Express Input Jack Grounding

[Ron Worley]

New Guy to the board, with perhaps some questions that have been covered before-

Pouring through the pictures of Francesca, I noted that the input jack appears to be grounded via the chassis, whereas the rest of the post and preamp board use the copper wire buss soldered to the pot backs.

I would seem that grounding the jack to the buss would be a good thing, or am I not thinking this through??

Did KF use copper wire with the sheath removed for this, and if so, what gauge?

I also see that there are resistors to ground off the heaters of V6- did this tranny not have a center tap, or is it just for hum reduction. None of the layouts or schematics seem to show this, is it a later mod for noise?

Thanks in advance!

Ron

[drhulsey]

I would seem that grounding the jack to the buss would be a good thing...
Did KF use copper wire with the sheath removed for this...
I also see that there are resistors to ground off the heaters of V6...

The input jack is grounded to the chassis, as are the pots.
The bare copper wire buss across the case of the pots was Ken's design, but others have used different grounding schemes. Gauge shouldn't matter.
Yes, the resistors create a virtual center tap for the heater supply.
Hope this helps

[Bob-I]

I'm not a fan of this grounding scheme. Washers and nuts can loosen over time, get dirty then you have a bad ground.

I once sent an amp across the USA, New Jersey to Califirnia. When it got there is had a nasty buzz. I had to have the customer return it at my expense. When it got here the footswitch jack was loose and ungrounded. I put in a plastic jack and ground wire, then for his trouble I added a mid boost to the footswitch.

He's had the amp over a year and except for a bad tube it's been perfect.

[Ron Worley]

Tim-

Thanks for the input. Part of what confused me was that this virtual center tap is no annotated on any schematic I've seen- in particular the Kelly 90 with Updates by UR12 (Dana), which is as far as I know the most accurate per the Francesca pictures.

I'm trying to work up a 100% accurate BOM per Francesca, and spent many hours pouring through the pictures. I will post that when I'm sure that it's not embarrassingly inaccurate.

It would seem to to me that adding a ground wire from the input jack to the copper buss makes a lot of sense...(which is actually shown in one of the nice graphical layout drawings in the files section).

Ron

[paulruby]

I'm not a fan of this grounding scheme. Washers and nuts can loosen over time, get dirty then you have a bad ground.

I once sent an amp across the USA, New Jersey to Califirnia. When it got there is had a nasty buzz. I had to have the customer return it at my expense. When it got here the footswitch jack was loose and ungrounded. I put in a plastic jack and ground wire, then for his trouble I added a mid boost to the footswitch.

He's had the amp over a year and except for a bad tube it's been perfect.

Yep. Still should run a ground wire from the jack to the board or the pot buss.

Editing to add some emphasis... It is a serious safety risk to omit a ground wire from the input jack to electrical ground. (And also in the connection from electrical ground to the chassis, if relevant to the particular ground scheme chosen.) The physical connection of the input jack to the chassis should NEVER be the only electrical connection of guitar-chord-shield to you chassis and, ultimately, earth ground.

[drhulsey]

... Part of what confused me was that this virtual center tap is no annotated on any schematic I've seen...

I understand. I spent a lot of time trying to understand the Express circuit before I built one. Most of the guys on this forum like Dana, Richie, Allyn, Paul, and others could draw the schem or layout in their sleep. For a newcomer like me, it took a little time understand it
Good luck

[Ron Worley]

Paul-

That is my experience as well that a simple chassis ground is not sufficient or safe as well. I find it odd that KF would have done that.

I am going to add the wire, guess my Wreck won't be as faithful to the original as I first thought!

Ron