I got shocked from this old tube amp. What do I need to do to fix it?

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It's an Airline/Wards 62-9021A. I just got it today and love the tone of this. However when I went to adjust the mic I put in front of it ZZZZAAAPPP. I got shocked.

Attached is the schematic:

Link to PDF: http://www.silvertoneworld.net/schematics/silvertone_1471.pdf

It has 2 transformers and the original 2 pronged power cable that I am aware I should change. I am comfortable with a soldering iron. I tend to learn tube amp stuff every couple years and then completely forget it all.

So I am humbly posting here to see if any of you very generous and kind folks can start my on my journey to making this safer to use. I have a little one at home now and I'm so worried she's going to wander into my studio and get hurt.

Thank you so much!
 

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Start by removing C5 the so called "death cap" which is probably responsible for the shock you received. Next install a 3 wire power cord and ground the chassis.

If you then have a ground loop issue take a look on the Rane website for their ground loop app notes which will detail several solutions.

Thanks! Do I need to replace it or is it fine if it's just totally gone?

Side note: as a note decays I can hear what sounds like a tremelo effect when it's very quiet. Is that something I need to address?

Is there a guide you'd recommend that can explain how to do the 3 wire power cord?
 

6L6

Moderator
Paid Member
2010-10-22 6:43 pm
Denver, Colorado
Please read this - it’s specifically about transformerless amps, which are genuinely dangerous and need to be modified with a transformer and 3-wire AC line as well as removal of the death cap and such. This post will become a sticky in the near future, and covers a lot of what you need to do to your amplifier. It’s huge, but it will save your life.

Guys, I recently got a neat old amp at a garage sale and want to fix it up. It's a Silvertone 1448 and would love to get it working. All it does now is make a buzzing noise.

I have done a little research and keep coming across people talking about the "Death Cap" and calling these "widowmakers." There were sold in the USA at mainstream stores, they can't actually be that bad, can they?

What's the worst that could happen? Is there actually a chance for a shock of some sort?

If something needs to be changed, how could I actually use this amp?

I found the schematic here -

http://el34world.com/charts/Schematics/files/silvertone/silvertone_1448.pdf

Tubelab.com responds -

So, what is a "Death Cap" and can my vintage guitar of HiFi amp really kill me?

A quick Google search with band name "Stone the Crows" will tell you that their lead guitar player, Les Harvey was electrocuted on stage before a live audience in 1973 due to a malfunction in his guitar amp when he touched a grounded microphone. Searching for "guitar player electrocuted" will bring up a few more names, notably Keith Relf of the Yardbirds. These are just the players famous enough to get written up in a major publication. So, yes your vintage amp, HiFi, guitar or PA can KILL YOU DEAD!!!!

There are three ways that a vintage piece of electronic equipment can kill you, one or two of them are likely present in nearly ALL electronics made before 1970. Let’s explore all three, and discuss how to make these old electronic devices safe to use.

The power system in most countries is connected to an earth ground for protection against lightning and accidental short circuits between 120 volt house current and high voltage distribution lines, say when a car hits a power pole and wires touch each other. In North America the NEUTRAL wire is connected or “bonded” to the metal water pipes in most buildings, or other conductive material that is buried in the ground. There is also a ground wire on nearly every power pole, reinforcing this ground system. In many areas there is a connection from NEUTRAL to GROUND at the fuse or breaker panel. This means that touching the HOT wire from the electrical outlet while simultaneously touching ANY grounded object will deliver a potentially lethal electric shock, just as if the person were to touch both the HOT and NEUTRAL wires. Many metal objects in the home are intentionally grounded and some are grounded due to the way that they were built or installed. For example many plumbing fixtures are grounded through copper plumbing. What is not obvious are some sources of unintentional grounding. Concrete, and cement based flooring (Terrazzo was the source of many shocks, some serious, during my childhood) contains enough trapped moisture to conduct enough electricity to cause a lethal electric shock. SO, to repeat touching a live (connected to power line HOT, even if through some electronic parts) circuit while standing on concrete with bare feet, or leaning against a concrete or steel wall can cause a serious or possibly fatal electric shock. This can also be true of the ground outside, damp wood, or other surfaces. I doubt that too many people would intentionally grab onto a metal object and stick it into the wall outlet while otherwise grounding themselves, however this is EXACTLY the scenario that can occur in one of three ways with our vintage electronic equipment.

