Quote:
Is it mandatory to have a resistor between the signal input and ground ? I’ve seen some design that use it, and some others that don’t (like the lacewood amp)
If the input to a tube is from a volume control pot, then the pot grounds the grid (no input capacitor between the volume control and the grid).
But if the control pot is before the coupling capacitor then there is no DC reference for the grid.
If you get any "even the smallest" amount of grid current through a pot then it will get a scratchy sound though the speakers.
The 100K is fine as a volume control but should be of a LOG taper to get some control over the sound.
So the bottom line is:
If the grid goes direct to a potentiometer then the pot grounds the grid.
If it goes to a capacitor then it should have a 1meg grounding resistor.
Grid stopper resistors are not critical in low gain circuits as Matt said a 5K will do, but to keep the high frequencies from being a PITA, then a 22K is probably the way to go.
You don't need a coupling capacitor on the input at all, and could put a 1meg direct from grid to Gnd and a 22K close to the tube as a grid stopper. ie 22K input to grid and a 1meg from grid to Gnd. If you fit a coupling cap then Gnd the grid. If I fit a Pot straight on the grid via a grid stopper then if you get any grid current the pot will go noisey very quickly. A 22K in seies limits grid current rolls off HF hash and can stop early pot failure. But all this is try it and see. A bit of a pain in the neck if the Pot is a mega bucks audiophile part and grid current destroys it in a few days.
If you look at your original "un-modified" circuit you will see that the input to the first tube is grounded through the control pot.
It goes through the wiper of the Pot to one side that is Gnd. You have removed this Gnd connection.
NB for interest your amp has a compensation capacitor in the anode of the tube to control frequencies on the output primary winding.
Regards
M. Gregg