Your getting it, but not all yet. I'll explain as best I can so you have enough to look for in Merlin's drawings. Look how he wires each node.
Lets get this out of the way 1st;
Fender use a 'random chassis' ground system. Fender used a random ground system. Many most Fender amps are pretty quite. But with a new build or a complete re-build want a 'wired' ground system.
And with that 'wired' ground system, there's a difference between 1.
wired star ground system, 2.
wired bussed local star system. We want 2, bussed local star with Doug's split buss. (See below.)
With a star grounding system, each B+ filter cap has all the grounds for the circuit it feeds grounded with that single B+ filter caps ground wire. It makes a single local ground star. Then they hook up a wire to that local ground star and do the same with all the other local ground stars and run all the ground star leads together to 1 single chassis ground.
That's a chassis star ground.With a
bussed local star system it's similar, but each local ground star is hooked up to a common buss wire, 1 at a time, in the order of the circuit, end to end in the chassis. Then the buss wire is grounded to the chassis at 1 end and 1 end only. If you ground it at both ends you get a ground loop and it will buzz. Merlin with the local star buss system only uses 1 chassis ground, at the input jack.
Doug is saying to use the buss grounding, not the single star ground. Doug then splits the buss into 2 sections; 1. Power amp 2. Preamp and has each grounded to it's own chassis ground. He does this so the current in the power amp ground can't modulate (cause noise/buzz) in the lesser current tubes.
What we're trying to do is reduce chassis grounds so there's not all these different ground paths going through the chassis crossing each other/disturbing each other. The less chassis grounds the better.
The power amp has the most current in the amp because of the power tubes. This is why we isolate the reservoir filter cap and PT B+ CT from everything else, because this makes it
THE noisiest ground in the amp. To deal with this we wire the PT CT directly to the reservoir filter cap's - lead, then run a wire from there to the chassis ground. That way it can loop all it want's without disturbing any other tube. See Merlin's drawings. See Merlin's drawing.
Power tube as an example; A tube pulls current up from ground through the tubes K (cathode) goes out through the tubes plate, then back through the OT's CT back to the B+ reservoir filter cap's + lead, back down through that cap, out it's - lead back to the power tubes K. And round and round it goes. All tube circuits we work with work this way.
My understanding is that I would run a separate wire to my power amp ground from nodes A, B, (and C?), and then ground node D to the preamp ground - should this be a short wire that runs from the negative end of D to the preamp bus, or a longer wire that goes to the preamp grounding lug (also connected to the bus)?
Look at Merlin's drawings. Look how he wires each node.
Local ground star preamp tube example; 1st preamp tube grounds; tubes input grid R - the grid leak R, the cathode (K) R, the input jack ground and the B+ filter cap ground that feeds that tube.
You ground each B+ filter cap with the circuit grounds from the circuit that filter cap feeds. They call it a local star. Each B+ filter cap has it's own local star ground. Then you string/daisy chain the individual stars to with a wire. So local stars bussed together with a buss wire.
Your going to end up with 2 chassis grounds for the ac circuit. 1. power amp ground, and 2. a preamp ground.
(The 3rd wire on the power cord, usually green safety ground wire, has it's own chassis connection. Nothing else gets connected with it. It's not part of the ac circuit grounds.)