In the early days of radio (1920's) most of them ran on batteries, as many homes and farms did not have electricity, especially in rural areas. These were relatively safe. In the 1930's, radios became available that ran from house current. The house current was fed to a transformer (often called the power transformer) which converted the 110 volt house power to lower voltages which powered the tubes. The transformer provides “galvanic isolation” which effectively isolates the internal electronics from the power lines, such that the user could not be shocked by the radio as long as he did not touch the stuff INSIDE the radio. Technically the power lines, and therefore the electrical outlet that powers the radio has a HOT wire, and a NEUTRAL wire. The third GROUND wire started appearing in homes as a safety feature in the 1960's, but a three wire cord for electronic equipment was not mandated until 1969.

This power transformer is connected to the HOT and NEUTRAL wires of the house power system. Usually a switch and fuse will also be in the circuit. Contact with ONE part of this circuit, and simultaneously touching an intentional, or unintentional ground can be deadly. The power transformer magnetically couples energy to its secondary side WITHOUT electrical connection. There is no physical electrical connection from the house power circuit to the electronics powered by the transformer, thus the transformer provides isolation from the power source. The voltages in use inside this electronic equipment may be high enough to be lethal, and this is often the case with vacuum tube equipment, but there should be no means of causing shock to the user by contact with the outside, or any accessible part, or anything plugged into it, if the equipment is properly designed.
Modern power transformers use specialized wire with very good insulation, plastic bobbins made for high voltages and temperatures, and space age plastics or coatings for the internal insulation and external wiring. This was NOT the case for much of our vintage electronics. Until the late 1960’s most transformers used ordinary brown paper similar to what’s found in a grocery bag for insulation. The bobbin was made of cardboard. The external wiring was often rubber coated wire with a cloth over coat. Higher quality transformers used paper that was wax or varnish coated, and good transformers were dipped or pressure (vacuum) sealed.

Some of these transformers are now 60 to 80 years old. The paper has deteriorated and absorbed moisture. Constant heat and cooling cycles have weakened the insulation, cracked up the varnish, or melted the wax coating out of the transformer core. The old transformer may be perfectly good, and capable of many more years of use, or its isolation abilities may have already broken down creating a serious shock hazard. It can be tested, but if good, there is no guarantee that it will remain good. Even a brand new modern transformer can have a manufacturing defect allowing loss of isolation. We need a failsafe mechanism.

How can we protect ourselves from this scenario? This one is pretty easy, remove the two wire power cord, and install a three wire power cord. The new green wire will be connected to the metal chassis or ground circuit of the equipment. The other two wires go to the places where the original two wires on the old cord went. One of those original wires went to a fuse or a on – off switch. Connect the new black wire there. If the amp has a DEATH CAP, remove it.

How does this fix the problem? For now assume that the original power transformer went bad and created a connection from the primary side which is connected to house power, and the secondary side which is assumed to be isolated and safe. This is NOT an uncommon failure! Say you had a guitar, CD player, turntable or other device plugged into this amp. The failed insulation inside the transformer creates a path for current to flow from the house power through the bad insulation into the electronic circuitry, to which you just plugged into. There is a high likelihood that the HOT side of house power is connected directly to your guitar, turntable or CD player. Most guitars have their strings connected to the pickup ground to reduce hum, likewise so do turntables and CD players. Any exposed metal will effectively be connected directly to the power lines! Now if your other hand were to grip a grounded metal microphone.....Think about how your hand is positioned on the neck of the guitar and the base of that microphone. If a serious electric shock occurs, your muscles will involuntarily contract making it impossible to release your grip on the guitar or the mic, and house power is connected from one hand to the other, which is a VERY BAD THING for your heart. Adding the third wire circuit to the power cord creates a short path for any current leaking through the power transformer to directly to GROUND. There should be NO possibility for current to flow from the electronics through the user to a grounded object, since both the guitar and mic (or other objects) are now grounded. If the leak in the transformer is bad enough, the fuse will blow.

