Widowmaker Amps & Isolation Transformers

[robrob]

The term "widowmaker" does seem to be a strong term for power transformerless amps wired like the 1430.

When we use the term "death cap" everyone knows what you're talking about. Call it an "RFI AC Ground Cap" and people say, "Huh?"

[Enzo]

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?

Not in my view. Hopefully the thing is in a metal box - chassis with cover, or largely covered by metal at least. Think about your guitar, the control chamber is often, if not usually, shielded, and grounded to the sleeve of the jack. All well and good plugged into a grounded amp. But in my experience, if I use a wireless belt pack, my guitar remains shielded even lacking a ground connection.

To me it is moot, because I'd not install an iso without also updating to a grounded cord.

[nickb]

The transformer might be there to protect the user in the case the 12AU7** heater goes open. If that were to happen one side of the heater would be live leaving only the heater-cathode insulation to protect from electrocution. For a 12AU7 the breakdown voltage is between 100-200V and the peak line voltage is 165V not really any margin at all.

Ironically, if the "death" cap were to be shorted, it would protect the user in this one case.

**12AU6 not 12AU7 - hk specs are the same.

[Enzo]

Hopefully that applies to 12AU6 as well, since that is what is in the amp.

[nosaj]

The term "widowmaker" does seem to be a strong term for power transformerless amps wired like the 1430.

When we use the term "death cap" everyone knows what you're talking about. Call it an "RFI AC Ground Cap" and people say, "Huh?"

Here in Fl, a widowmaker is also a rather large tree branch that has been broken in a storm and could fall at any time. Around here a widowmaker is understood to be some not to be messed with. A black widow is also a widowmaker.

May not have the word Death in it, but it is understood to bring about Death.

nosaj

[Justin Thomas]

"Widowmaker" is not some scientific term, it has no precise parameters, it is just a colorful term for a dangerous wiring scheme..."

This. What Enzo said.

That boutique amp from a few years back that someone put up here - that used an exposed PT with the HT secondary terminals right next to the output tubes - was also a widow-maker. Perhaps an 807-powered amp without a protective safety cage to guard the plate cap from wayward fingers could also be one...

As far as calculating how much current a "death cap" might pass, I don't care! 120VAC or 400+VDC sucks any way I look at it. After ten seconds of research, I found that 1-5mA is the level of perception. Too damn much for me. And I did once get my fingers clamped on my strings from an amp with a bad death cap and a non-polarized plug. I was also playing barefoot on a concrete patio on a hot and humid day. Before I knew about any of this.

And, why add an iso to the amp itself? Just make a stand-alone that you can take anywhere and plug anything into.

Justin

[The Dude]

The term "widowmaker" does seem to be a strong term for power transformerless amps wired like the 1430......

Not if you're this guy.

Keith Relf ? Electrocuted While Playing Guitar

[Enzo]

And, why add an iso to the amp itself? Just make a stand-alone that you can take anywhere and plug anything into.

Fine if you want to build something, but sometimes you want to operate the original amp, but safely.

[robrob]

The transformer might be there to protect the user in the case the 12AU7** heater goes open.

I believe you are right nick. Every schematic I found has the heater transformer for V1 except the Magnatone 107 (schematic above) and ironically it has a big Underwriter Labs "UL" stamp of approval on it. UL standards evolved so maybe the Magna is older than the other amps shown above.

[robrob]

Was Relf killed by a widowmaker?

[frus]

A friend of mine had an original US silverface fender champ. He also had a separate transformer that transforms 220V to 110V so he could use it here in Europe. He asked me if I could take a look at it, because he can feel "some mild electric shocks" when he played the guitar.

I noticed that someone has removed the third "prong" from the original (US) plug on the amp. The additional transformer of course had the ground prong on its socket (and the usual European grounding plug). I plugged it in and measured the voltage between the chassis and the ground connection on the wall socket. It was 220V (DC? I think). I installed a new US 3-prong cord (effectively connecting the chassis to earth ground), and using the same 220 to 110V transformer, got the chassis at ground potential, and the amp was working normally (as it did also with live chassis).

Regardless of the reason of removing the grounding connection on the plug, I would like to know why the chassis would be at such high potential, if the first transformer acts as an isolation? (does it?)

[The Dude]

Was Relf killed by a widowmaker?

So the story goes. I've never seen a story that said exactly what kind of amp it was. It would be interesting to know.
FWIW: He's not the only one. A Google search will land you several musicians that have been killed or electrocuted by amps.

[nickb]

A friend of mine had an original US silverface fender champ. He also had a separate transformer that transforms 220V to 110V so he could use it here in Europe. He asked me if I could take a look at it, because he can feel "some mild electric shocks" when he played the guitar.

I noticed that someone has removed the third "prong" from the original (US) plug on the amp. The additional transformer of course had the ground prong on its socket (and the usual European grounding plug). I plugged it in and measured the voltage between the chassis and the ground connection on the wall socket. It was 220V (DC? I think). I installed a new US 3-prong cord (effectively connecting the chassis to earth ground), and using the same 220 to 110V transformer, got the chassis at ground potential, and the amp was working normally (as it did also with live chassis).

Regardless of the reason of removing the grounding connection on the plug, I would like to know why the chassis would be at such high potential, if the first transformer acts as an isolation? (does it?)

It was more likely AC you saw due to capacitance between the windings.

It's the current than kills. The capacitance was a high impedance that limited the current. In such a situation again, connect a 100k resistor from chassis to earth and measure the voltage across that. Hooking the chassis to earth ensures that this small current goes safely to earth and more importantly, should other insulation fail, that large currents flow to earth, blow the breaker / RCD and prevent shock. That is why the earth impedance needs to be low.

[loudthud]

What hasn't been mentioned (nickb posted while I was writing this) in this thread is that in most transformers there is parasitic capacitance between primary and secondary(s) and/or between the primary and the core. So when you energize the primary, before you hookup anything else, you might measure an AC Voltage between the core and ground or between a secondary and ground. The capacitance varies but 100pF might be a typical value. Current through this capacitance is called leakage current. This is why you might measure a large Voltage (or feel a tingle) between an ungrounded chassis and earth ground. In the case of the old Fender amps with a ground switch, the Death Cap is intended to connect the chassis to the neutral side of the line and thus conduct the leakage current to ground and minimize the Voltage between the chassis and earth ground.

Aside: The parasitic capacitance between the HV secondary and the heater winding is why you need a grounded or virtual center tap. Without a center tap there is a very big AC Voltage between the heater winding and ground.

Hospitals were the first place the need for grounded outlets was seen. Without a ground, any connection to a patient could electrocute the patient, especially when two or more machines were connected.

[olddawg]

I haven't been a bench tech in over a decade, but (I think) I've mentioned this before. For many years on the bench the State of CA required me to perform an AC leakage test on every piece I worked on with an AC plug on it. It had to be less than 25ua and I had to sign and date a sticker on each unit. I always thought it was a bit of a scam since the shop charged a $2 "fee" to do it. I don't think it was really enforced much. But with a shop doing $2m+ in repairs a year, those fees added up to serious money. Sencore had a bench variac with the probe and meter built in. We had one on every bench.