Treble Bleed Circuits Introduction:
Stating the obvious, these treble bleeds only come into play when the volume control is off the '10' position, so if you never venture below 10, this is not for you!
A big advantage of active pickups and active circuits (see Ironstone's own Pre-Amp and Buffer Circuit) is that they can overcome this to a large degree, but its also possible to achieve a good solution with very low cost circuitry.
They all rely on the same basic principle - providing an additional path to the guitars output that is 'friendly' to higher frequencies ( commonly called a capacitor!). There are then several different variants and a range of values to consider, but the basic 'Capacitor Bypass' is always at the heart of things. The configuration options are summarised in the following diagrams (potentiometers shown as viewed from above).
This shows similar components to option 2, but this time the bypass capacitor has an associated resistor in series with it. This means the volume control taper is not modified strongly as in option 2, but the treble bleed itself is more 'restrained' than option 4.
The simplest option, with just a capacitor in place as the bypass circuitry. Favoured in Telecaster and Humbucker guitars where there may be a desire to maximise the treble at all volume settings. The effect on tone as the volume is rolled off, sounds like using a treble boost stomp box.
Of course, each of these options has its pros & cons, none are necessarily the best from all perspectives, and every individual will have their own ideas what sounds best for their style.
As the commonest implementation (particularly on Stratocaster single coil pickup styles), option 2 is probably the best all rounder. Functionally, the capacitor provides the treble bypass and the resistor modifies the effective potentiometer resistance taper to get a good blend of control over both the treble bleed and overall volume. The Seymour Duncan site recommends this configuration for example, with a 100K Ohm resistor and 0.002mF (2nF) capacitor. Values in the 100K - 200K Ohm and 0.5 - 2 nF crop up on the internet.
Option 3 uses similar values in a series configuration. For example the Kinman treble bleed calls for a 130K Ohm resistor and 1.2nF capacitor. In fact resistor values of between 50-100% of the volume potentiometer value will suit most ears and provide a less pronounced treble bleed than the 'capacitor only' option 4.
Option 4 is most commonly associated with Telecaster guitars, where the extra bite and presence of the Tele Bridge pickup (with its field focussing plate), was very much something to be preserved. Using the sort of capacitor values already quoted means that treble will be 'bled' to the output at almost all volume settings. This can mean that lower volume settings may sound 'over bright' to some ears as the bass rolls off against a more constant treble signal content. The desire to preserve treble makes this a common option for humbucker pickups too.
In terms of component values, there are some simple principles to follow. The larger the (bleed) capacitor value, the more it will transmit treble to the output for any given guitar set-up and volume control setting. So if there appears to be too much treble when the volume is rolled off, a smaller capacitor value will help (and vice versa). Similarly, in option 3, a lower value of R will allow more treble to bleed through to the output than a higher value of R (for any given volume setting).
The actual components required for treble bleed circuitry are very inexpensive and easy to fit (assuming you count de-stringing and re-stringing a Strat as easy). Because tone is very much in the ears of the beholder, there are no real rights and wrongs here, and the values given here are starting points only. To take some of the pain out of the vlaue decision process Ironstone strongly recommends a look at the following 2 YouTube clips where the effects of different capacitor values are explored in great depth by Mr John Cooper, along with issues with multiple volume controls;
In particular, he focuses on the capacitor only modification (option 4 above), and a range of values, settling on 1000pF (1nF). The demonstration of the effects of the volume settings on the G&L 'Tele bodied Strat' are particularly striking.
For completeness, its also worth exploring the 'Gibson 50's' (also called the Gibson Vintage) modification. It is not specifically a treble bleed modification, but one of its benefits is a perceived improvement in how treble rolls off with the volume potentiometer setting. There are plenty of references to this on the internet, and it simply involves a change in the wiring of existing tone control circuitry so that it connects to the volume wiper, rather than the volume pot input which is the normal stock wiring on most modern guitars.