OK, so what is a DEATH CAP?

A guitar amp, or other electronic device is subject to hum pickup from all the surrounding electrical equipment. The amp will be more immune to hum and noise pickup if it is properly grounded. The third GROUND prong on the house power outlet did not exist in the 50’s and 60’s so Fender and others came up with an ingenious idea to allow the amp to be partially grounded. Remember that one of the two prongs on the power plug is the NEUTRAL wire, and it is connected to ground, while the other is HOT and has house power connected to it. A switch was wired to connect the chassis, and circuit ground including the input jack that you plug your guitar into, to either side of the power plug through an electrical component chosen to pass a small amount of electric current if the HOT side was selected, and provide a partial ground if the NEUTRAL side was selected. This electrical component was a small capacitor. In theory the idea is sound.....but what happens if the capacitor fails? You have created a DIRECT PATH FROM THE GUITAR TO THE WALL OUTLET! It took a few years for the world to figure this out, but yes, capacitors DO FAIL, and quite often.

A modern capacitor is made by rolling up a sandwich of metal foil and some sort of insulation material. It can also be made by depositing the metal layer directly on the insulating material. Today this insulating material is a space age plastic often polypropylene, Teflon, or polystyrene. It could also be a ceramic material. Years ago the insulator was paper, often soaked in wax or oil. Tube amps get hot, the wax melts and drips out, the oil dries up or leaks out, leaving only the paper. Paper absorbs moisture. This leaves a piece of wet paper between you and the live electrical outlet!
People were killed by these Death Caps, and this can still happen. This switch is usually labeled “ground” or “polarity” but could go by another name. If your amp has this switch, it has a Death Cap. Again the fix is nearly as easy. Remove the DEATH CAP and install a three wire power cord.
This simple diagram makes most of the text above easier to understand.
Guitar Amp Shocks
In both of the scenarios above the amplifier, or electronic device had a power transformer to convert the house power into levels suitable for the electronics inside, and isolate the power line source from the user connections.

There exists another breed of electronics that runs directly from the power lines. These have proven to be extremely dangerous and often deadly. They were very common in the 1950’s and 1960’s when I was learning about electronics the hard way. You could literally be killed by a cracked knob from one of these radios, as I nearly was. Unfortunately a lot of radios, TV sets, guitar amps and phonographs have the same technology, and ALL contain a DEATH CAP, or WORSE!
Many of us have heard the story of how LES Paul “invented” the electric guitar by wiring a telephone pickup into an old radio which was used as an amplifier. Many of the vintage guitar amps and phonographs of yesteryear started out as a radio with the receiving circuits removed. Radios of the 1920’s through most of the 40’s used a transformer to power the tubes. The design was relatively safe as long as you didn’t go poking around inside the chassis.

The power transformer was the most costly component in the radio, so the manufacturers figured out how to eliminate it. This brings about several safety concerns which were never properly addressed. It is possible to design a radio and later a TV set that has no power transformer. EVERYTHING inside the radio is DIRECTLY or indirectly connected to the power lines. The first line of safety was to seal the radio up such that the user can not easily get inside it. The power cord was often interlocked so that it was disconnected from the radio when the back cover was removed to change the tubes. Some radios even connected one side of the power line DIRECTLY to the metal chassis inside. This is often called a HOT CHASSIS radio or TV set, for obvious reasons. A broken knob allowed exposed metal which was often connected DIRECTLY to the power lines.
The radio manufacturers attempted to improve safety by using polarized power plugs so that the power cord could only be inserted into the wall outlet one way since one blade was wider than the other. This would insure that the NEUTRAL side of the power line would always be connected to the chassis. Again, in theory this should work. In practice most homes did not have polarized outlets installed, so users simply ignored the warning in the manual and cut the plug to fit their outlet. Even if they installed the proper polarized outlet, there is no guarantee that it was wired correctly. HOT and NEUTRAL could be swapped without the user even knowing since appliances would operate normally, until someone got zapped.

