Talk about my grounding for a second

[hesamadman]

This week has consisted of a tremendous amount of reading for me. One of the main things is the Grounding chapter in Merlins book. I may have uncovered a couple things I have done wrong in my latest build as I read it.


I have read in the past that its good to have a designated buss bar for the power amp/PI and a separate designated one for the pre amp. In my passed builds I have had one large buss bar. All grounds (preamp and power) go to it. The buss bar was connected to chassis at preamp side. Not at PT lug like I should have.


I have since broken these up. On my last build I had power amp and supply on its own buss bar, terminated at PT ground lug. Pre amp on its own buss bar BUT terminated at chassis on opposite end of chassis. My question is, could this be a source of noise? Is it technically a ground loop? I guess it would be if my input jack is not isolated. So that would make my preamp be grounded via input jack AND the ground bus bar attached to chassis?


In addition, I am seeing filter caps for the triodes of the preamp placed on the board right next to the cathode cap and resistor. (or at least grounded with the associated triode) Is this a preferred method? I see it both ways. Hoffman AC30 has all filter caps on one buss bar, Tweed Deluxe for instance has preamp filter cap placed with preamp components on board and grounded with preamp buss bar. Generally my layouts have ALL filter caps right after the SS diodes on my board.


I am trying to elude any ground loops and find a method that works best for me. I have read that putting the buss bar on the back of the pots can inadvertently add a ground loop as well so my buss bar consists of a solid piece of 14awg wire secured on my board and grounded at one side.

[AZJimC]

One approach would be to use the Input Jacks as grounding for the preamp. On my latest build I have a wire soldered across the back of the pots, and that wire terminates at the input jacks. I see no need for a buss at all, other than this pot grounding wire, and I have other amps that don't even have that. Keeping it simple, do power supply and power amp grounds at transformer lug, and preamp grounding at input jacks. If you wish, you can put a screw or solder to chassis right by the jacks, and thus have a sure ground at two ends of chassis.

Ground loops would have to be much bigger than a buss a few inches away from the inputs, to become an issue, IMO. Simply keeping two separate grounds  at each end, one for power and output, the other for preamp is sufficient. I have several amps that are point to point, on the backs of sockets, where each tube has it's own ground via a terminal strip, and no issues with hum at all. At one point I had a hum in one, but rewired heaters, and that took care of it.

[hesamadman]

Thanks for your input. I grind my chassis down pretty good so maybe that will be sufficient enough if I use the input jack. Thats the rout I want to go because I don't like using plastic jacks. Junk IMO. Seems like any repair I do to a marshall, its replacing those jacks. Im going to focus very hard on my grounding on this next build.  And Im going to rewire the 30 watt thats giving me a bit of noise. It could be one of two things. Could be the leads from tubes to board is too long. Its a large chassis. The distance is pretty great. It could be the PT placement. Im going to try the headphone trick and see what it tells me. I doubt its my heaters. I do those pretty tighty but I am not opposed to checking them and seeing.

[AZJimC]

I would suggest a couple of washers, internal star type, on the jacks inside chassis. It makes for a better electrical connection.

[Fresh_Start]

Putting the filter caps close to the portions of the circuit they feed reduces the physical size of the loop current passes through IIRC. Kevin O'Connor and Merlin Bledsoe (Valve Wizard) both suggest this. It's part of a "galactic grounding" or "multiple star" approaches. You have to buy O'Connor's books but Merlin shares some of his thoughts:

http://www.valvewizard.co.uk/Grounding.html

Somewhere in the Library, Doug Hoffman has a pic of an amp with the filter caps running along the edge of the board instead of all bunched together at the power amp end.

Hoffman's grounding scheme works. Look in the Library. Power amp grounded near PT. Preamp on separate bus. Heater center tap doesn't matter. Bias supply can be separate or with power amp (I separate it). Hoffman's essentially is a "galactic" star approach. Moving the caps closer to their respective circuits makes sense to me and has worked well on several builds.

I isolate the input jacks and ground the preamp bus at one dedicated point near the input jacks. I don't like the idea of counting on inout jacks for grounding. The potential for mechanical movement resulting in a poor ground connection is easily avoided.

Cheers,
Chip


P.S. You don't have to use plastic jacks. Doug sells insulating "shoulder" washers for Switchcraft jacks here:
http://hoffmanamps.com/MyStore/catalog/MiscHardware.htm

[hesamadman]

Putting the filter caps close to the portions of the circuit they feed reduces the physical size of the loop current passes through IIRC.

I ha no idea what IIRC meant. I thought it was an abbreviation of a tube amp term I haven't heard. Through me for a hell of a loop. 


On the hoffman deluxe layout the preamp is being filtered by the same filter capacitor. It is also placed on the board in the preamp section. I like the idea of that, however, I use dual caps to save space.


If its 16uf/16uf, is it bad practice to use one half of it for power amp and one half for preamp?

I don't like the idea of counting on inout jacks for grounding. The potential for mechanical movement resulting in a poor ground connection is easily avoided.

This was my first thought. But I didn't know how I was going to keep my input jacks from making contact with the chassis. Thanks for the recommendation.

[hesamadman]

Ground loops would have to be much bigger than a buss a few inches away from the inputs, to become an issue, IMO.

