw Negative Bias supply (-100vdc) providing up to 6ma
  • 12vdc misc. supply for relays or other junk (unregulated)
  • Standardized Connector Pinout:
    1Chassis Ground
    2Raw Negative Supply
    4Poweramp Star Ground
    5Poweramp Star Ground
    7Preamp Star Ground
    8Preamp Star Ground
    9Preamp Star Ground
    11Low B+
    12High B+

    Intellectual Property Information

    The most current version of this document will be found at AX84.COM or one of its official mirror sites.

    The "AX84 Tone Brewery" name, concept and module naming conventions along with the text of this document are Copyright 2000-2002 by Chris Hurley, All Rights Reserved.

    This document may include ideas which were provided by participants of the AX84 project. For the purposes of Intellectual Property, Chris Hurley shall be the legal custodian of these ideas as they apply to the Tone Brewery project.

    Permission is hereby granted for this document to be distributed in its original form only to anyone provided that NO FEE OF ANY KIND is charged.

    Any other distribution is expressly prohibited without the express written consent of Chris Hurley.

    (Release Candidate 2 of TB Spec 1.0: 01/25/02 Chris Hurley )