I worked in TV repair from in 1969 and 1970. At that time hot chassis TV’s were common. Most were still built in fancy wood cabinets with virtually no exposed metal. The cheap TV’s often used plastic cabinets with limited exposed metal. Sylvania had designed a mid grade TV that worked quite well with excellent reliability for the times. It however had a metal cabinet and a hot chassis, with insulation between the chassis and the cabinet. In the first few years things were OK, but after a particularly hot and rainy summer in Miami we started getting a lot of calls due to customers being shocked by their TV sets. It seems that dust and humidity alone could create a good enough path for current to flow to deliver a substantial electric to a user from simply touching the cabinet of his TV. I have learned that red ants, mice, and other dead critters can also make for a shocking experience.

So we have a product, the hot chassis radio, which has already proven to be a safety hazard. How can we make things any worse? Easy, let’s put an input jack on it so that someone can plug in a turntable or guitar. Now, this jack must be connected to the electronics in order to function, and this electronics is already directly connected to the power lines, so how do we keep users from getting fried? We go back to the DEATH CAP!
Let’s examine the schematic for the Silvertone (Sears) 1448 guitar amp which borrows its design from an old radio. Look at the schematic:Error 404 - Page not found
Follow the line which represents a wire from the bottom prong of the power plug on the right side of the schematic diagram. It goes to the switch, then goes along the bottom to the left to a pair of components, resistor R5, and capacitor C3 (AKA, the DEATH CAP). From there it connects directly to the metal chassis, which is connected directly to the input jack. The only thing separating your guitar from the wall outlet is a cheap capacitor made of paper, which is now 60+ years old. Would you trust your life to a 60 year old piece of paper? I wouldn’t, and neither should you....that’s why it’s called the DEATH CAP.

Note that this particular amp has a small transformer to isolate the heater circuit of the input tube. This mitigates another common failure and possible path for power line current to flow to the input jack. It is possible and not uncommon for a short circuit to develop between the heater in a tube and its cathode. In a typical transformerless design, such a short would put power line current directly on the input jack, and thus the guitar cable. My obvious question here is that if you were willing to put a small transformer in this amp to eliminate one possible shock hazard, why wouldn’t you put a larger transformer in the amp and fix ALL of them?

So, how to we fix a transformerless amp? There is only one way....put a transformer in it! Fortunately for small amps such as this a small transformer that costs under $20 and a 3 wire power cord are all that’s needed. There are transformers made for this purpose called isolation transformers. They come in various power sizes, and you must choose one that will work with the power line voltage in your area. The schematic here shows 117 volts / 40 watts right next to the power plug on the diagram. That means that we need a transformer capable of 40 watts or more that works on a US spec 120 volt power line voltage. Here the Triad N-68X will work and it’s $15.81 at Mouser. A larger amp will require a larger transformer.

Once an isolation transformer has been installed in this Silvertone, the buzzing sound can be fixed by replacing the three section cap, C5 it has surely dried out inside, and many of the other caps are probably dead or dying.



More information on (non-grounded, hot chassis, DANGEROUS) Widowmaker amps here - Widowmakers
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
Attached shows the minimum change to reduce potentially lethal shock.

Since we now have a 60:40 chance of knowing Hot from Neutral, Kevin's plan to put switch and fuse "properly" is a wise choice, though not essential for playing. (It reduces fuse-change hazard, but IMHO you should *always* pull the plug before playing with the fuse.)
 

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Thanks. May I suggest a TL;DR or a table of contents at the top? It's a bit of a wall of text and I think beginners might ignore this very important info.

Please read this - it’s specifically about transformerless amps, which are genuinely dangerous and need to be modified with a transformer and 3-wire AC line as well as removal of the death cap and such. This post will become a sticky in the near future, and covers a lot of what you need to do to your amplifier. It’s huge, but it will save your life.



Tubelab.com responds -

So, what is a "Death Cap" and can my vintage guitar of HiFi amp really kill me?

