Where is to be considered to be at ground the pre section on Doug's Grounding ?

[kagliostro]

The preamp sections gnd, the input jack, the preamp filter caps are connected together to the bus of the pots ....

but all this where is better to be connected to ground ?

The ground connection seems missing



Franco

[sluckey]

The input jack provides the ground connection to chassis.

[kagliostro]

Yes, right, a not isolated input jack is to be used 


Franco

[kagliostro]

But the output jacks are isolated, correct ?

Franco

[Willabe]

But the output jacks are isolated, correct ?

No.

[kagliostro]

If the output jacks aren't isolated there is a ground loop, formed by the wire that connects the output jacks to the ground bolt

(and also if there are two jack both not isolated there is a ground loop)

?????

Franco

[jjasilli]

Yes there are physical ground loops.   This puzzled me when I joined the Forum.  But the ground scheme works.

[Mike_J]

I usually isolate the input jack(s) and make a ground near the input jack(s) where both the buss bar and input jack(s) are grounded.  In retrospect this is completely unnecessary so I will probably quit doing it that way and just ground the buss bar to the input jack and leave out the extra work of the isolation.  Same thing for speaker jacks.  Don't hear a difference in noise between grounding at the jacks versus isolating jacks and secondary black wire and grounding at the PT ground point.


Thanks
Mike

[shooter]

If the output jacks aren't isolated there is a ground loop

fwiw, I don't run a ground wire off the spkr jack, just the signal wire, so far no problems. 

[Fresh_Start]

If the output jacks aren't isolated there is a ground loop, formed by the wire that connects the output jacks to the ground bolt

(and also if there are two jack both not isolated there is a ground loop)

?????

Franco

Merlin and Kevin O'Connor will both tell you that the output transformer/jacks should be isolated and grounded with the stage where negative feedback is returned.  If no NFB, ground OT with power amp.  I've isolated all jacks and followed their advice. Those builds have been quiet. However, Doug Hoffman has made a heck of a lot more amps than I have, to say nothing of Leo Fender!

As far as I can tell, Fender used the brass plate as a preamp ground plane. The chassis is sort of a ground plane for the heavy current grounds like the OT and poweramp. It worked for Leo and it works for Doug.

I'll keep using the "galactic ground" Merlin and O'Connor advocate because I understand it clearly and it has worked for me. Fender and Hoffman grounding schemes really aren't much different if you study them very carefully - they're just more subtle and efficient.

Cheers,
Chip

[PRR]

Yes, SwitchCraft All-Metal input jacks. In the US there was no other kind. We saw plastic on some Marshalls but we could not buy them until the late 1970s? By then most US builders were committed input grounders.

> If the output jacks aren't isolated there is a ground loop

The *source* of the loudspeaker signal, the OT winding, "floats". The OT winding does not share ground with the rest of the circuit.

The speakers also float.

There's no "loop" having OT secondary go to chassis, even twice in the same place.

There is a minor issue when we take NFB from OT secondary. The power stage driver (where NFB returns) is grounded. This is rarely a problem, especially in Fender, because signal levels are very high here. It is not a problem in Doug's layout because the driver ground will return to the PT bolt ground, which is very-near the output jacks, and far away from sensitive input grounds.

[kagliostro]

Many thanks to ALL

I needed a really detailed explanation of how yours


Franco