# 60's Pepco Amp Grounding With No Power Transformer



## tomsy49 (Apr 2, 2015)

So I came across a cheap 60's Pepco amp converted to a head that i am considering buying. Apparently these amps were made without a power transformer and a 2 prong plug so they were considered a "hot" chassis. This one in particular has been converted to a 3 prong plug and was grounded to the Chassis. Can anyone give me info on whether or not there is still any danger of the chassis being a shock hazard?? I have attached a pic of the internals and am wondering if anyone can say if there are any red flags apparent in the circuit?? The tubes which you can't see are:

westinghouse 35Z5 GT 
sylvania 35L6 GT 
Westinghouse 12AX7A

Are the 35Z5 GT and 35L6 GT rare or is it just i have never heard of them??


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## cboutilier (Jan 12, 2016)

They were common tubes in those type of hot chassis circuit. 

Ideally to make that safe you'll want to wire up an isolation transformer


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## Lincoln (Jun 2, 2008)

I think you'll be safe with the three conductor cord, but I've never tested that theory. In a GFI plug it should be safe. The tubes you mentioned were common radio tubes at the time. I got a couple kicking around you can have if you need some.


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## traynor_garnet (Feb 22, 2006)

Nope, you need an isolation transformer.


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## dcole (Oct 8, 2008)

The only guarantee is an isolation transformer. Why? Cause this goes all the way back to the wall plug. Is the plug you are plugging into wired correctly? Hot has to be hot and neutral has to be neutral so that when it comes down the cord to your amp, the chassis is floating at neutral and not 120V referenced to ground. 120V to ground with you in series means electrical shock. Neutral to ground with you in series means no shock typically as neutral is tied to ground back at your breaker panel, hopefully. Lots of stuff out side of the amp that has to be right as well to minimize the risk of electrocution. An isolation transformer basically guarantees this by electrically separating the amp from ground so there is no path from the 120V to ground which you could possibly be a part of.

This type of circuit was typical of the old Bakelite farm radios. Everything was isolated from the end user electrically, so this was rather safe and cheap to produce. Converting this concept to a guitar amp was fatally flawed. Your guitar needs to be referenced to the circuit and it is by the shield wire of your guitar cable. All your strings and bridge are electrically connected to the shield wire of your guitar cable. So as you play the guitar, as you are touching the strings, you yourself become connected to the neutral reference in the amp. Now, if the wall plug you plug into is wired wrong, this neutral reference can be 120V instead, which is sent out to your hands. This is well and good as long as you touch nothing else electrically grounded, i.e. microphones or other amp chassis. As soon as you do, you complete the path from 120V to ground using your body as part of the path. How did they get CSA approval in the 60's to build these? By putting a resistor in series of the path to minimize the amount of current delivered when you get electrocuted. That is a shitty way of trying to protect a human from electrical shock.


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## sambonee (Dec 20, 2007)

These are skin fryers!!! I burnt my lip on a mic because of one of these. 

Stay clear man imo.


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## traynor_garnet (Feb 22, 2006)

They also typically sound rather "meh" so why bother. I wasted money having one safetied and modded by a tech, only to realize the tone isn't really there.


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## cboutilier (Jan 12, 2016)

They call them the Widowmakers


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## tomsy49 (Apr 2, 2015)

Haha all good info here!


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## dcole (Oct 8, 2008)

traynor_garnet said:


> They also typically sound rather "meh" so why bother. I wasted money having one safetied and modded by a tech, only to realize the tone isn't really there.


I had one with an unbelievably wicked sound. I put it in the corner of my bedroom and cranked it when no one was around. As I can repair my own things, I got it running up to snuff and left the isolation transformer off and touched nothing at all but my guitar when playing.


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## nonreverb (Sep 19, 2006)

Isolation transformer....period. It's a dangerous amp in the event of a short or inverted polarity. There's a reason CSA pulled the permit on these.


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## alwaysflat (Feb 14, 2016)

I was surprised by the 35L6 as I have a 201 chassis Pepco with a 50L6 which I think is more common. It is hard to see in your pictures, you may have a dropping power resistor there to eat up another 15 to 18 VAC from the heaters. It appears that you have one coming off pin7 of the rectifier heading to the 35L6 (can't tell the destination pin) . It would have cost pepco an extra power resistor to substitute the 35L6. 
I'm not sure the differences in other pepco 'widowmaker' chassis models, such as 801's I've seen online in perhaps other stencils. 
I thought mine was a good deal, so I sprang for the isolation transformer.


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## HattyDamp (Mar 28, 2017)

Hi...as per my knowledge this type of circuit was typical of the old Bakelite farm radios. Everything was isolated from the end user electrically, so this was rather safe and cheap to produce. Converting this concept to a guitar amp was fatally flawed. Your guitar needs to be referenced to the circuit and it is by the shield wire of your guitar cable. All your strings and bridge are electrically connected to the shield wire of your guitar cable. So as you play the guitar, as you are touching the strings, you yourself become connected to the neutral reference in the amp. Now, if the wall plug you plug into is wired wrong, this neutral reference can be 120V instead, which is sent out to your hands.

printed circuit assembly


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## Lincoln (Jun 2, 2008)

this is a really awesome product.......gives new meaning to the term "plug & play" 

Hammond 171A - Isolation Transformer, 115VAC-115VAC


and welcome HattyDamp!


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## copperhead (May 24, 2006)

The cool thing about the plug & play is you don't have to leave it in the amp if you sell it


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## jb welder (Sep 14, 2010)

That's a great non-invasive solution Lincoln!
Just to note it's for smaller amps that have fuses of 1amp or less. But most amps that need isolation transformers aren't any bigger than that anyway.


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## traynor_garnet (Feb 22, 2006)

Yeah, but when you factor in the cost of the isolation transformer your cheap amp is hardly cheap anymore. As I mentioned before, I think these amps are not worth it. By the time they are safe they are not cheap and still don't sound great. In the end, I would just go with a different used amp; there are a billion small tube combos for sale at any given time.

TG


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## loudtubeamps (Feb 2, 2012)




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## sammyr (May 7, 2013)

A quick way to notice these widowmaker circuits is to just add up the first two numbers of the tube complement. If the sum is close to 110v you can assume its a series filament amp with no transformer. In this case, 35+35+12=82. There is likely a ballast resist that eats up the remainder of the line voltage.


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## jb welder (Sep 14, 2010)

I suspect any amp with any tube that starts with anything but 5,6, or 12.


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