# Added grounded cord - now loud hum



## whywhyzed (Jan 28, 2008)

I have a 1955 Harmony H204
Schematic here:
https://reverb-res.cloudinary.com/i...75,w_620/v1435452346/spc42jybeevoi4zbch7q.jpg

Unlike that schematic, mine did not have a grounded cord, so I added one. I wired hot to fuse and neutral to pwr switch, and ground to chassis (at transformer mount stud).
Here's the power supply part of my amp's actual schematic.

The death cap is still there - I made that the hot side to the fuse. 
I'm thinking I might clip the death cap out and see what happens. Or, replace it with a modern safety cap but haven't done anything there yet. 

Now I have a very loud 120HZ hum at all volumes. It was very quiet before the new cord in the same outlet.

Should I have wired the other way? I was following the black as hot and white as N on the original cord, but maybe they paid no attention (and why would they with a 180 deg. flippable cord....) .


----------



## jb welder (Sep 14, 2010)

Try it the other way temporarily to see if it makes the hum go away (reverse hot & neutral).
Then you can wire it up correctly from there.


----------



## whywhyzed (Jan 28, 2008)

Found a wall lump with a broken off ground pin and tried it without ground and flipped both ways with no change...

So, I think my issue was crappy soldering. After giving the solder connections at the fuse and switch a heavy crimp with needle-nose, the hum decreased. 

I will take it back apart tomorrow and do better work, clean everything shiny and resolder.
Paying for laziness I think.


----------



## jb welder (Sep 14, 2010)

That's good that you tried it back like it used to be to see if something else you did may have created the hum. That is what I would have suggested to start off with.
So you should get rid of the death cap, run the neutral straight into one wire of the transformer, ground to chassis, and hot to switch then to fuse then to other transformer wire.


----------



## 4575 (Jan 18, 2009)

The hot should travel like this. From the wall to the ring side of the fuse holder, then the tip of the fuse holder to the power switch, then to the transformer. The neutral should go directly to the transformer. Cut out the death cap. Make a nice new ground stud and tie in your ground with a crimped and soldered ring terminal. The ground lead should be longer than the other 2 wires. In case someone pulls the cord out of the chassis. NEVER SWITCH OR FUSE THE NEUTRAL.


----------



## jb welder (Sep 14, 2010)

Dave, the info I gave is standard configuration for modern amp manufacture. I put the switch before the fuse as that is how Fender now does it. I guess there may be some manufacturers that put the fuse before the switch? I'm not aware of any.
On the 2 fuse terminals, the tip (not the ring) should always go to the most "hot" point of the circuit. This is done for safety purposes. If by accident someone removes the fuse while powered up, the tip is the first thing to disconnect. As the fuse is removed, the far end of the fuse can touch the ring so the ring should not be the most hot point as that is a shock hazard.
If there is some reason you do it in the way you described, I'd be interested to hear why.


----------



## 4575 (Jan 18, 2009)

jb welder said:


> Dave, the info I gave is standard configuration for modern amp manufacture. I put the switch before the fuse as that is how Fender now does it. I guess there may be some manufacturers that put the fuse before the switch? I'm not aware of any.
> On the 2 fuse terminals, the tip (not the ring) should always go to the most "hot" point of the circuit. This is done for safety purposes. If by accident someone removes the fuse while powered up, the tip is the first thing to disconnect. As the fuse is removed, the far end of the fuse can touch the ring so the ring should not be the most hot point as that is a shock hazard.
> If there is some reason you do it in the way you described, I'd be interested to hear why.



Of course, I had it backwards. The tip should be the hot from the line voltage. My mistake. However, the line voltage should always go to protection first (IE the fuse). If for some reason the switch malfunctioned you want it to be fused before hand. If not, you are relying on the line voltage protection downstream (panel box in your house etc.). Not to mention that older amps most likely won't have a power switch that will handle 15 amps. Most modern amps do. 
Just my 2 cents.


----------



## jb welder (Sep 14, 2010)

Good point. And I forgot that some IEC receptacles have the fuse built in, so it's before the fuse. And looking further, I see Yorkville has fuse first, then switch (Fender & Marshall with switch first).
Mainly it's good to see we're on the same page as far as hot to tip on the fuseholder.


----------



## whywhyzed (Jan 28, 2008)

Awesome - thanks guys. I like the idea of the switch and fuse in series and both hot. 
So, hot to fuse tip first - then off ring end to switch then to xfmr. Goodbye death cap. I will report back.


----------



## whywhyzed (Jan 28, 2008)

Well. it's useable now. Quiet with volume below 3 1/2
Above 4 or 5 it hums pretty loud still. 

It turned out to not be loud enough for jams anyhow. 3 volume is great for home use. May go back in and tidy the wiring some more at some point.


----------



## dtsaudio (Apr 15, 2009)

Did you remove the death cap?


----------



## whywhyzed (Jan 28, 2008)

yes


----------



## mccormickanalog (Aug 11, 2015)

120 cycle hum, eh? Maybe try changing the heater wiring to the "two wire" style, and lift one side from ground (assuming the schem is accurate to the amp still) and add a 100 ohm resistor to ground of both heater leads.
I've had this issue with an old bogen PA I converted to a bitchin' guitar head.


----------



## alwaysflat (Feb 14, 2016)

I was going to ask about a good star washer with clean chassis metal under it, but 120Hz sounds more like secondary side noise, like caps in your B+ or cold solder. Hum increases with volume level increase now ?


----------



## jb welder (Sep 14, 2010)

He said he had a hum that increases as the volume knob is increased. I'm not sure where the 120hz came in? Could be 60hz being picked up by the guitar for all we know from the info we have.
So the grounding & star washer is a good suggestion.
And if it's heater wiring related, that would also be 60hz rather than 120hz.


----------



## epis (Feb 25, 2012)

Maybe you could post some pics of internals ? IMHO problem lays in improper grounding scheme. Most of these old amps use chassis for ground, grounding points placed everywhere and one wire heater string doesn't help neither.


----------



## whywhyzed (Jan 28, 2008)

here's a couple pics of the layout. Why is there a center tap off the xfmr secondary supply to the rectifier tube grounded to the chassis I don't get....that matches schematic, but it doesn't look well bonded. I could clean it shiny or lift it. Not sure which would make more sense. I don't see them using half voltage for anything from it.


----------



## epis (Feb 25, 2012)

That center tap must be grounded, for full wave rectification done with only two diodes (your rectifier tube). Basically two high voltage secondaries are wound out of phase each one rectified with single diode. End result - full wave rectifier


----------



## epis (Feb 25, 2012)

It looks to me that heater wiring is changed comparing to diagram.Can you measure AC voltage from each heater wire toward ground (chassis) ? It should be 1/2 of heater voltage. I can't see that heaters are referenced to ground.


----------

