Widowmaker Amps & Isolation Transformers

[Enzo]

And when the cap shorts?

The whole concept of "death cap" was about when they short. The hot chassis era was before the advent of safety caps.

[SoulFetish]

That 68k resistor will pass less than 2 milliamps at 125 volts so it has less impact than the death cap's 2.4 milliamps at 60Hz and 125v.

until either of those have significantly drifted in value, in which case they may present a potetial grave danger. Or until they fail short, in which case you probably go tits up.

[robrob]

Guys, I understand the danger of a shorting cap in both "normal" amps with two prong cords and in the Silvertone above but what is it about "widowmaker" amps without power transformers that warrants that title?

[nosaj]

Guys, I understand the danger of a shorting cap in both "normal" amps with two prong cords and in the Silvertone above but what is it about "widowmaker" amps without power transformers that warrants that title?

Because even with the deathcap functioning correctly. The two prong cord incorrectly plugged in can be a shock hazard that can kill you. Often times for me it's just been a tingle that warns me. I have never been seriously shocked but I am cautious.
on another not just read an article about Electric shock drowning near piers and marinas. AC paralyzes the muscles which presents as just a normal drowning guessing it's becoming more common here in the South.

nosaj

[R.G.]

R.G., I wired my barn for 240V 200 amp service so I understand hot, neutral and safety ground wiring and how neutral and ground are tied together at the service entrance.

I intended no slight. I over-explained to you (man-splained, my female relatives would say) because I have no idea how much experience or background posters have on these forums. The level of tech content will now go up and the level of background content will go down for replies to you.

Guys, I understand the danger of a shorting cap in both "normal" amps with two prong cords and in the Silvertone above but what is it about "widowmaker" amps without power transformers that warrants that title?

It's the fact that widowmaker amps were made in a way that allowed a single failure to make them deadly. In some cases, one of the single failures could be a user action, as in touching the wrong screw holding the covers on. I don't know that any guitar amps had the screwheads-live issue that the radios did, but the problem in radios indicates that it was viewed as OK at that time.

Modern safety practices grow like coral reefs. They accumulate new rules not out of theory, but out of new Darwin-Awards candidates finding new and interesting ways to die electrically. The standards folks examine the cases and see if a new standard would have prevented the deaths, and if it would have, may add it to the standards.

It's worth noting that the "widowmaker" practices in wiring up electrical stuff still kind of exist. They're just cased up in the standards for double insulation. You can still as a designer meet the international electrical standards by using a two-wire, non-polarized cord and a hot chassis and get safety certification. You just have to show that you have provided two layers of insulation, either one of which would prevent making user accessible surfaces from becoming live if the other layer failed completely.

[robrob]

No disrespect taken R.G., I'm a fan of your work.

It's the fact that widowmaker amps were made in a way that allowed a single failure to make them deadly. In some cases, one of the single failures could be a user action, as in touching the wrong screw holding the covers on. I don't know that any guitar amps had the screwheads-live issue that the radios did, but the problem in radios indicates that it was viewed as OK at that time.

I haven't been able to find any schematics of widowmaker guitar amps with the neutral wire wired directly to the chassis like the widowmaker radio I posted above. If a guitar amp was wired that way the guitar strings would be hot with mains voltage when plugged in backwards.

Every power transformerless guitar amp schematic I've found has a death cap between the chassis and neutral just like all the Fender amps of yore. Are "widowmakers" any more dangerous than an amp with a death cap?

The bottom line is I don't understand the benefit an isolation transformer would add to a "widowmaker" with a death cap between neutral and chassis. It seems to me a three prong power cord with a chassis safety ground would make the amp safe to play.

This Kay 703 widowmaker has the same chassis isolation resistor & cap (R7 & C5) as the Silvertone above:

[robrob]

Lectrolab #10 shows the cap and resistor to chassis:

[nosaj]

No disrespect taken R.G., I'm a fan of your work.



I haven't been able to find any schematics of widowmaker guitar amps with the neutral wire wired directly to the chassis like the widowmaker radio I posted above. If a guitar amp was wired that way the guitar strings would be hot with mains voltage when plugged in backwards.

