I am finally going to remove the .047uf 600V death cap off the polarity switch in my 72 Marshall

H

harddriver

Well-known member
I've read some differing views on this and wanted to ask some questions, I have researched some post online about vintage Marshalls and the death cap and intalling ground plugs on them. I have already cut the old cord out and have a new 16/3 ready to go in. The amp did have a 3 prong grounded plug with the black(hot wire) feeding the input side of the bottom lug of the polarity switch and has a jumper wire over to the + side of the power switch, the white neutral wire feeds the mains fuse and the green is grounded to the chassis but the .047uf death cap is connected to the center lug of the polarity switch. The top lugs of the polarity switch has a light blue wire from the power transformer soldered across the lug to a yellow wire that connects to the output lug of the mains fuse.

Some say just clip the .047uf death cap out and you are good to go and leave the polarity switch as is wired. I will unsolder the death cap cleanly regardless but I may want to move my PPIMV into the old polarity switch hole so I can re-install the speaker jack I pulled out for the PPIMV years ago so I would need to unsolder the blue and yellow wires from the upper polarity lugs resolder them and apply some heat shrink tubing thus allowing removal of the polarity switch all together. But if I were to keep the polarity switch in for aesthetic reasons then the blue an yellow wire just remain soldered across those upper lugs.

We will leave the polarity switch in for the sake of this discussion. Unsoldering the .047uf death cap and leaving the blue and yellow wires soldered across the top lugs of the old polarity switch.

Here is where this other method I came across online differs from the first one and how the amp is wired right now. It states the the black(hot ) wire must be connected to the input lug of the mains fuse holder and the white neutral must be soldered to the power switch input lug and then of course green wire to the chassis ground lug as seen in the current pictures with the .047uf death cap removed of course.

Usually I would wire black(hot to input side of the power switch) and white(neutral) to the mains fuse as I have done on all my builds so far.

So which is the most correct method for the hot, neutral connection? Black(hot) to fuse/White(neutral) to power switch lug or Black(hot) to power switch lug/White(neutral) to fuse.

Of course green to chassis ground for both scenarios.

I have attached some pics as the amp was done by a tech before I got the amp and I just left things be. It was time for a new power cord and seen it still had the death cap in it so I figured it was high time to pull the death cap out.
 

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Always fuse hot, never fuse neutral. If you fuse neutral and have a switch failure that still allows black to stay live, a person can get shocked conducting black to ground through their body.

Black wire to fuse immediately and then to power switch. Neutral to other side of mains switch (mains switch should be appropriately rated DPDT). Green soldered to a steel tab to chassis ground with the steel solder tab having a steel lock washer. Don’t use aluminum tabs to solder copper wire - it’s a fire hazard from expanding and contracting disassociated metals. Technically code states green should be dedicated to its own steel solder tab/lug.

Cut the death cap and leave the polarity switch but have it disconnected for aesthetics only.
 
glpg80 said:
Always fuse hot, never fuse neutral. If you fuse neutral and have a switch failure that still allows black to stay live, a person can get shocked conducting black to ground through their body.

Black wire to fuse immediately and then to power switch. Neutral to other side of mains switch (mains switch should be appropriately rated DPDT). Green soldered to a steel tab to chassis ground with the steel solder tab having a steel lock washer. Don’t use aluminum tabs to solder copper wire - it’s a fire hazard from expanding and contracting disassociated metals. Technically code states green should be dedicated to its own steel solder tab/lug.

Cut the death cap and leave the polarity switch but have it disconnected for aesthetics only.
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Thanks for looking over this glpg80, I appreciate it. :2thumbsup: Fusing the hot(black)120ACV wire does make sense and that's what I see when I watched some death cap videos regarding Fender amps, the techs were fusing the black(hot) wire.

I even consulted my old Metro amp build instructions and it clearly shows the Black(hot) 120ACV connected to the switch and the white(neutral) to the mains fuse. I had not really questioned it until now.

