By principle and theoretically joey is right - the best grounding scheme is:
- each stage does have its own filter cap
- all grounds of every stage are connected to their corresponding filter cap negative (sub star)
- all sub stars are routed together in the order, as the circuit is running (ground bus) - still isolated from the chassis (!)
- the ground bus is connected to the chassis at only one point
That's the way, how I'm doing the grounding in my own amps.
But in Marshalls, Fenders, aso. and in their derivates & clones not seldom are some (or more) designer bugs - and sometimes you may think, that the amp designers better should have been selling burgers at McD, instead of designing tube amps.
But anyway, there's a way, not to make all the efforts of a chassis free ground w/ a separate ground bus in a marshall-esqe amp, to get it (almost) dead quiet. The way is, to USE the chassis instead of a ground bus as a ground plane. The different 'sub ground spots' have to be well choosen and proven by empiric, what I've done and refined over years in a lot of different Marshall amps, which I've modded, upgraded or just serviced - and that's, what now is known as the "Larry's grounding scheme".
To get the idea, how my grounding scheme is done, first read some:
http://forum.metroamp.com/viewtopic.php ... 25&start=0" onclick="window.open(this.href);return false;
http://forum.metroamp.com/viewtopic.php ... 1&p=335678" onclick="window.open(this.href);return false;
http://forum.metroamp.com/viewtopic.php?f=15&t=27641" onclick="window.open(this.href);return false;
http://forum.metroamp.com/viewtopic.php?f=4&t=25351" onclick="window.open(this.href);return false;
And some additional hints:
NEVER use a PT bolt as a ground spot!
ALWAYS run the OT's secondary common to the speaker jack! Don't ground it anywhere to the chassis!
Run a regular (22 AWG) cable from the speaker jack's shield to the spot, where the PI is grounded.
Use lock washers between the ground lugs and the chassis, to enable good contact.
One of the designers bug mentioned beyond is the double filter in the 2204 for screens & PI w/ common ground.
Here I'd recommend to give this filter (one or both halfes - try it, what you like better!) to the screens and I'd mount an additional filter only for the PI, what then will be grounded to ground #3 of my grounding scheme.
Larry