3 Prong installation on a Sound City Concord help?

[DarrylC.]

ok, so I decided I didn't want to take my life into my own hands every time I play this guy and started putting in a 3 prong. This is the first time I've replaced a 2 prong with a three prong cable. I couldn't find much info about this amp but I did find a schem. I got it installed the way that I thought was correct and everything powered on but I couldn't get any output. The amp seemed to be permanently stuck in standby regardless of how I switched it. I wired my hot straight to the on/off switch. My neutral went to the tip of the mains fuse connector and the side lug went to the opposite lug on the on/off switch. I removed a .047 cap that was wired from the ground switch to pins 7&8 of one of the EL34s (which were wired to chasis ground). I assumed that this was my "death cap."  I did notice that on the strip board terminal where the black wire from the mains transformer is terminated that I had a wire that I disconnected to install my 3 prong left unterminated. any quick tips on where I may have gone wrong?

[DarrylC.]

I think I may have spotted my own mistake. I see that nowhere in the circuit is my neutral connecting to the common of the mains transformer. I am assuming if I just wire the common to the other side of the neutral on my switch I should be in business. I just don't know 100% if this is what I should do. Can anyone verify that this is what I need to do?

[DarrylC.]

Turns out I'm an idiot. Forgot that things need both sides of 120vac to work. 

[HotBluePlates]

Turns out I'm an idiot.

I don't think so. You found your error in just over an hour, including time to fire up the computer and ask the question here.

I took longer than that to figure out how I turned my last amp build into a 25w multivibrator (oscillator).

Congrats on finding the problem! 

[catnine]

You found it and that's all that matters . I have made many mistakes mostly using the wrong value resisters but reading the color codes rather than checking them first with an ohm meter . Took me  hours to figure out why a simple champ build that I did so many times had no volume .

 With power cords it would help if every company that sells them would use the same color leads .

 Give yourself credit for being able to find and remove the death cap . Seems most times the simple things are the ones that sound the alarm bell.

[DarrylC.]

Well, I thought this would be easier than it turned out to be. I searched the web to see what I could find on the subject but it seems like most of the info out there that is super detailed (instructions and pictures) are for fender amps. I surmised that it shouldn't be too difficult to just study those examples and translate it to this amp, but it turned out there was a whole lot to be lost in translation (from a fender to a sound city). There were some funky things to get passed, too. The sound city's schematic (at least the one that I found) shows a three prong AC cable??...that threw me for quite the loop. Why would someone actually replace a sturdy three prong cable with this flimsy "lamp cord" two prong? There was even a random solder pool in the chassis. Nothing else in the amp indicated any modifications of any kind and everything for the most part appeared all original. I'm still trying o wrap my mind around that. Kinda reminds me of my buddy's 1965 Vibrolux where it would have been all original but someone replaced the two prong with a three prong...... but they only used two leads and wired it like a two prong leaving it ungrounded as it was before.

[DummyLoad]

last amp build into a 25w multivibrator

maaan! i hate it when that happens! esp. when it's not OT phasing related...   

--DL

[sluckey]

The quickest/simplest way to replace a 2 conductor power cord with a 3 conductor power cord is to...

1. Cut the 2 conductor cord where it enters the amp. This will leave 2 dangling wires inside the amp.

2. Remove about 6" of the outer jacket of your 3 conductor cord and securely attach the cord to the chassis.

3. Connect the green wire to the chassis with a screw and kep nut (or star washer and hex nut).

4. Splice connect the other two power cord wires to the dangling wires (see step 1).

The amp will function as it always did but now it will also be safe because the chassis is properly bonded to earth ground (assuming proper house wiring). The term "death cap" is now irrelevant and can be replaced with it's intended term "line filter". No need to remove it.

There are neater ways to do this including rewiring the PT primary to some "standard", but the above 4 steps will git 'er done safely.

[DarrylC.]

^ Man....I sure wish I had gone with that method..

[catnine]

even though it's AC shouldn't the black lead called the hot and the white lead called the neutral still be wired to the PT the same way? I know the green is always grounded so it provides a path to ground and the larger slot on a wall out let is the neutral / white and the smaller slot is the Hot and that was when they can out with polarized outlets and plugs so you can plug in a lamp and be assured another lamp is touched does not creat a ground path. Most later PTs have the black and white and green leads . I still place the black lead to the fuse first then to the power switch of the amp then to the black lead of the PT and the PT's white lead to the white lead of the power cord and green to a solid chassis ground.

[PRR]

> I know ... the larger slot on a wall out let is the neutral / white

This was UN-true for 60% of the outlets in my old kitchen.

What you say is only how it is "supposed" to be. But never assume it is true.

Inside the amp, there's a green wire and two others. The others may as well both be black.

And if the White _does_ happen to be Neutral, and you use the old-fashioned fuseholder, there's good reason to run it to the fuse first.

[catnine]

 I checked this old apt and installed outlets with the third ground which is green and made sure each socket had the white lead on the silver screw and the black lead on the brass screw . I know the white in the neutral or common and is the return with close to zero volts yet the black is the hot and has 120VAC . Basically if you just had the black and the green and no white you could power a lamp.

 In an amp I can see why one would want the white on the fuse with an old style fuse holder which is what most amps have at least fender.

 When working on an amp plugged in checking voltages and you had a hand on the chassis and touched the exposed black on the fuse holder you are part of the path . If the white were on the fuse holder you are not part of the circuit.

 I always have seen the black to the power switch and the fuse between the black line in and the power switch . Even in hoffmans schematics it is this way. The PT's I have have a white and black so I wired them the same .

 Am I missing something why it's better to fuse the white lead? I would think it's better if the fuse blows to kill the hot lead . I always checked outlets when I gigged so the mics and other bandmembers did not create a path to ground. 

[sluckey]

Basically if you just had the black and the green and no white you could power a lamp.

No, you should use black and white. Green should NEVER be used as a current conductor. NEC says so. They also say white/neutral shall not be interrupted by a switch or fuse.

But these regulations apply to residential and commercial building wiring or power distribution systems. They do not apply inside a device or appliance. However, many people, including myself, will carry the NEC code into the amp chassis just for convention. Black/hot to fuse to switch to PT primary. White/neutral direct to other PT primary lead. Green direct to chassis.