Grounding Question

[lvbluesboy]

Hello All,
I have a Sunn Spectrum 1 (1967 vintage) with the two wire power lead and the polarity switch. Is it feasible to switch out the power lead for a three wire grounded line? And if so, would the grounding line be attached to the chassis or?

I remember getting zapped a few times back in the day and would just like to make this old boy a little safer if it is feasible.

Any feedback would be greatly appreciated. I know the two wire lead was fairly common on vintage amps and this happens to be one of those. By the way, I have had this baby since it was new and it still works great. Kind of sparse on the controls compared to current equipment, but simple can be good.

[JoeArthur]

Yes, the ground wire would be attached directly to the chassis.

I don't know your amp, but I usually solder a circular lug onto the ground wire and use one of the transformer attachment nuts to secure it. If all else fails, you can drill a hole and use a screw/washer/nut to secure it.

I don't recommend soldering the ground wire directly to the chassis, but that is a possibility as well. The problem is getting enough soldering iron heat to do it and cleaning the area to be soldered well enough for it to stick.

[lvbluesboy]

Thanks very much for the feedback. I was planning on using some type of grounding lug rather than trying to solder the connection.

FYI - the Spectrum 1 is a separate head and cab combination. It is rated at 50 watts and has a single D130F 15" JBL speaker in the cab. The only controls are volume, treble, bass and contour. It also has three rocker switches and those are standby, polarity and power. I put some new tubes in it recently and that really brought it back to life.

Once again, thanks for the information.

[Soundmasterg]

To be correct with the UL requirements, the ground wire should attach to the chassis within about an inch of the recepticle on the back of the amp. You should leave the wire just a little long in case the recepticle gets torn out of the amp, and the wire should be green, or green with a yellow stripe. Attach it with a lug that is held down by one of the transformer bolt screws as mentioned, and nothing else should attach to this point.

You will also have to rewire the switch and fuse, because currently they have one on each side of the AC line, and both should be moved to the hot side (black wire) for safety reasons.  From the AC recepticle on the back of the amp, it should go to the fuse first, and then the power switch, and then to the power transformer. The other side of the AC line (white wire) should go direct to the power transformer wire. To do this correctly and safely, you may have to drill a hole and mount a terminal strip that you can solder each wire to on the white wire side. You definitly don't want to just solder the two wires together and use tape to isolate them!

The ground switch should also just be completely disconnected as it isn't needed with a 3 wire system. In my 200S, I used Kevin O'Connor's galactic ground system and utilized the former ground switch as a ground lift switch, but to do this particular mod requires extensive changes to the amp grounding system. If you want to find out more about his grounding system, it is in his TUT (The Ultimate Tone) series of books. You can find them at his site www.londonpower.com, or many other retailers. If you want to just do the simple setup, then disconnect the ground switch and rewire as I mentioned above.

Greg

[lvbluesboy]

Thanks for the recommendation Soundmasterg. I am going to check out the ground lift system you mention. I may just go the simple route, but the alternative sounds interesting. What is the benefit of the ground lift configuration? I am sure that question is answered in the reference material from Kevin O'Connor, but I was just curious and you obviously have experience with the system.

When I bought my amp Sunn was making some noise and Hendrix was using their equipment. I was lucky enough to see him in Seattle in from of a wall of Sunn amps and it was pretty cool. Very loud, but cool. The next time it was a wall of Marshalls.

My little Spectrum looks just like it did when I brought it home and it does get some looks.

[Soundmasterg]

The main benefit to the ground lift switch, and the galactic ground system that Kevin O' Connor talks about is that it makes the hum level on the amp a lot quieter, and it allows you to work in a studio environment or any environment where you have multiple pieces of equipment plugged into the same ground source without ground loops. For example, if you have 3 amps that you're using at the same time, and all three have a 3 wire AC line, the chassis of each will loop together through the common ground and cause excessive hum. Being able to lift the signal ground in one of the amps makes the whole system hum less overall. In order for that ground lift to work however, every ground in the amp has to be isolated from the chassis ground except for at one point. So the input and output jacks have to be isolated from the chassis, and all the circuit grounds likewise have to be isolated from the chassis. The actual circuit ground connection is made through the lift switch, and through a resistor and cap in parallel with each other to ground. This makes an AC grounding point only when the lift switch is lifted, and when in normal mode and lift mode, it offers RF suppression too.

Honestly I wouldn't bother. Its a ton of work, and it does provide some great benefits, but I'd only do it if you were totally rebuilding the amp like I did. If you just did the 3 wire AC conversion, and just disconnected the ground switch, and rewired the fuse and AC switch to the correct side of the AC line, you'd be fine. That said, it is interesting reading, and his books have lots of other great info in them. If you want to learn about working on and modding amps, his books are a good resource.

Greg

[lvbluesboy]

Thanks again for all the info Soundmasterg. I actually found the original specifications sheet for the Spectrum I and it shows the amp rated at 125 watts.

I will be doing the conversion to a grounded plug. It just makes sense to take that type of precaution. I do plainly remember getting zapped a few times and would really like to avoid that in the future.

Once again, thanks for your help.

[EdBass]

Thanks again for all the info Soundmasterg. I actually found the original specifications sheet for the Spectrum I and it shows the amp rated at 125 watts.

Was that spec sheet rating for three of them?

[rick.heil]

125 watts A/C draw, perhaps?

[loudthud]

I've done the three wire conversion on several amps. Because of the safety issues, this is not something for someone without experience. Before you start the conversion, be prepaired to deal with the following:

The first problem is the convienience outlet is usually broken. This is where the black and white wires from the line cord terminate. If you can't find a replacement outlet, I suggest you leave it disconnected and install a terminal strip by drilling a hole in the chassis near where the cord enters.

The second problem is the fuse holder usually needs to be replaced. Make sure that the black wire goes to the center terminal and not the one on the side. Otherwise it is possible to touch the hot side of the line (OUCH!) when you are changing a fuse.

Lastly, the strain relief for the line cord won't accommodate most three wire cords unless you modify it. Use a rat-tail file but don't take off too much or the line cord could pull out. I think UL tests this with a 25 pound weight on the cord.

Note that to make the pilot light work, there is a wire that needs to be run to the power switch from the white line cord wire. It goes to the terminal closest to the ground switch.

Remove the "Budroc" cap and any wires to the ground switch. You'll need the switch at some point when either the standby or power switch fail. You can just connect wires from the failed switch to it.

As soundmasterg wrote, the fuse and power switch should both be on the hot (black) side of the line.

[Soundmasterg]

Another thing that you could do, and I did this on my 200S, is to use an IEC power cord connector like what is used on most things nowadays instead of the hard-wired plug. This is the same as what is used on your computer. I just did away with the convenience outlet since I wouldn't be using it anymore and put the IEC in place of that, and in place of the hole for the power cord. It worked out pretty well and looks fine. I like using the IEC setup on heads anyway. I used a dremel to cut the hole in the chassis larger.

Greg