A quick Google search with band name "Stone the Crows" will tell you that their lead guitar player, Les Harvey was electrocuted on stage before a live audience in 1973 due to a malfunction in his guitar amp when he touched a grounded microphone. Searching for "guitar player electrocuted" will bring up a few more names, notably Keith Relf of the Yardbirds. These are just the players famous enough to get written up in a major publication. So, yes your vintage amp, HiFi, guitar or PA can KILL YOU DEAD!!!!

There are three ways that a vintage piece of electronic equipment can kill you, one or two of them are likely present in nearly ALL electronics made before 1970. Let’s explore all three, and discuss how to make these old electronic devices safe to use.

The power system in most countries is connected to an earth ground for protection against lightning and accidental short circuits between 120 volt house current and high voltage distribution lines, say when a car hits a power pole and wires touch each other. In North America the NEUTRAL wire is connected or “bonded” to the metal water pipes in most buildings, or other conductive material that is buried in the ground. There is also a ground wire on nearly every power pole, reinforcing this ground system. In many areas there is a connection from NEUTRAL to GROUND at the fuse or breaker panel. This means that touching the HOT wire from the electrical outlet while simultaneously touching ANY grounded object will deliver a potentially lethal electric shock, just as if the person were to touch both the HOT and NEUTRAL wires. Many metal objects in the home are intentionally grounded and some are grounded due to the way that they were built or installed. For example many plumbing fixtures are grounded through copper plumbing. What is not obvious are some sources of unintentional grounding. Concrete, and cement based flooring (Terrazzo was the source of many shocks, some serious, during my childhood) contains enough trapped moisture to conduct enough electricity to cause a lethal electric shock. SO, to repeat touching a live (connected to power line HOT, even if through some electronic parts) circuit while standing on concrete with bare feet, or leaning against a concrete or steel wall can cause a serious or possibly fatal electric shock. This can also be true of the ground outside, damp wood, or other surfaces. I doubt that too many people would intentionally grab onto a metal object and stick it into the wall outlet while otherwise grounding themselves, however this is EXACTLY the scenario that can occur in one of three ways with our vintage electronic equipment.

In the early days of radio (1920's) most of them ran on batteries, as many homes and farms did not have electricity, especially in rural areas. These were relatively safe. In the 1930's, radios became available that ran from house current. The house current was fed to a transformer (often called the power transformer) which converted the 110 volt house power to lower voltages which powered the tubes. The transformer provides “galvanic isolation” which effectively isolates the internal electronics from the power lines, such that the user could not be shocked by the radio as long as he did not touch the stuff INSIDE the radio. Technically the power lines, and therefore the electrical outlet that powers the radio has a HOT wire, and a NEUTRAL wire. The third GROUND wire started appearing in homes as a safety feature in the 1960's, but a three wire cord for electronic equipment was not mandated until 1969.

This power transformer is connected to the HOT and NEUTRAL wires of the house power system. Usually a switch and fuse will also be in the circuit. Contact with ONE part of this circuit, and simultaneously touching an intentional, or unintentional ground can be deadly. The power transformer magnetically couples energy to its secondary side WITHOUT electrical connection. There is no physical electrical connection from the house power circuit to the electronics powered by the transformer, thus the transformer provides isolation from the power source. The voltages in use inside this electronic equipment may be high enough to be lethal, and this is often the case with vacuum tube equipment, but there should be no means of causing shock to the user by contact with the outside, or any accessible part, or anything plugged into it, if the equipment is properly designed.
Modern power transformers use specialized wire with very good insulation, plastic bobbins made for high voltages and temperatures, and space age plastics or coatings for the internal insulation and external wiring. This was NOT the case for much of our vintage electronics. Until the late 1960’s most transformers used ordinary brown paper similar to what’s found in a grocery bag for insulation. The bobbin was made of cardboard. The external wiring was often rubber coated wire with a cloth over coat. Higher quality transformers used paper that was wax or varnish coated, and good transformers were dipped or pressure (vacuum) sealed.