Every power transformerless guitar amp schematic I've found has a death cap between the chassis and neutral just like all the Fender amps of yore. Are "widowmakers" any more dangerous than an amp with a death cap?

The bottom line is I don't understand the benefit an isolation transformer would add to a "widowmaker" with a death cap between neutral and chassis. It seems to me a three prong power cord with a chassis safety ground would make the amp safe to play.

This Kay 703 widowmaker has the same chassis isolation resistor & cap as the Silvertone above:

[ATTACH=CONFIG]43998[/ATTACH]

Do you have a schematic for the Danelectro( or was it silvertone) Guitar amp that was in the guitar case? I remember having one that behaved that way. And wasn't part of the issue something to do with playing the guitar and using a metal microphone and getting shocked on the kisser?

nosaj

[robrob]

C3 & R6

[robrob]

Any amp with a death cap with the plug in backwards will shock you like that but the current is limited to about 2.4 milliamps--that's what the death cap will pass with 125 volts at 60Hz.

[robrob]

Cap and resistor to chassis ground in center of schematic:

[Enzo]

Every power transformerless guitar amp schematic I've found has a death cap between the chassis and neutral just like all the Fender amps of yore

You might want to lose the term neutral when discussing hot chassis amps. There is no guarantee which prong of the plug is going to be neutral each time you plug it in. Just because it is drawn that way on a schematic doesn;t mean it will be plugged in that way. Better to say all these amps have a cap from chassis to one side of the mains. Could be neutral, could be hot.

Yes, old Fender amps with two-wire cords had a cap to chassis from one side of the mains. That was why the switch was there, so we could get the cap to neutral.

Back in the 1960s, the first thing we did after setting up a stage was turn everything on and get all the ground switches to agree. You lightly touched each amp to see who gave you shocks. It wasn't always enough to just get the switches right. Turning the plug over and flipping the switch were not the same, sometimes you had to do both. Usually the less shocking setting was also the least hummy. But shocks were more important than hum to eliminate.

"Widowmaker" is not some scientific term, it has no precise parameters, it is just a colorful term for a dangerous wiring scheme. Yes, the old Fenders were also dangerous, but knowing that doesn't make widowmaker chassis any less dangerous. I recall one gig where my guitarist was complaining of shocks from his microphone. He tried not to touch the mic, but during one solo, he raised the guitar and the high E string touched the mic, and melted in two with a flsh of light. SHocks indeed.

[nosaj]

Cap and resistor to chassis ground in center of schematic:

Power transformers were not just about isolating you from mains but several other factors also I would imagine. Electrical line noise, not having various voltages for tubes. The fact that the country was moving completely to AC. Some of the transformerless designs allowed equipment to be power by DC which was more common in rural and farm areas. I think there were attachments for tractors that would run a generator to charge up 6volt battery common to the radios of that time.

nosaj

[robrob]

I'm really trying to nail down three points.

Does adding an isolation transformer to a power transformerless amp really make it safer? The only benefit I see is the transformer will reduce the maximum amperage available for a shock. The death cap would still tie the mains to the chassis.

Are power transformerless "widowmaker" amps any more dangerous than old two prong Fenders with the death cap still installed?

Will adding a three prong power cord to a power transformerless amp make it safe to play? There is a risk of plugging into a miswired outlet without it's safety ground connected + a death cap short + plugged in backwards. A $7 outlet tester would remove this scenario.

[Enzo]

A tester will tell you if the outlet is miswired, but the vast majority of people don't carry them and we can't rely on them having one. Plus you have to use it EVERY time you plug this in. So no, just a three wire cord does not make these safe.

Is one type more dangerous than the other? That is a distinction I don't bother to make. They are both dangerous. But in the case of Fender, I CAN simply replace the power cord and snip the cap and have safety. There is no such simple solution for the widowmaker. And the hard wired aspect of the widowmaker makes it seem more benign than it is. The average amp owner/user is not looking at schematics and counting milliamps.

Yes, an iso makes the widowmaker safer, it removes ANY direct connection to the mains.