I just finished doing a Mark Moyer mod on a Fender Custom Vibrolux Reverb and the difference is astonishing. The mod involves removing the reverb from the Normal channel, lowering the gain of the pre-amp, increasing the caps on the reverb to make it fuller, replacing the tone stack capacitors with higher quality caps, removing the protection diodes on the 6L6s, adding negative feedback, and changing the ground reference resistors on the heater lines.
The mods really open up the sound of the amp and give it much more clean headroom. The distortion changes from a pre-amp based fuzz to a power amp crunch. The reverb has a lot more depth and realism. The mod also takes care of the constant hiss in the amp.
Anyone interested in doing this mod, contact myself or Degroovia Guitars in Dekalb, IL
Jason
P.S. Some versions of the mod suggest swapping V3 (reverb driver) 12AT7 with V6 (Phase inverter) 12AX7 to improve the dwell of the reverb and decrease the gain of the phase inverter. While some classic Fender designs did use a 12AT7 for the phase inverter, none used a 12AX7 for the reverb tube. The 12AX7 is not suited to drive reverb. It will not damage the amp or tube in any way, but the 12AX7 does not have the needed power to properly drive a reverb tank. I found the tube swap made the reverb sound lifeless and it took away the sparkle of the overall tone. Feel free to experiment.
JDP Amplifiers 
May 25th, 2016 at 8:38 am
Hi looking in the back of my cvr which has been modified, the tube diagram shows v6 as a 12ax7 but it has a 12at7 in the slot. Is this right?
May 25th, 2016 at 10:17 am
Some people put a 12AT7 in V6 (which is the phase inverter) to lower the gain. I don’t think it sounds good like that. Some people also put a 12AX7 in V3 (which is the reverb driver). While this is what the original ’63 Vibroverb used, it was the only Fender amp to use a 12AX7 to drive the reverb. After that, Fender used a 12AT7 to drive the reverb in all amplifier models. The 12AT7 has lower gain, but higher drive capability. It is much better suited
May 25th, 2016 at 10:22 am
for driving reverb than the 12AX7 is. Some will blindly copy original Fender amplifiers, assuming they are perfect in every way. Don’t assume that Fender never made mistakes. Using a 12AX7 to drive reverb was one of them. After all, Leo Fender was a man walking uncharted territory.
July 13th, 2016 at 6:02 pm
I agree with your assessment…I had the mods done to my CVR and the difference is amazing. I replaced the stock Jensens with an Emi Ragin Cajun and an Emi Ramrod as well. I like this amp a lot now.