For a more professional sound you can replace the cheap tubes that often come in consumer level audio gear (such as mic preamps) with some NOS tubes.
What is NOS? It means "New Old Stock." Basically the idea being that the new tubes being made today are crap, so you're better off buying a refurbished older tube.
When shopping for tubes make sure you get the right kind that fits well with your gear. 12AX7 is the most common type of tube used in audio gear.
Buy NOS Tubes Online
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 5 Raytheon 12BV7 Vacuum Tube NOS NIB 12BY7  US $19.99
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 NOS 6KN6 vacuum tube Sylvania US $14.00
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 NOS 6KN6 vacuum tube RCA US $14.00
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 RCA 117N7 Vacuum Tube Amplifiers Tested NOS US $3.95
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 NOS 6LR6 vacuum tube Philips ECG US $24.00
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 NOS 6KD6 vacuum tube RCA US $29.00
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