For those that change first stage gain preamp tubes to change the taste and character of their amps (remember, this can be done in less than a minute without tools, adjustments, or a "tech"). This is one tube that should be in your basic toolbox. The basic tube sets generally are the 12AX7C, ECC83, and 7025 to cover a wide range of styles and tastes for most amplifiers.
Groove Tubes GT-12AX7-C (code 55026)
Guitar Amplifier Tube, Great for Marshalls and Fenders
£16.43 (inc vat)