Very practical mod at practice and low volume environments.If you pulled the V1 12AX7 tube you may use it as V6 phase inverter.
That variable resistance is applied to the audio signal on the Vibrato channel, creating a pulsating increase and decrease of that channel's volume.
The speed controls varies the rate of the oscillator.
The reverb knob on the vibrato channel will have no effect any longer.
This mod is not applicable together with the Pull V1 mod, as you need the normal channel preamp tube.
These tubes have different frequency responses than 12AX7, particurlarly when distorting.