Best Place to Ground Output Transformer?

[The New Steve H]

I'm wiring up my 56fa clone. I have an output transformer with three hot leads: 2, 4, and 8 ohm. There is only one ground lead.

Instead of a switch, I decided to use three separate output jacks. Now I'm installing them. I connected the ground lugs with 20 gauge solid wire; it's the heaviest wire that will fit.

Now I'm wondering...do I run the black OT lead to one end of the wire on the ground lugs, or should I just ground it to the chassis? I keep hearing that you want grounding attachments to the chassis to be as few as possible, so I'm thinking maybe I should connect this to the star or bus or whatever I end up using as a ground for the circuit board.

[The New Steve H]

More genius questions.

I'm looking at the Aiken amps site, and it suggests using the same dedicated ground the power transformer uses. That leads me to a power transformer question. My transformer has a center tap between the green heater leads.

I hooked the heaters up so one solid green wire goes in, and one solid green wire comes out. But what happens to the center tap? I assume I just ground it. If so, should I ground it at the common grounding point by the power transformer?

Finally, do I need to run any kind of ground wire to the output jacks? They appear to be self-grounding. Maybe I was stupid to connect them to each other. It does look pretty, however.

[Cliff Schecht]

The absolute best place to ground the output transformer center tap is at the very first b+ cap ground. This is where the return currents eventually go anyways and so by connecting directly to it, you prevent these large currents from getting into other places in the amp. For the power transformer HV CT I do the same thing, put it at that first caps ground lug (or as close as you can get to it).

The heater CT is a lot less picky but I tend to put this near that first B+ cap as well. If I'm doing an artificial center tap with resistors I will put the resistors on the first tube socket and connect the ground end somewhere near the input. If you do a buss bar along the pots (my favorite method as of now) then the artificial center tap goes near the input where the currents are the lowest. Or you can elevate the CT with some DC to help reduce hum..

Have you investigated buss-bar based grounding schemes. The ones I prefer look like a modified star grounding scheme that keeps the grounding for each amp section at a localized point only meeting up at the buss bar. Seems to make for some very quiet builds..

[The New Steve H]

Here's what annoys me about Mr. Fender's schematic. The B+ caps are labeled "A."

I already put a ground at the same place where I grounded the transformer, but it's very close to the place you mentioned, so I can move it if it hums. That screw now holds the red wire from the OT and both center taps from the PT.

I have read up on star grounding. I don't see how it's possible unless you have at least two "stars"! I'm going to do what I can. I don't know too much about buss grounding. I have Merlin Blencowe's book, though, so I will try to become an expert before soldering the last joint.

[The New Steve H]

Could still use advice on the output jacks. I don't see any point in grounding them with a wire, because an ohmmeter reads 0.6Ω between the jacks and the chassis.

[diagrammatiks]

the common wire on the transformer secondary needs to go somewhere.

the simplest place would be the output jack.

that way it is close to it's signal, the speaker out wires.

however, that ground section can cause problems if you have issues else where in the amp.

it's sometimes necessary to run insulating washers on those jacks and ground the secondary back at the filter.

[martin manning]

I like grounding the OT secondary directly to the speaker jacks... less chance of a poor connection taking the load off the power amp. A wire could be run from there to the main filter ground if the jacks are isolated from the chassis, but usually its fine to let the jacks take care of grounding the secondary to the chassis. They are usually located well away from the preamp ground(s).

[KT66]

Not to make things more complicated for you, but a tip that I picked up from Aiken's site that I find works well is to ground the ground reference of the PI at the same point that the OT jacks are grounded if the amp has negative feedback. So, in this case that would mean that you should run the ground wire from the presence pot over to the speaker jacks and connect it to the ground wire that you have soldered to the jacks.

[Firestorm]

I already put a ground at the same place where I grounded the transformer, but it's very close to the place you mentioned, so I can move it if it hums. That screw now holds the red wire from the OT and both center taps from the PT.

Err ... red wire goes to B+, not ground.

[The New Steve H]

Did I say red? I better go back and look at that.

[The New Steve H]

I have no idea why I said red. I have it wired right.

Looks like I made a mistake with the bias circuit, however. I capped off the red/green wire that was supposed to go to the bias circuit! Dang it. Now I have to fix that.