I’m finishing putting together a Hagerman Clarinet preamp that I was lucky enough to find a pcb for. It’s going in a wood case with an aluminum top plate, a rear aluminum plate bolted to the outside, and an aluminum plate inset inside the front to mount the controls to. Which plate would be the best to tie safety earth to? I plan on running short pieces heavy awg wire between the plates and bolts securely. The original design had a ground jack on the rear plate that Jim had the safety ground and a single pcb ground going to in an all metal chassis, fwiw. I could sure use some help to make this as safe as possible.
If you interconnect the plates with pieces of wire it is considered a single object. According to NEN1010 (the EU version of installation code) those wire straps should not be bolted by bolts which hold the construction together. Those should be separated bolts used for grounding wire only. Use lock washers so you pinch through oxide.
Having all that it does not matter where you connect the safety earth.
Having all that it does not matter where you connect the safety earth.
Yep. Thank you.
The plates all bolt to the wood frame. The volume and other controls mount to the front plate, the rca jacks and power entry module to the rear plate, and the pcb is on the top plate along with the power transformer. The connecting wires would just be to “extend” the safety ground connection. All bolted with tooth washers and double nuts. The upright hammond transformer will have at least one mount with tooth washers to make a good connection of the frame to the top plate.
That was my original thinking unless there’s a better way.
The plates all bolt to the wood frame. The volume and other controls mount to the front plate, the rca jacks and power entry module to the rear plate, and the pcb is on the top plate along with the power transformer. The connecting wires would just be to “extend” the safety ground connection. All bolted with tooth washers and double nuts. The upright hammond transformer will have at least one mount with tooth washers to make a good connection of the frame to the top plate.
That was my original thinking unless there’s a better way.
If the transformer cannot be touched from the outside it is sufficient (and not even required for safety) that the transformer has an adequate connection to safety ground. So your idea to have a toothed washer on one of the mounting screws is just fine.
For completeness, if the transformer could be touched from the outside, a separate ground wire must be connected to the chassis. The ground wire is not allowed to be bolted with one of the transformer mounting bolts.
For completeness, if the transformer could be touched from the outside, a separate ground wire must be connected to the chassis. The ground wire is not allowed to be bolted with one of the transformer mounting bolts.
I’m not sure how you’d do that with a Hammond 370BX. I’ve always put a flat washer then toothed washer on top of one of the mounting legs, one toothed washer between the leg and the top plate, and one on the other side of the plate, then another flat washer and double nuts. I’ve never seen a separate wire done directly from the transformer even with the several Bottlehead kits I’ve made or guitar amps I’ve worked on. I never tie the safety ground directly to a transformer bolt though. It’s always by itself. But I’ve never mounted things on separate plates like this before either though.
You would use a connection to one of the 4 bolts holding the transformer together. Often a manufacturer attaches a spade connector to one of those bolts. That's all. However, like I said before, what you do on the mounting bolt is fine.
Unfortunately, I had had two of the pieces (top plate and rear plate) already powder coated a long time ago and now that I'm finally getting around to putting the thing together I'm questioning what the heck I was thinking.
The original manual for the Clarinet from Hagerman had the safety ground going to a ground jack on the rear of a Lansing metal chassis and the PCB ground was tied to that jack as well, which definitely isn't best practice.
I've been away from this project for years so I'm pretty sure I was going to put a long #10 bolt on the top plate right behind the power transformer (about 2" from the IEC module) and secure the connection from the earth ground there using lock washers and two nuts. Then on the same bolt, after the safety ground was secure, make a connection to the rear plate ground jack and PCB ground. Essentially following what Jim had laid out with one extra wire. I'd have another connection from a bolt at the front of the top plate to the controls plate. I only see that I drilled one hole on the top plate at the front and rear. I could drill another hole and keep the safety ground from the power entry separate, which is the correct way. It could be a smaller #8 bolt for connecting the rear plate and PCB, right beside the safety ground bolt.