Some of these transformers are now 60 to 80 years old. The paper has deteriorated and absorbed moisture. Constant heat and cooling cycles have weakened the insulation, cracked up the varnish, or melted the wax coating out of the transformer core. The old transformer may be perfectly good, and capable of many more years of use, or its isolation abilities may have already broken down creating a serious shock hazard. It can be tested, but if good, there is no guarantee that it will remain good. Even a brand new modern transformer can have a manufacturing defect allowing loss of isolation. We need a failsafe mechanism.

How can we protect ourselves from this scenario? This one is pretty easy, remove the two wire power cord, and install a three wire power cord. The new green wire will be connected to the metal chassis or ground circuit of the equipment. The other two wires go to the places where the original two wires on the old cord went. One of those original wires went to a fuse or a on – off switch. Connect the new black wire there. If the amp has a DEATH CAP, remove it.

How does this fix the problem? For now assume that the original power transformer went bad and created a connection from the primary side which is connected to house power, and the secondary side which is assumed to be isolated and safe. This is NOT an uncommon failure! Say you had a guitar, CD player, turntable or other device plugged into this amp. The failed insulation inside the transformer creates a path for current to flow from the house power through the bad insulation into the electronic circuitry, to which you just plugged into. There is a high likelihood that the HOT side of house power is connected directly to your guitar, turntable or CD player. Most guitars have their strings connected to the pickup ground to reduce hum, likewise so do turntables and CD players. Any exposed metal will effectively be connected directly to the power lines! Now if your other hand were to grip a grounded metal microphone.....Think about how your hand is positioned on the neck of the guitar and the base of that microphone. If a serious electric shock occurs, your muscles will involuntarily contract making it impossible to release your grip on the guitar or the mic, and house power is connected from one hand to the other, which is a VERY BAD THING for your heart. Adding the third wire circuit to the power cord creates a short path for any current leaking through the power transformer to directly to GROUND. There should be NO possibility for current to flow from the electronics through the user to a grounded object, since both the guitar and mic (or other objects) are now grounded. If the leak in the transformer is bad enough, the fuse will blow.

OK, so what is a DEATH CAP?

A guitar amp, or other electronic device is subject to hum pickup from all the surrounding electrical equipment. The amp will be more immune to hum and noise pickup if it is properly grounded. The third GROUND prong on the house power outlet did not exist in the 50’s and 60’s so Fender and others came up with an ingenious idea to allow the amp to be partially grounded. Remember that one of the two prongs on the power plug is the NEUTRAL wire, and it is connected to ground, while the other is HOT and has house power connected to it. A switch was wired to connect the chassis, and circuit ground including the input jack that you plug your guitar into, to either side of the power plug through an electrical component chosen to pass a small amount of electric current if the HOT side was selected, and provide a partial ground if the NEUTRAL side was selected. This electrical component was a small capacitor. In theory the idea is sound.....but what happens if the capacitor fails? You have created a DIRECT PATH FROM THE GUITAR TO THE WALL OUTLET! It took a few years for the world to figure this out, but yes, capacitors DO FAIL, and quite often.

A modern capacitor is made by rolling up a sandwich of metal foil and some sort of insulation material. It can also be made by depositing the metal layer directly on the insulating material. Today this insulating material is a space age plastic often polypropylene, Teflon, or polystyrene. It could also be a ceramic material. Years ago the insulator was paper, often soaked in wax or oil. Tube amps get hot, the wax melts and drips out, the oil dries up or leaks out, leaving only the paper. Paper absorbs moisture. This leaves a piece of wet paper between you and the live electrical outlet!
People were killed by these Death Caps, and this can still happen. This switch is usually labeled “ground” or “polarity” but could go by another name. If your amp has this switch, it has a Death Cap. Again the fix is nearly as easy. Remove the DEATH CAP and install a three wire power cord.
This simple diagram makes most of the text above easier to understand.
Guitar Amp Shocks
In both of the scenarios above the amplifier, or electronic device had a power transformer to convert the house power into levels suitable for the electronics inside, and isolate the power line source from the user connections.

