I'm a believer, in de-coupled grounds

[Bob-I]

I've been chasing a ground loop in my amp for weeks. Since all my amps have the filter grounds to one point at the PT bolt, I ignored this as a problem. Finally I seperated these ground, the first filters to the chassis bolt near them and the rest coupled together to a chassis bolt at the other end of the filter board.

No more ground loop, no more hum.

[MarkB]

I'll be damned if I can figure this out. I've seen explanations that sound great at first, but always seem to have holes on second thought. We need an amp-building Einstein to figure it out.

[Icetech]

Bob.. i tried the whole star ground thing.. and i never got it quiet... i do it like fender did.. and i have no probs:)

[Bob-I]

Bob.. i tried the whole star ground thing.. and i never got it quiet... i do it like fender did.. and i have no probs:)

I hate star grounding... and my middle name is Starr

I've never had a star ground system work out. In this case I used a bus across the filter caps and that was the cause of my hum.

[tictac]

I tried a star ground once and had the same problems with hum.
A "Galactic Ground" is the way to go. You have a Main Buss (I like 16awg tinned solid) and you "Star" ground groups of sections of the amp, something like...
1) PT/CT, 1st Filter, Power tube grounds, screen filter ground, neg. bias ground.
2) Heater grounds
3) PI grounds, OT secondary ground
4) OD circuit grounds
5) clean Preamp grounds

The ground buss and the chassis ground (from the AC cord) should be as far apart as possible. I ground the chassis ground near the AC cord entrance and the ground buss near the input jack.

Ideally each filter should be grounded to it's respective section it it filtering but the Dumble layout doesn't make this very practical. I ground the preamp filters together then ground them with another wire somewhere in between the OD and Clean star. Haven't had any problems with hum with this setup.

TT

[Bob-I]

What I have on this amp now is working great. Quite simply the quietest tube amp I've ever used.

1) First stage filtering grounded to the chassis 1" away
2) All other filters tied together grounded 1" away

All filters are on the back side of the chassis like a BF Fender.

3) Ground bus bar connected to the chassis near the input, bolted at the other end using a nylon bolt, so it's isolated. All controls are grounded to the bar

4) Gain stage cathodes are tied together and grounded to the same bolt as the bus bar.

5) Reverb grounded to a bolt near the reverb jacks. Return jack is isolated from the chassis.

6) PS grounds to the PT bolts using lugs.

7) Heaters grounded to a bolt on V1.

Like I said, quiet as a mouse, even with the gain cranked.

[llemtt]

I agree with TT "galactic ground" is a good one, I do exactly the same.

Very important the only two chassis grounding points as far as possible.

Teo

[BobW]

Congrats Bob, Am glad you now have a quiet amp! What is key in making your amp quiet was the separation of the Main Filter from the other secondary B+2,3,4 filters. I couldn't tell from your pic, but your PT is now most likely grounded closer to the Main filter? So now the return path of the bus bar is closer to the secondary filters than the PT or Main return.

This is basically how my layout is also connected, except:
1. Heater CT, PT CT, and B+1 return connected to the chassis via one of the PT bolts.
2. From there, a wire attaches to the far left of the bus bar.
3. Attached at the bus bar from far left to far right is the bias, B+2, B+3, B+4, B+5.

This is basically the same, as your ground scheme and proves the point of keeping the secondary filters at a lower ground than to the Main filter and PT.
A good experiment would be to connect the secondary and bus bar grounds to a low value resistor (5 Ohm 5W), then connect the resistor to the PT/Main filter ground. This would accomplish the same effect.
cheers

[MarkB]

I agree with TT "galactic ground" is a good one, I do exactly the same.

Very important the only two chassis grounding points as far as possible.

Teo

Which goes against what Bob posted just above you. He has multiple chassis grounds.

[pedro]

can I be pedantic and suggest that the Galactic ground should really be named the "binary star system"

these amps really are out of this world.........

couldnt resist that

Pete C

[Bob-I]

I agree with TT "galactic ground" is a good one, I do exactly the same.

Very important the only two chassis grounding points as far as possible.

Teo

Which goes against what Bob posted just above you. He has multiple chassis grounds.

Exactly. I have 4 chassis grounds.

1) Controls and cathodes
2) Reverb return
3) B+ Filters
4) All other filters

I had fewer but also lots of hum. Go figure.

[tictac]

The "Galactic Grounding" setup is not original but is explained by Kevin O'Connor in his Tons of Tone (TUT) series of books, especially in TUT3.

One point he makes is that multiple chassis grounds are not a good idea because of the possible interaction of ground currents of different stages though the chassis, and hence noise.

That being said, if your present setup works for you why change it? Just because just because some amps with multiple chassis grounds have noise problems doesnt mean it can't be done sucessfully. I think O'Connor's point is that "Galactic Grounding" is a repeatable method that an inexperienced builder can use to get consistent results, time after time, from one amp to the next; not that it is the "only way" a quiet amp can be built...

TT

[MarkB]

I think O'Connor's point is that "Galactic Grounding" is a repeatable method that an inexperienced builder can use to get consistent results, time after time, from one amp to the next;
TT

Which would be nice if it were true. But it's not. Hang around amp boards long enough and you'll have the evidence. Sometimes it works, sometimes it doesn't. If it were that simple, the question wouldn't keep coming up.

[Tracy]

I'll be damned if I can figure this out. I've seen explanations that sound great at first, but always seem to have holes on second thought. We need an amp-building Einstein to figure it out.

I think the best thing is to keep the plate and screen caps grounds together with the center tap and the power tube cathodes and bias supply . this is the high current star. connect it to the chassi near the safety ground wire. then run a buss the other way connecting preamp supply cap grounds to driver , reverb, and preamp grounds all the way to the input jack. and ground the buss there. making the high current star and the bus to chassi connection as far away as possible has worked well for me.

[Allynmey]

Hi all, Proper grounding can be a real pain in the a**! The thing to remember is that the voltage runs in paths across grounds and chassis back to the source. If your signal grounds cross the direct path of a AC or any noise voltage path it will pick up noise. I ground my Chassis ground close to where it comes in the chassis. I ground my CT's and PS filter caps at a tranny bolt. I usualy ground my preamp wires on a ground buss bar across the pots and terminated to a chassis bolt below the pots. Every amp is consistantly quiet with this configuration. If you use your eyes and follow direct paths of circuit grounds, make sure they don't criss-cross each other. I think that is the best wasy to explain it.

Allynmey