Is this something Marshall did in the past that wasn't technically correct? I can't imagine George Metropolous would send out thousands of instructions and no one ever called it out, I just checked all of the instructions for the Metro 50, 100 Superleads all are running the black(hot) to the switch and the white(neutral) to the mains fuse.

The JCM800 instructions with the IEC connector does show the hot leg(black) going directly to the fuse first then another black wire is routed up to the mains switch along with the neutral wire to the other lug on the JCM rocker switch exactly as you describe.

Here is a pic of a 1975 JMP with black bat switches
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So if I were to remove the polarity switch all together then the robin egg blue wire from the PT winding and the yellow wire coming from the mains fuse would then be moved over to the upper lugs of the main power switch. I know the black wire from the PT is for the 120 VAC selection. What is that robin egg blue wire from the primary side of the PT that shows 0 V according to a Drake wiring ID and why is it connected to the yellow wire coming from the mains fuse completing a 120ACV fused circuit???? Sorry...........I don't mean to be a bother..... I just want to understand why this amp was wired the way it was. The hot does appear to be fused on the schematic with the IEC connector but Marshall appears to do the opposite with their amps prior to using the IEC connector?????? WTF?

So every Marshall amp that has the hot on the switch is a potential shock hazard if the switch fails, that's probably alot of amps out there wired like this.:shocked:

I do think I want to remove the polarity switch all together to make room for a PPIMV which is why I am asking where to move the robin egg blue and yellow wire are seeing 120ACV on the polarity switch as wired and then 120ACV if moved over to the mains switch. From what I can see they would be moved to the upper lugs of the power switch as I seen in that pic of 1975 JMP with the same PT.

Drake PT 1202-324/7 Wire Colors:
Mains VAC Primary;
240 Yellow
220 Orange
120 Black
0 Blue
Secondary;
High VAC;
Red
Black CT
Red
Heaters 6.3 VAC;
Black
Brown CT
Orange

Here's a 79 2203...
1735416933074.jpeg
 
I’m just saying what’s technically correct and the way I wire amps. I don’t really care why someone does something unsafe or wrong - I only care that people do it correctly. I’d assume at some point there were requirements to get UL certifications which prior versions didn’t have.
 
glpg80 said:
I’m just saying what’s technically correct and the way I wire amps. I don’t really care why someone does something unsafe or wrong - I only care that people do it correctly. I’d assume at some point there were requirements to get UL certifications which prior versions didn’t have.
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I agree fully, I had just never questioned it before. I have an NOS black bat switch coming yet to replace the metal one in the amp so I have time to map out exactly what I am doing

What exactly is the 0V blue wire coming from the primary side of the Drake PT then and why is it connected to make a closed circuit with the fuse circuit and receiving 120VAC?
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I do think I want to remove the polarity switch all together to make room for a PPIMV which is why I am asking where to move the robin egg blue and yellow wire are seeing 120ACV on the polarity switch as wired and then 120ACV if moved over to the mains switch. From what I can see they would be moved to the upper lugs of the power switch as I seen in that pic of 1975 JMP with the same PT.

Drake PT 1202-324/7 Wire Colors:
Mains VAC Primary;
240 Yellow
220 Orange
120 Black
0 Blue
Secondary;
High VAC;
Red
Black CT
Red
Heaters 6.3 VAC;
Black
Brown CT
Orange
 
Blue is the other side of the switch that matches up with neutral. Blue is the neutral equivalent connection for the PT. Black is the other side of the PT primary which matches with switched black from the inline mains fuse.
 
Yeah man this is super important to get right. You only get one life and I hate when people fuck up the safety stuff even if it’s by accident. Part of my mods are always making these changes to amps if they aren’t already wired this way. You can see the IEC calls out a 3A fuse for 120V. I usually use 3A fuses and not 4A for 120V for high gain builds but if the amp is a Van Halen type of design where the owner is known to variac down, I’ll use a 4A (lower voltage will pull higher current - the amp will demand the VA it wants regardless assuming bias is correct)
 
Yeah, always fuse Mains hot. Not neutral. It's a pretty common "must do" mod to vintage amps. I have a '71 Super Lead incoming that will require this as well.

Any photos of your '72?
 
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