Ah... been reading some more. I think i finally get it. What I described would not effect anything as there are no components inside that small loop anyway. Its just two paths to chassis at the very very end of the buss anyway.

[Willabe]

If its 16uf/16uf, is it bad practice to use one half of it for power amp and one half for preamp?

Read through the Merlin grounding page again, he talks about your question.

The problem is in a cap can you can have 2 or more caps in it but still only 1 ground connection per can.

If you are going to ground the power amp section by the PT and ground the preamp section by the input jack where will you ground that cap? 

[PRR]

> connected to chassis at preamp side. Not at PT lug like I should have.

*Either* way will work.

BOTH ways at the same time begs for a loop.

There are many ways to skin a cat. Gut up. Neck down. Scalpel. Chainsaw. Some folk become very attached to what they think is the "best" way. It may be best for them, their situations, their problems, their tool-chest.

You can also run every return separately to a "Star". This makes a mess of green/bare wires, and can have hidden pitfalls.

Combination of local star and overall bus tends to work fine.

The big buzz "ground loop" in wall-power stuff is usually the PT-Rect-1st Cap loop. This should work alone, and the rest of the amp returned at the *capacitor*, not many inches away along a wire carrying huge 120Hz pulses.

The choice of chassis-bonding in front or at the end is often "mechanical".

With US SwitchCraft jacks, the input is naturally bonded to chassis. Bus from here to your main cap and rectifier/PT return, and do not bond that end to chassis. (However I too do not trust SwitchCrafts to stay tight.)

With plastic jacks, it is convenient to let the input float, bus to the PT, and bond to chassis there. (While a foot of wire is fairly low-Z at audio, it may be poor grounding for radio. You may sometimes like to put 0.001uFd from input jack shell right to chassis, so radio signals get a short-cut and don't flow so much in the audio bus.)

You would generally want all returns in one stage to come together nearby. This idea can usually be violated in tube audio systems. Copper conducts SO much better than vacuum that going a few inches over on the bus makes little difference.

In Any Case. Your 3rd-pin wall-plug green wire MUST go direct to chassis. Agency Best Practice says it should have a dedicated screw so it won't be disturbed if parts have to be replaced (though in our world, a PT bolt is common and reasonably safe). Sheet-metal screws are bad mojo, they don't have long-term bite in the chassis. If you know electrician's toys, you know there is a fine-thread green screw just for grounding. Plain nut-bolt with star washers is a good choice. Doug has pezz-nuts with embedded tooth-washer, you can really honk these tight. A fine detail: you like more slack in the green than the hot wires so when you rip the cord out the green is that last to go (you stay safety-grounded to the end of your rope).

[Ed_Chambley]

I do not solder to the back of the pots as I have not found it to be necessary, but it does work.  I bend an eyelet and make a buss bar and bolt it to the preamp end of the chassis.  Then I simply arrange my lead dress with shorter ground wires to hold it suspended.  I have been using electric fence wire instead of copper and it is more rigid.  Sluckey and I both used it on the AC15 builds and it worked well.  I now prefer it.

I make a separate earth ground very close to my IEC and ground everything in the power section to a star on a convenient transformer bolt.  Here I use a locknut.  I am also very fond of threadlocker and use it on most every nut, even tube sockets.  All in all I have 3 connections to the chassis and have never had any problems with noise coming from ground problems.

I like to use Switchcraft Long Bushing style jacks.  This allows me to get a star washer in the inside and on the outside I use 2 nuts and blue thread locker.  Tighten the first one, all a tad of thread locker and tighten the jam nut.  I never had one get loose since and they have more solid feel when I plug in.

I like cap cans as space savers too, but only in the PA.  Lately I have been using radial electrolytic caps with teflon tubing in the leads.  I was looking at all the different styles being made now and some are no taller than a axial on its side.  Makes fitting them easy especially if adding components.  Although, I have been doing true point to point not because I believe it makes better tone or has more mojo, but simply because it is enjoyable to me.

One day I will be able to build PTP like Darryl.

[hesamadman]

If its 16uf/16uf, is it bad practice to use one half of it for power amp and one half for preamp?

Read through the Merlin grounding page again, he talks about your question.

I actually read that right after I posted. An exception would be the caps before and after a choke, wouldnt ya say? I would thing a dual cap there would be ok but im hoping you can tell me one way or the other.

[Willabe]

Since you and a few other members this week have posted questions on grounding, I started this thread on the grounding topic;

http://el34world.com/Forum/index.php?topic=18828.0

[sluckey]

An exception would be the caps before and after a choke, wouldnt ya say? I would thing a dual cap there would be ok but im hoping you can tell me one way or the other.

I prefer to use a dual cap in this position, even if there is a resistor instead of a choke. I consider screens to be part of the power circuit and as such should use the same ground point. I always ground the PT HV CT, 1st, and 2nd filter caps, and PA tube cathodes to the same ground point so using a dual cap in this position makes plenty of sense to me. Doing this keeps all the high current, 'dirty' DC from actually flowing in the chassis.

I also sometimes use another dual can for other parts of the circuit if those grounds would be connected to the same point.

I do prefer to use a single cap for the first preamp tube and it gets grounded near the input jacks.

I've had good results with this but I'm also not afraid to vary it a bit if the layout or other factors call for a different scheme.