TONE STACK: James

The James tone stack is one of the more common bass/treble tone controls. You most often see it in hi-fi equipment, or in guitar amps that sit outside the usual Fender and Marshall mould - although both Fender and Marshall did use similar circuits on rare occasions.

Probably the best-known guitar amp examples are Orange amplifiers and some Ampegs.  Closer to home, Australian builders, such as Goldentone and Vadis used this style of tone control.  Marshall used it in the Artiste, and Fender used a related bass/treble arrangement in the blonde Twin, model 6G8.

It’s hard to mention the James circuit without mentioning Baxandall - same same, but different, so I won’t get too bogged down in that here.  The original Baxandall circuit was an active hi-fi tone control, incorporated into a gain stage.  The James circuit came a few years earlier and is passive: 1949 for James, compared with 1952 for Baxandall.

The James is a simple and useful design. It mostly leaves the midrange alone, while giving reasonably independent control over bass and treble. Think of it as two tone controls working in parallel, rather than a Fender-style tone stack where the controls interact heavily.

For this example, ignore RIN and RL. These simply represent the source and load connected either side of the tone control.

ORANGE, GRAPHIC MKII

link to TSC for frequency response

FILMOSOUND 385

Despite the impossibly difficult to read schematic with dual 3meg pots on a single tone control, this is actually a James eq.    You probably can't find 3m dual gang pots anymore, but you can adjust values to a 1m dual pretty easily - just divide the resistance by 3, and multiply the capacitance by 3.  

Link to TCS

AMPEG B15

FENDER TWIN, 6G8

note: this uses a tapped pot.  i.e. it has fourth lug - good luck finding one of those.

MARSHALL ARTISTE

MARSHALL: 1974X AKA the Marshall 18w one knob tone controls

Another simple one knob tone control, this time from the Marshall 18w amplifier, or the model 1974x, whatever you prefer...   

Marshall used two different tone controls in the amp.  A lot of folks build the 18w lite, which is just the normal channel of the amp, and they often use the trem channel tone control as it has a bit more range to it (18w lite II, it's a great little amp).  The trem channel is basically a Fender one knob tone control using 500k pots.

Normal channel - a little less bass than the trem channel, no top boost

Trem channel - more bass than the normal channel, a bit of top boost

Combined

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)