These tubes have different frequency responses than 12AX7, particurlarly when distorting.
People describe these tubes to have less harsh and buzzy distortion.
Vice versa; If you’re using the Normal channel, you can pull out the V2 tube.
All AB763-similar circuits (Deluxe Reverb, Super Reverb, Pro Reverb, Twin Reverb, Vibroverb, Vibrolux) are designed so that they share the cathode cap and resistor (25 u F/1500 ohm) and pulling one of the tubes will changes the effective value of the resistor they both share.
But the delivered current is doubled with four tubes. Power (Watts) = Current (Amperes) x Voltage (Volts).
This mod is a must-know survival trick for all Twin owners. If you want cleaner and spankier preamp distortion charcteristics, you may replace the V1 or V2 12AX7 preamp tube with 12AT7 and 12AY7.
This is exactly what the amp is made for, being played unmiked in in a gospel band in a 300-seat church every Sunday.Sadly, we are seldom offered the opportunity to turn the volume above 3 on a Twin Reverb.Instead we have played many nights with a thin and unpleasant tone, even worse with a sparkling, clean sounding strat.If you are one of these players, you should find some mods on this page interesting.With just a few simple tricks you can make your Twin break up earlier like the more versatile Pro Reverb 2×12″ and Deluxe Reverb 1×12″.
Five fundamental tricks to create the holy grail of Fender tone Here is a video demonstrating the effects of some of the mods and tricks described here.