There exists another breed of electronics that runs directly from the power lines. These have proven to be extremely dangerous and often deadly. They were very common in the 1950’s and 1960’s when I was learning about electronics the hard way. You could literally be killed by a cracked knob from one of these radios, as I nearly was. Unfortunately a lot of radios, TV sets, guitar amps and phonographs have the same technology, and ALL contain a DEATH CAP, or WORSE!
Many of us have heard the story of how LES Paul “invented” the electric guitar by wiring a telephone pickup into an old radio which was used as an amplifier. Many of the vintage guitar amps and phonographs of yesteryear started out as a radio with the receiving circuits removed. Radios of the 1920’s through most of the 40’s used a transformer to power the tubes. The design was relatively safe as long as you didn’t go poking around inside the chassis.

The power transformer was the most costly component in the radio, so the manufacturers figured out how to eliminate it. This brings about several safety concerns which were never properly addressed. It is possible to design a radio and later a TV set that has no power transformer. EVERYTHING inside the radio is DIRECTLY or indirectly connected to the power lines. The first line of safety was to seal the radio up such that the user can not easily get inside it. The power cord was often interlocked so that it was disconnected from the radio when the back cover was removed to change the tubes. Some radios even connected one side of the power line DIRECTLY to the metal chassis inside. This is often called a HOT CHASSIS radio or TV set, for obvious reasons. A broken knob allowed exposed metal which was often connected DIRECTLY to the power lines.
The radio manufacturers attempted to improve safety by using polarized power plugs so that the power cord could only be inserted into the wall outlet one way since one blade was wider than the other. This would insure that the NEUTRAL side of the power line would always be connected to the chassis. Again, in theory this should work. In practice most homes did not have polarized outlets installed, so users simply ignored the warning in the manual and cut the plug to fit their outlet. Even if they installed the proper polarized outlet, there is no guarantee that it was wired correctly. HOT and NEUTRAL could be swapped without the user even knowing since appliances would operate normally, until someone got zapped.

I worked in TV repair from in 1969 and 1970. At that time hot chassis TV’s were common. Most were still built in fancy wood cabinets with virtually no exposed metal. The cheap TV’s often used plastic cabinets with limited exposed metal. Sylvania had designed a mid grade TV that worked quite well with excellent reliability for the times. It however had a metal cabinet and a hot chassis, with insulation between the chassis and the cabinet. In the first few years things were OK, but after a particularly hot and rainy summer in Miami we started getting a lot of calls due to customers being shocked by their TV sets. It seems that dust and humidity alone could create a good enough path for current to flow to deliver a substantial electric to a user from simply touching the cabinet of his TV. I have learned that red ants, mice, and other dead critters can also make for a shocking experience.

So we have a product, the hot chassis radio, which has already proven to be a safety hazard. How can we make things any worse? Easy, let’s put an input jack on it so that someone can plug in a turntable or guitar. Now, this jack must be connected to the electronics in order to function, and this electronics is already directly connected to the power lines, so how do we keep users from getting fried? We go back to the DEATH CAP!
Let’s examine the schematic for the Silvertone (Sears) 1448 guitar amp which borrows its design from an old radio. Look at the schematic:Error 404 - Page not found
Follow the line which represents a wire from the bottom prong of the power plug on the right side of the schematic diagram. It goes to the switch, then goes along the bottom to the left to a pair of components, resistor R5, and capacitor C3 (AKA, the DEATH CAP). From there it connects directly to the metal chassis, which is connected directly to the input jack. The only thing separating your guitar from the wall outlet is a cheap capacitor made of paper, which is now 60+ years old. Would you trust your life to a 60 year old piece of paper? I wouldn’t, and neither should you....that’s why it’s called the DEATH CAP.

Note that this particular amp has a small transformer to isolate the heater circuit of the input tube. This mitigates another common failure and possible path for power line current to flow to the input jack. It is possible and not uncommon for a short circuit to develop between the heater in a tube and its cathode. In a typical transformerless design, such a short would put power line current directly on the input jack, and thus the guitar cable. My obvious question here is that if you were willing to put a small transformer in this amp to eliminate one possible shock hazard, why wouldn’t you put a larger transformer in the amp and fix ALL of them?

