Components: 22uf cap (min 25V), 820R resistor (1/4 watt) - optional
1) Optional step - remove the 1K resistor in position R24 (just beside the right hand tube) and replace it with the 820R resistor
2) Solder the 22uf cap to the empty C13 position
3) Remove R7
The resulting sound gives you your traditional plexi mod gain between 7 o'clock and 12 o'clock, and extra between 12 o'clock and 5 o'clock.
You can make it switchable pretty easily too.