NFB & Ground Connections

[davent]

Hello, I've a small Edcor, PP output transformer for a build that's slowly going together, has solder lugs for all connections. Where the leads for the OT will enter the chassis is close to the eyelet board and where, on it, the NFB and speaker ground will be connected.

Any issues with running the NFB wire and ground wire directly from the OT lugs rather then running them from the speaker jack back to the eyelet board? (Would be separate leads from the OT to the speaker jack). It would be a much shorter/neater run from the OT then traveling back from the speaker jack.

Have only ever used OT's with flying leads so the speaker jack is where i've made the connection for NFB and ground.

Thank you!
dave

[xtian]

Sounds like a smart plan to me.

[Structo]

Well safe margins and wiring would probably say you should keep those leads away from the preamp.

Try to adjust lead dress for the least noise if possible. (chop sticking)

The output jack is a viable attachment point for NFB.

[JazzGuitarGimp]

I would suggest you use an isolated jack for the speaker output to avoid a ground loop.

[davent]

Thanks guys.

The output jack's a Cliff-style Neutrik so no worries over a ground loop.

I hope i'm ok on distance, the OT lead entry point is 4" away from the PI tube and related components then another ~3.5" to the next preamp tube.

Low power amp using a pair or 6AQ5's so not very high voltage either. (HoSo 56)

Only pic i have on this computer, the OT leads come into the chassis just to the right of the small eyelet strip. NFB connects to the second eyelet from the right on the big eyelet board.

Speaker jack is to the left of the power tubes along the bottom of the picture.

[IMG:800:600]http://i216.photobucket.com/albums/cc30 ... 2f928d.jpg[/img]

dave

[Firestorm]

NFB comes off the transformer so shorter is better. In some of my amps there's an unused transformer tap I run straight to the feedback resistor. Ground is another matter. If you're not using the chassis as a ground (except for the safety ground which you must do) then all grounds in the amp should be isolated and you can make ground connections where they should logically be. This will not work for most single-handed amps, which essentially require the chassis to be in parallel with one side of the signal. There are workarounds, but you also have uncancelled heater hum and other issues in SE. For push-pull, if you can keep all signal grounds off the chassis, the amp can be quiet as a mouse, as long as the chassis itself is connected to the AC ground (three prong cord, or UL approved capacitor from the neutral). Somebody could write a whole book on amp grounding. Not me though.

[Malcolm Irving]

.... This will not work for most single-handed amps, which essentially require the chassis to be in parallel with one side of the signal. ....

Can you expand a bit on that? I've not heard of that before. (Presume single-handed is a typo. for single-ended.)

[Firestorm]

Yeah, single-handed was autocorrect. SE amps have no common mode cancellation so every little bit of preamp noise winds up in the output. NFB might help some, but you sacrifice gain which don't have much of. Unless you elevate the heater supply, there will always be 60Hz hum from the output tube, and likely from the preamp as well. Fender Champs (and a lot of early tweeds) put one side of the heaters on the chassis, which makes the whole thing "broadcast" 60 cycles to the enclosed circuitry, hopefully out of phase, thus cancelling the inherent hum. The signal has its ground side attached to same point, so ground hum is reduced in the same way. If you isolate the ground in an SE amp, the guitar cord becomes an antenna for the 60Hz hum that is all around us. There are any number of ways to deal with it. Fender went for the easiest one.