[ZLS]

    I am asking what is the first tube you would look to replace that would give you the maximum sound improvement. 
    I am not talking about the brand of the tube, but rather what purpose the tube is serving in the component. 

                                                          Thank You

[Ericus Rex]

It really depends on the circuit but a good rule of thumb is for preamps, the input gain tubes make the most difference and for amps the power tubes make the most difference followed by the voltage gain tubes and rectifier tube (if used).  This is, of course, based on my experience and YMMV.

[Guy 13]

Hi all.
I have tried different combinations on my Nitesahde Audio SE-15 from the 6SN7 NOS Motorola vs Radio Shack (England) to the 6L6WGC vs 6L6WXT+ Sovtek and finally with the 5U4GB Raytheon vs 5AR4 Sovtek and up to now, I have not seen any major improvement or degradation in the sound quality. I have to do more listening...
Guy 13

[rollo]

 I would agree for amps the output tube and actually the same for preamp. Usually one change leads to another. Then again some driver tubes make a huge difference.
   As we all know everything makes a difference. google "Joes Tube Law" and get an idea of the character of the different brands. Makes choosing a little easiar to achieve your goal.

charles

[galyons]

My experience has always been, preamp or amp, driver tube rolling typically has more impact than power tube changes.  Driver tubes do most of the amplification, do it first and, therefore contribute most to the sonic signature.

Cheers,
Geary