Some grounding questions

[Tdale]

I read the aiken tips on grounding, and have a couple of questions.

Aiken writes:

"the output transformer secondary "common" should be run directly to the shield connection of the output jack (preferably using an isolated jack), not to the buss. Then run a second wire from the output jack shield connection over to the buss at the point where the global negative feedback is implemented (usually the phase inverter ground point). If the amp is not using global negative feedback, just run the wire over to first filter cap ground. This keeps the heavy output stage currents flowing in a loop from the secondary of the output transformer to the speaker and back, keeping them out of the sensitive preamp or buss ground circuits, and off of the chassis. The wire back to the phase inverter carries no significant current, but provides the "reference" ground for the feedback loop to work properly."

I have the PI components and the GNF resistor grouned at one point, on the ground buss. The first part of the quote tells me that I should run a wire from the sleve of the output jack, to this ground point on the buss. Is this how you do it?

Also. Is there any critical placements of the groundings on the buss? I have grounded them along the board (input jack, cathodes components on tube 1, tube 2, PI parts. The pots are grounded along the buss bar, where they are located. Should things be in a different order?

The buss is grounded to the chassis, at the input jack side. The power supply, output jack, bias supply, screen grids and PT center, are grounded together, at one of the PT bolts..

Tommy

[Bob-I]

I read the aiken tips on grounding, and have a couple of questions.

Aiken writes:

"the output transformer secondary "common" should be run directly to the shield connection of the output jack (preferably using an isolated jack), not to the buss. Then run a second wire from the output jack shield connection over to the buss at the point where the global negative feedback is implemented (usually the phase inverter ground point). If the amp is not using global negative feedback, just run the wire over to first filter cap ground.

I always run it to the filter cap ground after going to the jack. I agree that you want to run this neutral directly to the jack, not to the ground. The GNF is typically grounded to the bus along the board.

This keeps the heavy output stage currents flowing in a loop from the secondary of the output transformer to the speaker and back, keeping them out of the sensitive preamp or buss ground circuits, and off of the chassis. The wire back to the phase inverter carries no significant current, but provides the "reference" ground for the feedback loop to work properly."

That's a tad confusing. My goal in keeping the secondarys direct to the jack is to keep the high voltage swings away from the rest of the amp. I don't really care about the NFB, not enough current to worry about IMHO.

I have the PI components and the GNF resistor grouned at one point, on the ground buss. The first part of the quote tells me that I should run a wire from the sleve of the output jack, to this ground point on the buss. Is this how you do it?

That's not what I read there. Only the wires direct from the OT need to go to the jack. The rest can go to the bus.

Also. Is there any critical placements of the groundings on the buss? I have grounded them along the board (input jack, cathodes components on tube 1, tube 2, PI parts. The pots are grounded along the buss bar, where they are located. Should things be in a different order?

I use 2 busses. One on the pots and one on the board. Cathodes and any other grounds on the board are seperated from the input and pots.

The buss is grounded to the chassis, at the input jack side. The power supply, output jack, bias supply, screen grids and PT center, are grounded together, at one of the PT bolts..

Tommy

That's how I do it, and my amps seem pretty quiet.

[Tdale]

I might try two busses...

Tommy

[Tdale]

Is there any point is soldering the buss to the casings of the pots? Aren't the casings grounded to the chassis, as long as they are boltet to it?

I see some solder the bus to the casings, and some don't...

Also..when we talk about the GNF resistor being grounded to the bus.. we mean the grounding of the presence pot, right? The wire from the output jack would have to go to the place on the bus where the presence pot is grounded, right?

tommy

[mhuss]

Is there any point is soldering the buss to the casings of the pots? Aren't the casings grounded to the chassis, as long as they are boltet to it?

There is no need to solder anything to the pot casings, that is strictly an old, traditional way to mount a ground buss wire, and ensures a sub-optimal grounding scheme!

Also..when we talk about the GNF resistor being grounded to the bus.. we mean the grounding of the presence pot, right? The wire from the output jack would have to go to the place on the bus where the presence pot is grounded, right?

yep!

--mark h

[drz400]

Is there any point is soldering the buss to the casings of the pots? Aren't the casings grounded to the chassis, as long as they are boltet to it?

I see some solder the bus to the casings, and some don't...

Also..when we talk about the GNF resistor being grounded to the bus.. we mean the grounding of the presence pot, right? The wire from the output jack would have to go to the place on the bus where the presence pot is grounded, right?

tommy

Dont ground to chassis except for 1 place, In the middle front of chassis is good with an RF cap at the input jack or use a point right next to the input jack. Do not use the chassis as a ground path.
A ground buss and local stars going to the buss, Keep filter cap grounds for each stage as the local star, then go to the buss. If you have GNF then do it the way Randall says, it works. The OT ground goes to the output jack, then run the ground back to the place at the buss where the GNF ground goes (yes presence pot and a resistor usually). Keep third pin ground away from circuit ground. Float your CT on the heaters to 75~80VDC and you will have 0 hum at any level. everything that Randall says is also said by London Power books and it really does work. Pick up The Ultimate tone 3, it is all explained there as well.

[David Root]

DrZ400, could you explain the rationale for dual bussing and "keeping #3 pin ground off the board"? I have TUT3, I guess I should review that section again. I have always used a single galactic star ground with DC heaters, which is very effective. Never had any luck with the DC offset on AC heaters though, always got 120Hz buzz for some reason.

[drz400]

DrZ400, could you explain the rationale for dual bussing and "keeping #3 pin ground off the board"? I have TUT3, I guess I should review that section again. I have always used a single galactic star ground with DC heaters, which is very effective. Never had any luck with the DC offset on AC heaters though, always got 120Hz buzz for some reason.