So, how to we fix a transformerless amp? There is only one way....put a transformer in it! Fortunately for small amps such as this a small transformer that costs under $20 and a 3 wire power cord are all that’s needed. There are transformers made for this purpose called isolation transformers. They come in various power sizes, and you must choose one that will work with the power line voltage in your area. The schematic here shows 117 volts / 40 watts right next to the power plug on the diagram. That means that we need a transformer capable of 40 watts or more that works on a US spec 120 volt power line voltage. Here the Triad N-68X will work and it’s $15.81 at Mouser. A larger amp will require a larger transformer.

Once an isolation transformer has been installed in this Silvertone, the buzzing sound can be fixed by replacing the three section cap, C5 it has surely dried out inside, and many of the other caps are probably dead or dying.



More information on (non-grounded, hot chassis, DANGEROUS) Widowmaker amps here - Widowmakers
 
Attached shows the minimum change to reduce potentially lethal shock.

Since we now have a 60:40 chance of knowing Hot from Neutral, Kevin's plan to put switch and fuse "properly" is a wise choice, though not essential for playing. (It reduces fuse-change hazard, but IMHO you should *always* pull the plug before playing with the fuse.)

Thanks. Is it important to know which of the wires on the transformer are what? I couldn't figure out how to tell them apart.

Also, I wired the switch in series with the fuse like kevin suggested. BUt your diagram is different.
 

6L6

Moderator
Paid Member
2010-10-22 6:43 pm
Denver, Colorado
Thanks. May I suggest a TL;DR or a table of contents at the top? It's a bit of a wall of text and I think beginners might ignore this very important info.

Not altogether a bad idea, but I did preamble - I added the bold now. :)

6L6/ said:
Please read this - it’s specifically about transformerless amps, which are genuinely dangerous and need to be modified with a transformer and 3-wire AC line as well as removal of the death cap and such. This post will become a sticky in the near future, and covers a lot of what you need to do to your amplifier. It’s huge, but it will save your life.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> Is it important to know which of the wires on the transformer are what?

No. Same as an old-style lamp-plug with two equal blades: it works either way round. US-only market power transformers are usually two black wires.

Kevin's plan for fuse and switch *may* reduce the hazard of fuse-changing *if* the H and N leads are for-sure right. In my last kitchen, 60% were wrong. His fine plan with my miswired kitchen would often leave the part-open fuse-hold HOT. My policy is to UN-PLUG before messing with a fuse-cap. This is indeed what we are supposed to do with "plug connected appliances", which do not have to follow special rules for permanently wired loads.
 
> Is it important to know which of the wires on the transformer are what?

No. Same as an old-style lamp-plug with two equal blades: it works either way round. US-only market power transformers are usually two black wires.

Kevin's plan for fuse and switch *may* reduce the hazard of fuse-changing *if* the H and N leads are for-sure right. In my last kitchen, 60% were wrong. His fine plan with my miswired kitchen would often leave the part-open fuse-hold HOT. My policy is to UN-PLUG before messing with a fuse-cap. This is indeed what we are supposed to do with "plug connected appliances", which do not have to follow special rules for permanently wired loads.

Thanks so much! I've got everything wired up and it sound great! No shocks and hopefully no dead kid because lord knows my wife would REALLY complain when she sees a new guitar or amp if that happened.
 

thoglette

Member
2008-12-20 5:59 am
Think of it as like selling a car with a leaking fuel hose. Not acceptable.
Think of it as selling a car with single circuit brakes, all round drums (which need manual adjusting), no ABS and no airbags: That is, stock and legal c. 1969.

"The past is a foreign country, they do things differently there"
- L. P. Hartley’s "The Go-Between"

Now, I do agree (as someone who's had two engine bay fires from leaky fuel hoses :eek:) that such amps should be made safe whenever they pass by a technician. It's not clear to me how pocket_change came to be in possession of this amp. May be it was a hard-rubbish item or yard sale find?

@pocket_change. Do spend some time reading through the rest of Rob Robinette's Tube Amp Stuff
 
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