In terms of physical layout, this amp will be modular. Each significant function block will be built as a separate module. Each module comprises a rectangular region of the front panel, containing the associated knobs and/or switches. The module also contains a circuit board (not necessarily a PCB, however; wiring will probably be done point-to-point, using so-called turret boards); and on the back panel are the associated tubes. Most preamp modules have one or two 12AX7 tubes; some may have up to four. The modules are housed in segments of extruded aluminum "U"-channel, which forms the front and back panels as well as the "floor". Thus, quite a bit of EMI shielding is provided to the electronics, even though I plan to build the outer case of wood. The modular design is to facilitate incremental development, construction, and testing, as well as to encourage others to develop their own modules. Each module will come with "flying leads" (wires), which are soldered to connection points on the amp chassis; this choice favours reliable connections over the convenience of rapidly swapping modules in the field.
The heart of this (or any) guitar amp are the gain stages. This is where the bulk of any desired distortion is produced; even with so-called "clean" guitar tones, the mild distortion produced by tube gain stages is a critical element of the sound. A Fender amp can produce a nice "clean" tone for a guitar; plug that same guitar directly into a hi-fi stereo amp or mixing board, and you'll hear a very thin and disappointing tone, lacking in the interest and "sparkle" of the Fender amp tone. An electric guitar is really only part of a musical instrument; the formants and non-linearities imposed upon the signal by the amp or other electronics, become crucial components of the complete musical instrument whose sound we call "electric guitar".
In my amp, rather than having a single high-gain section to produce the distortion, I have elected to provide several lower-gain sections, which can be cascaded to produce just about any desired level of distortion. Each section has gain, tone, and level controls, allowing subtle control over the character of the distortion as it is built up. Alternatively, although this is technically a single-channel amp, different gain sections can be set up for different tones and can be switched in and out by the footswitches, affording some of the flexibility of a multi-channel amp, while also providing the ability to run some or all of the sections in series, simultaneously, for extreme levels of gain.
Initially, I plan to build two types of gain stage: the "Type M", which copies a Marshall 2203 preamp, and the "Type F-Plus", which is a Fender AB763 preamp with an additional switchable gain stage (i.e., either one tube with two triode sections, or two tubes with four sections). The amp will have space for three gain modules, which I'll probably populate as F+ -> M -> F+.
The footswitch board will have six buttons, each a regular push-on/push-off metal footswitch, with an associated status LED. The LEDs will be replicated on the amp front panel, with each LED colour-coded and located with its associated preamp module. Each footswitch activates one of the six bypass relays: two EQs, three gain stages, and the final stereo effects loop. For convenience in cabling, the footswitch unit will connect to the amp using a standard 8-conductor "CAT-5" cable, of any desired length (six switched lines plus power and ground).
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