Widowmaker Amps & Isolation Transformers

[nosaj]

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.

Point 1 reducing total available amperage from 15-20amps(common outlet circuit breaker rating) to 2amps or under ? Less likely death scenario more likely survival rate still deadly.

Point 2 they both are dangerous

Point 3 2 prong power plug some used a polarized plug so you could only plug in one way. Having an outlet tester what audience are you shooting for? The technically inclined or the guitarist who will plug something in because it fits?

Overall it is just unsafe because other external factors can come into play.
Enzo's example is a great one.

nosaj

[robrob]

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

The death cap and return resistor would still tie the mains to the chassis. What are the benefits of removing ANY direct connection to the mains other than limiting max shock current?

It seems to me a power transformerless "widowmaker" amp with the addition of an isolation transformer and three prong power cord is still unsafe. The amp circuit needs to be modified by removing the death cap and return resistor but I've never seen that advice given, it's always, "add an isolation transformer".

[nosaj]

The death cap and return resistor would still tie the mains to the chassis. What are the benefits of removing ANY direct connection to the mains other than limiting max shock current?

It seems to me a power transformerless "widowmaker" amp with the addition of an isolation transformer and three prong power cord is still unsafe. The amp circuit needs to be modified by removing the death cap and return resistor but I've never seen that advice given, it's always, "add an isolation transformer".

The return resistor you mention isn't that just to bleed voltage off the cap when it is off?
Adding an isolation transformer making it a 3wire and removing the death cap are all I've ever done.
nosaj

[robrob]

The return resistor returns the V1 cathode current to the mains. That's why you can't simply remove the death cap and return resistor to isolate the chassis in a widowmaker.

[R.G.]

What are the benefits of removing ANY direct connection to the mains other than limiting max shock current?

It removes the "death trap" nature of failures.

Quite a lot of the world around you has places that are electrically connected to the planet. Concrete floors, for instance. They conduct a modest amount of leakage current well. A hot chassis setup that either doesn't shock you at all or gives the mildest tingle with shoes on will conduct ventricular-fibrillation levels of current if you're barefoot. Practically everything that follows modern electrical standards that has user-touchable metal parts has those parts firmly grounded to safety ground, which is intended to carry at least 25A of current back to the breaker panel, as it's intended to blow fuses if the AC line touches it. Most plumbing fixtures. All three-wire amplification equipment and hifi equipment with three wires. Computer cases. Microphones. The list goes on. Third wire safety grounding is intended to (a) make the surfaces positively safe to touch and (b) convert a subtle leakage situation into a blown fuse for bigger leakages.

The danger is that a single failure leaves the chassis sitting at a dangerous fraction of the AC power line voltage, and that anything that touches that and another grounded item conducts AC leakage current. And that single failure makes touching things that were just fine to touch suddenly - and silently - potentially (sorry - but it's the right word here ) lethal.

Limiting current isn't too much protection. It only takes on the order of 20ma of 50 or 60 Hz through a human chest to start ventricular fibrillation. Once that starts, if you don't have someone handy with a defibrillator and the smarts to use it, you don't recover.

[robrob]

An isolation transformer has to be used to allow a "normal" chassis grounding scheme. There's no way to ground the input jack safely in a power transformerless amplifier. The chassis and guitar cable must be referenced to ground to limit RFI and that can't be safely done using the mains neutral line.

This is the summary of my research:

All widowmaker amps should have their circuit modified to remove the RFI AC Ground Cap (death cap) and the Return Resistor which tie the mains to the chassis. An isolation transformer should be installed so the amp can be grounded to the chassis like a "normal" modern amp. A three prong power cord must be added to provide a chassis safety ground. The removal of the death cap, Return Resistor and the addition of an isolation transformer and three prong cord are the only way to make these widowmaker amps as safe as any other modern amplifier.

[R.G.]

And I'll take a swing at these.

Does adding an isolation transformer to a power transformerless amp really make it safer?

In my opinion, yes. It removes the "death trap" you can get by a single part failure.

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.

"Death Caps" are almost, but not quite as bad as hot-chassis amps. "Hot chassis" was the term the industry used for transformerless equipment back in the day. That was because repair personnel had to treat the chassis as thought it was always connected to the line side of the incoming AC. On that smaller-current thing, I had a thought while typing this up. Smaller current really isn't the issue. The deadly current is quite small, so there's no benefit there, as you're wrestling with.

The secondary of the PT is a hazardous voltage, OK. But it only exists (or should only exist) on two wires, the HV leads coming out of the secondary. These run to the rectifiers and not all over the world. The only way to touch these things is to open the chassis, power the unit on, and reach in to touch the ends of those two wires.

Granted, you can also touch the high voltage DC, and that's not pleasant, but it doesn't have the lasting-death issue of ventricular fibrillation. In general, high voltage DC contracts all the muscles so much that it "knocks you loose". AC mains frequencies can set your heart into a non-rhythm that it does not self-recover from.

