Fender Amp Preamp Tube Layout and recommendations

Fender 2 channel reverb amp preamp tube layout (as seen from the back of the amp)
Tube Designation V6 V5 V4 V3 V2 V1
  12AT7 12AX7 7025/12AX7/A 12AT7 7025/12AX7A 7025/12AX7A
Specified Function Phase Inverter Tremolo 2nd gain stage for channel 2 and reverb return amplifier Reverb driver First gain stage for channel 2 First gain stage for channel 1

Typical examples: Deluxe Reverb, Vibrolux Reverb, Super Reverb, Twin Reverb, Showman Reverb, Pro Reverb and Dual Showman Reverb.

V1

If you don't use channel 1, don't worry too much about this tube, any 12AX7 or 7025 will work though noisy or microphonic tubes in this position can be a problem even if you're not using that channel. You can also opt to remove this tube entirely. Doing so will give you increased/earlier overdrive in channel 2.

If you do use channel one, this tube is the most critical to tone for that channel. Pick the highest quality 12AX7A or 7025 (7025s are nearly impossible to find). Highly recommended: JAN GE 12AX7WA,  RCA 12AX7A, ANOS RCA 12AX7A or , RFT 12AX7. Excellent: Raytheon 12AX7.
Best new production tubes for this socket are Svetlana (sound most like NOS RCAs),  Electro Harmonix JJ ECC803SSovtek 12AX7LPS.

V2

This tube is critical for the best tone for channel 2 (your reverb/tremolo channel). Experiment with different types to find your tone. Get the best tube you can afford. This position is particularly sensitive to microphonic tubes due the high gain of channel 2. Highly recommended:JAN GE 12AX7WA,  RCA 12AX7A, ANOS RCA 12AX7A or , RFT 12AX7. Excellent: Raytheon 12AX7.
Best new production tubes for this socket are Electro Harmonix , JJ ECC803SSovtek 12AX7LPS.

12AY7/6072s (as used in the front end of many Fender tweed amps) or 5751s (a lower gain 12AX7 great for smoothing out any harshness sometimes caused by humbucker pickups in Fender amps) are often substituted in the V1 and V2 sockets. Each will give reduced gain and a different tone. Often, one of these substitutes is used to reduce harshness in the first stage of an amp. Highly recommended: JAN GE 5751, RCA 5751, JAN GE 6072 (12AY7) or any other NOS 12AY7.

V3

Because of the high demands placed on this tube by the Fender reverb driver design, it's important to get a good quality 12AT7 for this slot. Some 12AT7s that will work just fine as phase inverters (V6) will crackle and hiss in this position. Better 12AT7s (notably, the Mullard CV4024, Mullard ECC81 or RCA 12AT7s) will also improve the tone of your reverb. Some customers say that the CV4024s remove an overly "surfy" reverb tone which can result from using a low end 12AT7.

V4

The 2nd most important socket affecting the tone of channel 2. If you have a tube that's microphonic in the V2 position, try it here. This is a much lower gain stage and won't be as sensitive to a microphonic tube. Highly recommended:JAN GE 12AX7WA,  RCA 12AX7A, ANOS RCA 12AX7A or , RFT 12AX7. Excellent: Raytheon 12AX7.
Best new production tubes for this socket are Electro Harmonix , JJ ECC803SSovtek 12AX7LPS.

V5

This is the least critical tube position in your amp. This is used as an oscillator for the tremolo circuit and has absolutely no effect on tone. This is a good place for microphonic or cheap 12AX7s.

V6

V6: A new production phase inverter should often be replaced when replacing power tubes if it is more than a year old. NOS tubes last longer. Opinions on how the phase inverter affects tone vary. Some say it has little or no effect while others claim it's critical to have a high quality 12AT7 in this slot. I highly recommend the Mullard CV4024 or ECC81, or RCA 12AT7 for V6. Some players prefer a 12AX7/A in this postion as it allows an amp to "break up" or distort at lower volumes due to being "mis biased" in the 12AT7 circuit. This is a perfectly safe substitution to make. Try it, you may like it!