The Custom 100 Watt Amp uses 4 EL34's in the output stage and has a custom voiced preamp.  This results in an early natural overdrive.  The master volume, if used, is for darkening the tone and adding a little dirt at lower volume settings.  Custom wound output transformers are used to recreate the classic tones of the 60's British Amplifiers.  Check out our Custom 80 Sound Bytes