I'm not following you there pin 3 ground off board?

You mean you use 1 star for the whole amp? I tried that on a few marshalls and it made things worse. I did have pretty good luck with 3 stars to the chassis but the chassis is not a reliable ground

The thing about floating the heater center tap is that if the rest of your grounding isnt excellent it can make it worse. It is hit or miss adding it to a non rebuilt ground path (like a Marshall) IF the rest of your grounds follow that buss with independant stars to the buss it works every time, almost as good as DC heaters.
Read his chapter again and then read Aiken, they same the same thing.

[David Root]

No, what I meant was I use multiple local stars (in this case 4) with a single wire to the chassis from #4 star. The filter cap for each local star is grounded there (local star) too. But I mix the board and the pot grounds on each star.

On the #3 pin, I thought you were talking about preamp tubes, not power tubes. Now I'm confused.

[drz400]

No, what I meant was I use multiple local stars (in this case 4) with a single wire to the chassis from #4 star. The filter cap for each local star is grounded there (local star) too. But I mix the board and the pot grounds on each star.

On the #3 pin, I thought you were talking about preamp tubes, not power tubes. Now I'm confused.

Yes that is a great way to do it if I'm reading you correct, Take a big old piece of fat buss wire or a length of fret wire works nice, float it from chassis with stand offs going from the input jack area to the power tube area, this is your buss, Or Chain with 18gauge from one star to the next, dont connect your individual stars to one point but to the buss and if you make the chassis ground right near the input jack you will kill RF as well. If you dont then run a 51ohm in series with a .01 to chassis ground from the input jack ground to dump RF.

Take in consideration that with feedback loop the OT ground goes to the Speaker Jack Ground which then goes to the ground point of the phase inverter if it is a Marshally/Fender type circuit. Dont use chassis as a ground and try and keep a volume pot ground grounded to the stage it is feeding, this avoids funny buisiness like the amp being quieter or noisier at certain settings of the volume. IMO the Pot casing belongs grounded to the chassis and only the chassis unless you are going to try and isolate each pot case from the chassis which would be a real pain.

Pin 3? What did I say about pin 3? I dont remember

[David Root]

Looks like I'm doing it the same way you are, except my only circuit ground is at the power end, not the preamp end. I have the 100 ohm balancing resistors on the heater line at the preamp end i.e. furthest away from the supply origin and I didn't want to have that ground point going to chassis at the preamp end because it's a high current circuit.

However, the way I'm doing it now, which is putting that heater balance ground into the first preamp star, sort of comes to the same thing I think, given that I'm going to circuit ground at the other (power) end of the buss.

Would the optimum setup be to ground the heater balance point at the supply end i.e. star 4 in my case, and have the single circuit ground coming off star 1 to chassis, as you do?

Or would it be better to balance the heaters at the supply end, connect it to Star 4 and then take star 4 to the power end of the chassis? That would keep all potential high current sources at the power end of the buss, with a nice sequence from low at star1 to highest at star 4.

What do you think?

On the pin 3 thing, I think I remember you saying to keep pin 3 ground off the board, but this was in the context of two busses, one for the board and one for the pots, unless I misunderstood something, which seems most likely since we are both now confused about this!

[bueller]

Interesting thread, I was just playing around with the same ground points myself over the weekend by chance. I have one clone that sounds phenomenal and another that is close but does have that "magic" quality. Having checked all components and values, I noticed the ground paths I chose were sublty different in the two amps. The first has the OT common, the first filter cap, and the presence control, all grounded to the same point (only by chance). The second amp was differently grounded with a non-isolated speaker out and the GFB to the buss and I think all the filter caps were together on the same point.

I re-ground the second amp, but mistakely chose to ground all the supply filter caps and the OT common together, but not the GFB (presence). Result was not excessive noise, but ghost notes that sounded harsh coming through. As soon as I placed the ground points back away from each other, ghost notes were gone.

Tonight I will try the OT common, GFB, and first filter cap to the same point.

[KGW]

I've been reading Aiken's article and TUT#1 (I don't have #3 yet).
I've started a drawing for a grounding scheme. This version has 3 star points: power, PI and preamp. I used a "simplified" TW express layout in the drawing that shows 3 filter caps for simplicity (I realise that there are 5 B+ voltages on the Express). Let me know what you all think and I can work on a "best" version.

[drz400]

I've been reading Aiken's article and TUT#1 (I don't have #3 yet).
I've started a drawing for a grounding scheme. This version has 3 star points: power, PI and preamp. I used a "simplified" TW express layout in the drawing that shows 3 filter caps for simplicity (I realise that there are 5 B+ voltages on the Express). Let me know what you all think and I can work on a "best" version.

That looks weird, I'm not sure what I am looking at.

You should have one main buss like a real piece of buss wire suspended off the chassis and only grounded to chassis at 1 place.
Each stage should have it's local star consisting of the supply decoupling filter cap with lets say the volume pot ground and the cathode ground that the volume pot is feeding and any grid resistors etc, these go to the filter cap ground and then 1 wire to the buss, take each stage like this. OT ground goes to the speaker jack ground, then 1 wire to the phase inverter ground and 1 wire to the buss, all jack isolated, You need TUT3 it is spelled out

[bueller]

I finished re-wiring my other clone that didn't sound right. First filter cap ground next to but not on the CT of PT, common OT to isolated speaker jack and then to the same point as the filter ground, with the PI ground on top. Instant mojo, changed the character in a way I would have not thought possible. Of course in this build I could not have picked a worse ground scheme using a non-isolated jack and the whole chassis as a loop for the feedback. Doh! FYI this is a sort of clone of the "Blue" channel of a Bogner XTC in a 50-watt version. Sounds very close to the real thing.