FENDER: One knob tone control

This is the classic single-knob tone circuit made famous by a number of Fender amps—most notably the 5E3 Tweed Deluxe.  Despite not inventing it, Leo must have thought that he nailed it when he first dropped this into a guitar amplifier: it’s simple, clever, and very musical.  Despite using only a handful of parts, it offers surprisingly flexible tone shaping. Best of all, it barely saps any gain—there’s virtually no signal loss, which is rare for a tone circuit.

So how does it work?

At full clockwise, the 500p cap connects directly between lugs 3 and 2 of the volume pot.  That’s effectively a bright cap, allowing treble to bypass the pot's resistance and reach the output.  Turn it counter-clockwise, and you’re grounding high frequencies through a 5n cap, taming the treble response. It’s a beautifully balanced design.

Right now I’m mapping how different pot values affect the response.  Why bother?  Well, on some old amps I refurbish, the original pots aren’t always 1M.  This might be a handy reference in future, as I'm sure to forget at some stage.

All my tests use a 22nF coupling cap from the previous gain stage, and both the 500pF and 5nF caps in the tone circuit (though 470pF and 4n7 are perfectly valid substitutes).

1meg volume, 250k tone

1meg volume, 500k tone

1meg volume, 1m tone (5E3 sytle)

FENDER: Blender

Something of a classic, the Fender Blender is a monster octave fuzz.  I'd be tempted to remove the Blend pot myself, as I don't like fuzz pedals with clean blend.  Probably advisable to add LEDs to the Tone boost switch too.   Layout has been designed for a wide enclosure to accomodate the switching and 4 x knobs.  Should be enough room to add a few extra columns for mounting points.  

FENDER BLENDER - VERO LAYOUT

FENDER BLENDER - SCHEMATIC / LTSPICE

Looking at the schematic, essentially what you have here is a Foxx Tone Machine, with a bit more going on.  Certainly more extreme on the fuzz side of things, and unlike the Foxx, you can't turn off the octave (unless you wanted to add a switch to disconnect C6 from the emitter of Q3).

Signal on the collector and emitter of the inverter stage, before being joined together to form the octave

Post diode clipping - the signal is throughly destroyed by this stage

The Tone Boost switch - notch filtered or raw fuzz

Looks a little different by the time it hits the output - tone control in circuit

Tone Boost on, taken from the output

If you want to ditch the blend, it's as simple as this - no effect on the circuit / frequency response.  

In case your wondering - I have no idea on what's happening inside a Kevin Shields Blender.  I've seen a gut shot and it has about 6 x ICs in there in addition to the regular transistors, so there's a lot going on outside of the stock circuit.

FENDER MDX Circuit / AION FX Elite boost / preamp

There's a few variations of this circuit, so I'd recommend reading through the Aion FX build docs or maybe just skip the vero all together and buy the board from them.

The build docs also include LTspice simulations of the frequency response, so that saves me a bit of time plotting them out.

I've labelled a few of the components where variations occur, but not all.  You should be able to work out the ones that I haven't labelled easily enough.

FENDER MDX CIRCUIT / AION FX ELITE BOOST / PREAMP VERO LAYOUT

Vague explainer / demo here