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)
So you can turn this into a pedal format?
ReplyDeleteIn theory, yes.
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