Basic Safety

All:

The most recent thread (threat) with a mechanical hum within an SP14 has me writing this, and not for the first time, and not just from me. But like "Wet Paint" signs invite touching, this subject bears repeating:

1.  Lethal voltages exist within most tube equipment, absolutely including most everything discussed here. B+ voltages from 300 - 500 VDC are no fun at all.
2. Age, errors in assembly, mechanical damage or other unusual or untoward conditions may put that voltage anywhere, at any time, on the chassis.

If anyone is even the slightest bit serious about working on, restoring, testing, assembling or otherwise messing about with tube equipment in its naked state, that person MUST invest in an ISOLATION TRANSFORMER. This device will reduce (not eliminate) dangers from inadvertent exposure to B+ (or any other voltage) coming from the chassis. If one deliberately inserts one's self into a circuit.... you get the picture. But from the item-to-ground, you are protected.

DO NOT cheap out. Get either: a Medical Grade device, or an industrial-grade device. One designed for the purpose and not cobbled from two conventional transformers back-to-back.

I keep two (2) of these (Thank you, Steve!):  http://www.byan-roper.org/steve/steve-at-play/antique-electronics-and-2/heathkit-ip-5220-variable.html    One is permanently mounted to the bench, one is in the traveling kit. Nothing (electronic) gets tested without isolation. Full Stop.

Peter,

Thanks for the nice safety reminder.

I run a couple of Variac’s, but not always used on the bench.

Other than your vintage Heathkit units, could you recommend any modern medical or industrial isolation transformers in a reasonable price range that would be a safe bench addition?

Cheers,
Bluewater

aguaazul wrote:Peter,

Thanks for the nice safety reminder.

I run a couple of Variac’s, but not always used on the bench.

Other than your vintage Heathkit units, could you recommend any modern medical or industrial isolation transformers in a reasonable price range that would be a safe bench addition?

Cheers,
Bluewater

https://www.tripplite.com/isolator-series-120v-1800w-ul60601-1-medical-grade-isolation-transformer-6-hospital-grade-outlets~IS1800HG At the top of the range. Unlikely you would need 1,800 watts, but if so, this will do nicely. I have seen them go on eBay in the $350 range, about half-of-new.

https://www.digikey.com/product-detail/en/triad-magnetics/MD-500-E/237-2078-ND/5813844&?gclid=CjwKCAiAqOriBRAfEiwAEb9oXVuj2d1i6R6oXsS6FuHNVGN6MHndrVn5D5dvG7B6vrT_S0UQBr1ZvBoCC5gQAvD_BwE

A mere 500 watts, but under $200 new.

https://www.ebay.com/itm/TRIPP-LITE-500W-MEDICAL-GRADE-ISOLATION-TRANSFORMER-Y424/183672171637?epid=21005246380&hash=item2ac3b6ec75:g:aqsAAOSwatBb4lpC:rk:340

Used, 500 watts.

There are many.

For those who want to dig deeper into this topic, below are links to discussions of some pros and cons.

Isolation transformers do not always isolate ground from the secondary side, which many assert does not provide protection to the user. There is much discussion available about this online, with some recommending GFCI protection, others recommending disconnecting secondary ground connections, etc. There is much debate over relative safety of isolation transformers with connected grounds for protection of oscilloscopes (and users). Worth reading, I think.

http://sound.whsites.net/articles/iso-xfmr.htm
https://www.allaboutcircuits.com/technical-articles/transformer-isolation/
https://www.slideshare.net/sustenergy/isolation-transformers
https://electronics.stackexchange.com/questions/17496/how-is-using-a-transformer-for-isolation-safer-than-directly-connecting-to-the-p
https://www.powerinspired.com/isolation-transformer-need-know/
https://electronics.stackexchange.com/questions/204835/why-does-isolation-transformer-protect-from-shock
https://electronics.stackexchange.com/questions/247198/will-the-use-of-an-isolation-transformer-improve-the-safety-of-my-power-supply
https://www.toruspower.com/plain-white-papers/why-use-an-isolation-transformer/
http://www.reo.co.uk/files/safety_8_-_isolation_transformers.pdf
https://electronics.stackexchange.com/questions/42541/cutting-the-ground-wire-in-an-isolation-transformer

Great info in this thread. I don't see where having one would hurt anything. Unless there were any sound issues.

Why the medical grade?
Why do some say to do the mods, others are adamant against.

Busy week at work, will take the time to look at the links suggested by gkt when I can come up for air.

Bluewater

Generally, I illustrate the value of an Isolation Transformer by taking a VOM and measuring AC and DC volts within the workpiece, and then to any given ground up and downstream of the Iso. I have seen too many discussions dive deep into theory, but entirely ignore the actuality.

As I previously noted! An isolation transformer is a SAFETY ITEM, that REDUCES the possibility of an unintended shock. It is NOT absolute protection against shock, deliberate carelessness and congenital silliness.

Medical-grade isolation transformers are additionally rated so as to protect patients who may be hooked up to some highly sensitive instruments, or for whom even a small shock could be fatal. I work in (but not for) a hospital, and the Medical Engineering folks are very good about explaining their use. Industrial units are just a bit more rugged. What is to be avoided are the cobbler's solutions, consisting of two conventional transformers set up back-to-back.

Peter W. wrote:What is to be avoided are the cobbler's solutions, consisting of two conventional transformers set up back-to-back.

Why? What's the difference in having two transformers with their secondaries hooked together and taking your output off the second transformer primary?  Other than losses what is the safety issue?  I ask out of genuine curiosity.

Tim L.

Rod Elliot has a discussion about this in the first link in my previous post:
http://sound.whsites.net/articles/iso-xfmr.htm

Tubes4ever wrote:

Peter W. wrote:What is to be avoided are the cobbler's solutions, consisting of two conventional transformers set up back-to-back.

Why? What's the difference in having two transformers with their secondaries hooked together and taking your output off the second transformer primary?  Other than losses what is the safety issue?  I ask out of genuine curiosity.

Tim L.

Tubes4ever wrote:

Peter W. wrote:What is to be avoided are the cobbler's solutions, consisting of two conventional transformers set up back-to-back.

Why? What's the difference in having two transformers with their secondaries hooked together and taking your output off the second transformer primary?  Other than losses what is the safety issue?  I ask out of genuine curiosity.

Tim L.

As already noted in the Rod Eliot link. And why it is that I specify Medical or Industrial. By the way, the transformer supplied in the Heath is an industrial-rated device.

But, cutting to the chase - I do work on vintage radios, including a great many AA5 types. I also have seen far to many slagged transformers shorting to the chassis, and "death-caps' replaced with non-rated types and much more. What the Iso allows me to do is 'get' to all those things, identify them, and make necessary repairs. And then, I will test whatever the whatizit as if it were in actual use.

An AA5 is essentially irredeemable, but can be made as safe as when it was new, even a bit safer, by adding a polarized plug so that "OFF" means OFF, running it through a GFCI-rated receptacle and such. But it is a KNOWN danger. A piece of audio equipment with a failed transformer or shorted cap is not.