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RE: That's very dangerous

Posted by cpotl (A) on October 24, 2016 at 14:52:28

In Reply to: That's very dangerous posted by Triode_Kingdom on October 24, 2016 at 11:33:33:

"It's extremely hazardous to build high voltage power supplies and amps without a power transformer. No one should be doing this."

Hi TK. Normally, I find myself on the same side of the argument as you, but over this one, I do feel a need to disagree. There is a long and venerable tradition of direct mains connected radios and TVs. The classic AA5 radios were ubiquitous for a long period, and then direct mains-connected TVs were similarly, until the demise of the vacuum tube television. I grew up in England in the 1960s, playing around with direct mains-connected TVs, dismantling them and repairing them, from age about 10.

A couple of years ago I came across a paper by Dickie and Makovski, dating from 1954, describing an OTL amplifier with no transformers at all; direct mains connected, with the heaters connected to mains, in a string like Christmas lights, and the HT+ and HT- derived directly from the mains. I thought about it for a while, and decided I'd make a modern day copy of that amplifier. I thought about the safety aspects, and figured out a way of making it reasonably safe. The chassis is connected to 3-prong socket ground. The signal ground is (as it has to be) neutral. I use massive 35 amp back-to-back (anti-parallel) rectifiers to ensure signal ground can never deviate from true ground by more than +/- 1.5V. If it were ever plugged into one of the dreaded reverse-wired sockets, the breaker at the fusebox would blow instantly.

I think there is a tendency to exaggerate the dangers of direct mains connection. Especially in the US, the mains voltage is actually quite low, certainly in comparison to the secondary side of a power transformer for a typical vacuum tube amplifier. It is true the maximum current available from the secondary of an HT transformer is a lot less than that from the direct mains supply, but the available current is still way more than the lethal threshold.

I maintain that with care, and with thoughtful attention to possible fault modes, it is not that dangerous to make direct mains connected gear.

I would be interested in a realistic discussion of hazards with a design such as I have built, rather than just blanket assertions that it is dangerous and one should not do it.

Chris

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