NOS stands for New Old Stock, and it refers to vacuum tubes that were made decades ago but never used. Some audiophiles and musicians prefer NOS tubes over modern ones, because they believe they have better sound quality, durability, and reliability.
The function and importance of each slot in your preamp depends on the type and model of your amplifier, as well as your personal preference and taste. However, here are some general guidelines that may help you:
• V1: This is the first gain stage, and it is usually the most important for shaping your tone. It amplifies the signal from your guitar or other input source, and it affects the amount of distortion, clarity, and dynamics. A good NOS tube for this slot would be a Mullard long plate F92, which is known for its warm, rich, and smooth sound. It has low noise and microphonics, and it can handle high voltages well.
• V2: This is the second gain stage, and it is usually used for the overdrive or distortion channel. It adds more gain and saturation to the signal, and it affects the character and intensity of the distortion. Half of this tube is also used for the reverb circuit, if your amp has one. A good NOS tube for this slot would be a Mullard I63, which is similar to the F92, but has a shorter plate and a different getter. It has a more aggressive and crunchy sound, and it can produce a nice reverb effect.
• V3: This is the third gain stage, and it is usually used for the tone stack or the equalizer. It shapes the frequency response of the signal, and it affects the balance and color of the bass, mid, and treble. It also acts as a buffer between the preamp and the power amp. A good NOS tube for this slot would be a Mullard I61, which is a low-noise and low-microphonic tube. It has a clear and balanced sound, and it can preserve the tone and dynamics of the signal.
• V4: This is the phase inverter, and it is usually the second most important for your tone. It splits the signal into two opposite phases, and it sends them to the power tubes. It affects the power, headroom, and feedback of the amp. A good NOS tube for this slot would be another Mullard I61, which can handle high currents and voltages well. It can also create a symmetrical and harmonic distortion, which can enhance the tone and feel of the amp.
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