Input level and overdrive 

Old vintage guitar amps without a master volume knob and your guitar have one feature in common: their respective volume knobs allow you to control - in addition to the volume itself - the amount of distortion as a function of volume. In fact, both the volume pot on your guitar and the volume knob on your vintage amp determine how much level is sent into the preamp. The higher the level, the more overdriven the sound becomes. Conversely, this means that by turning down the input volume, you can achieve a clean sound with an amp that has no master or dedicated gain control. Problem: Since the treble is also reduced when the volume is turned down, the sound quickly becomes dull.

Bright caps

For this reason, the volume controls of some vintage amps have so-called "bright caps" that function as high-pass filters, enabling frequencies above a specific threshold to bypass the volume pot. By allowing these high frequencies to reach their full volume at an early stage of the pot's rotation, vintage non-master-volume amplifiers were able to produce a clean sound with a vibrant and shimmering top end when the volume knob was set to a low level. 

Bright caps are frequently utilized in vintage and vintage-style amplifiers, including notable examples like the Fender Blackface Deluxe, the Marshall JTM45, and the Vox AC30's "Brilliant" channel. But also modern amps like the BluGuitar AMP1 Mercury Edition make use of this traditional concept.

Today: overdrive pedals and gain controls

These vintage amplifiers were not originally designed with overdrive pedals in mind; instead, achieving overdrive was simply a matter of cranking up the amp's volume  (see above: a higher input level meant more overdrive). 

However, in modern times, many players prefer using an overdrive pedal in conjunction with a relatively clean amplifier to create a secondary, overdriven sound. In this scenario, the bright cap unintentionally amplifies and emphasizes the high-frequency harmonics present in the output of the overdrive pedal. Consequently, this often results in a harsh, unnatural, and raspy tone

In this case, the "LOW" input is recommended for vintage amps. And smart modern units like the AMP1 Mercury Edition offer a distinctive operation mode for this purpose: the Low Gain Mode. 

Treble Bleed

The "treble bleed" circuits on some guitars's volume pots follow exactly the same principle as bright caps: A capacitor wired across the volume pot preserves the treble components of the signal when the pot is turned down.

In this way, the guitar's volume pot can be used to tease out a clean sound with sparkling treble from a distorted amp channel. The sound does not become muffled and dull, but it remains nice and silky. In fact, you'll get a very specific treble spectrum - quite different from a clean sound with a volume pot turned all the way up.

The treble bleed capacitor is one of the many modifications Thomas Blug made to his '61 Strat. Of course, the BluGuitar '61 Masterbuilt also comes with treble bleed: What is the value of the treble bleed capacitor in the volume pot of the '61 Masterbuilt?