"I spoke to Mark Cameron tonight and he said it was okay to reveal the mods he gave me for the 5150.... so here is what has been done to my amp:
V5B(middle preamp tube add 10pf silver mica cap to pins 7 and 8. This is the main mod!!!
R9: piggyback a resistor on top of the 1 meg resistor. I used a 120k based on feel. You can try out 68k, 100k, 120k, 150k, etc. This tightened up the map a lot!
C26: cut one leg for less bottom end on the rhythm channel
C15: bypass cap for better "feel"
I tried tinkering with R15 and using Marshall vlaues, but it took too much away from the original sound. The 39k is the SLO value as well.
There you have it. Have fun getting a killer 5150 sound