The stock amp had a coupling cap between (each) input jack and grid of first gain stage. Initially, I kept that in there. Used shielded wire to board-mounted cap, and added a 33k grid stopper on pin 7 of the tube socket.
My wiring colour code for tube sockets:
- brown for unshielded grid wires
- yellow for cathode
- blue for plate.
Follow the wires to find:
- first gain stage cathode: 2k7 R with bypass cap
- first gain stage anode/plate: 220k plate with bypass cap to remove some high end hiss
- plate coupled to front panel gain pot.
- output of gain pot fed to input of second gain stage (with shielded wire and 91k grid stopper)
- second gain stage cathode: no bypass cap. lower value than Marshall's 10k cold clipper (4k7, I think)
- second gain stage anode: 100k R
anode Rs: 1W
cathode and most other Rs: 1/2 W