Do you know that...
The best all around indication of the effectiveness of a tube as an amplifier is its grid-plate transconductance ? (also called mutual conductance or gm) It is the change in plate current divided by the change in grid voltage that caused the change. It can be found by dividing the amplification factor by the plate resistance. Since current divided by voltage is conductance, is measured in the unit of conductance the siemens. Practical values of trans conductance are very small, so the microsiemens is the commonly used unit. Different type of tubes have transconductance values ranging from few hundred to several thousand. The higher the transconductance the greater the possible amplification. (source arrl handbook)

Here you can find and download tube data sheets for the most common audiophile tubes like Mullard EL34 , EL84 , 6550, 12AX7 Mullard, 807, 811A, 6L6, Svetlana 811-3 etc.

All data in PDF except stated

• GL 807 tube data General Electric
• 807 tube data RCA
• 807 tube data STC
• 811 tube data RCA (zip file)
• 811 svetlana tube data
• SV811-3A tube data svetlana
• SV83 tube data svetlana
• 845 tube data
• 6L6 tube data
• EF86 tube data
• EL34 tube data
• KT88 tube data
• SV300B tube data svetlana
• Mullard 12AX7 tube data (zip file)
• 5U4GB tube data
• EZ-81 philips tube data

click on the "attachments" to open and download the files