In a transformer amp, the chassis is still a shock hazard with respect to the secondaries of the PT, but the possible exposures are much smaller and hard to get to. It's different from being "hot" with respect to the entire rest of the planet.

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

Yes. There are situations where touching a widowmaker amp's chassis ground and then touching something else on the planet can kill you. On the two-prong amps, it also takes the failure of the death cap, and the setting of the plug rotation and line reverse switch to do the same thing.

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.

Interesting question. I can see a situation where the chassis is tied to safety ground, the neutral is floated on a cap/resistor from chassis, and both the neutral and line sides of the incoming AC wire are fused. One might make a case that this, in combination with a GFI circuit in the amp to catch those 20ma-heart-stopping leaks, you might convince yourself that it was less-deadly (note the change of terms there).

The miswired outlet is a big problem, and although I'd never play in any venue where I had not tested the outlets, most guitar players are not so careful, and even if they have a tester, they'd skip testing. Shoot, most guitar players don't even have spare tubes. Relying on a guitar player to do something technical and non-music related to save themselves is a positively Darwinian approach to amp design but I'm guessing that there would not be a lot of compliance with the procedures. And we WANT those creative "don't bog me down with the tech stuff" folks to be making music. Some of them are good at it.

In any case, the situation gets more hypothetical every day. There were a limited, if possibly large, number of widowmaker hot chassis guitar amps made, and I've converted every one I personally had my hands on to an isolation transformer. So do many techs and owners. Eventually, we'll get them all.

[nosaj]

The return resistor returns the V1 cathode current to the mains. That's why you can't simply remove the death cap and return resistor to isolate the chassis in a widowmaker.

It may be a dumb question but when you remove the resistor what then returns the V1 cathode current to ground?

Thanks,
nosaj

I love how your site is evolving. Looking at your writing style in the order of books I think you might like the how and why of tube amps by Gar Gillies.

[SoulFetish]

The death cap and return resistor would still tie the mains to the chassis. What are the benefits of removing ANY direct connection to the mains other than limiting max shock current?

It seems to me a power transformerless "widowmaker" amp with the addition of an isolation transformer and three prong power cord is still unsafe. The amp circuit needs to be modified by removing the death cap and return resistor but I've never seen that advice given, it's always, "add an isolation transformer".

Rob, no one would upgrade a widowmaker amp and install a "death" cap, just like you wouldn't wash your ass with sewer water. No one is saying that fender's 2-prong amps are safe, because... the that lone capactor was a failure away from - no isolation. Not only that, but the US mains comes in as a 240V 2 phase system with a shared neutral line. If a point of failure happens, lifting the neutral open, then you now have 240V coming in two hot connections and you no longer get the current limiting you expect. Think about this: when the current in your body reaches 20mA, you muscles seize and physically cannot do anything to save your own life. 20mA! that is not a lot. That's nothing compared to what we are usually working around.

[Enzo]

The death cap and return resistor would still tie the mains to the chassis. What are the benefits of removing ANY direct connection to the mains other than limiting max shock current?

On an isolation transformer, the mains do not enter the amp, other than the power leads. The circuit cap and resistor still connect to the 120v circuit, but that is no longer part of the mains, it is a secondary voltage isolated from the mains. The 120v inside is referenced to the chassis via the cap and resistor, but the chassis is not referenced to the outside mains wiring, and would normally be grounded by the three wire plug we add at the time we add the iso. But in the absence of that third prong ground, if we link the secondary of the iso to the chassis, it still does not present a voltage to earth.

In other words, if I touch one side of the mains and then the other while standing on wet soil, I get a shock off the hot lead. If I connect an iso to the mains and then touch either end of the iso secondary, NEITHER side will shock me.

[robrob]

Rob, no one would upgrade a widowmaker amp and install a "death" cap

They don't need to be "upgraded", widowmakers already have a death cap.

My point is we should tell widowmaker amp owners that they need to alter the circuit to remove the death cap and return resistor, install an isolation transformer, ground the circuit in a "normal" manner and install a three prong power cord. The amps should also be re-wired so the power cord hot wire goes to the fuse first then to the power switch. Most, if not all, widowmaker amps have the power switch connected to the neutral wire which leaves the amp circuit hot even when turned off.

Telling widowmaker owners to just install an isolation transformer is wholly inadequate.

[robrob]

Enzo, by adding an isolation transformer without a three prong cord wouldn't we would lose the RFI shielding for the chassis and guitar cord since there would be no tie to earth ground?

nosaj, after installing an isolation transformer both the death cap and return resistor can be replaced by a jumper wire to tie the chassis to the amp ground.

[SoulFetish]

They don't need to be "upgraded", widowmakers already have a death cap.

"Update" was probably a better word, but I see your point.

Telling widowmaker owners to just install an isolation transformer is wholly inadequate.

Word.

[DrGonz78]

Well Robrob once you finish your web page on this matter I would add a link to this thread too. That way someone reading your site can also follow along with all the great advice given by the MEF members.

[Tom Phillips]

I sure have come to dislike the terms "Widowmaker" and "Death cap." This thread has sealed the deal