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Old 3rd February 2019, 10:16 AM   #31
Kay Pirinha is online now Kay Pirinha  Germany
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Were there really guitar amps without mains isolation anywhere?

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Old 3rd February 2019, 01:49 PM   #32
DF96 is offline DF96  England
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Quote:
Originally Posted by Gnobuddy
But guitar amps operating on AC mains without power transformers?
Yes, that would be foolish. I was commenting on radio and TV without power transformers, which are much safer than guitar amps.
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Old 3rd February 2019, 02:50 PM   #33
stephen_keller is offline stephen_keller  United States
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There were tons of them. The Harmony H400 is a pretty typical example. A routine upgrade is to add an isolation transformer.
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Old 4th February 2019, 12:13 AM   #34
PRR is offline PRR  United States
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Quote:
Originally Posted by Kay Pirinha View Post
...Should they have said »Sorry, no radio!« to those who only had DC power?...
Actually, DC-only customers were so few that nobody really cared. Except they sold much more expensive radios to farmers with only 32V windmill battery lighting.

The only real point of "AC/DC" radios was CHEAP. No power transformer.

And yes, sadly, "AC/DC" guitar amps existed. And evolved as new test requirements tightened-up the loopholes which allowed them. Fairly early the preamp stage was stood-off on an RC "ground" network which reduced leakage to a few mA, which was allowed in those days. There were other workarounds for possible heater breakdown. Put your lips on the mike while holding an axe on some of these amps and it will knock you down.
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Old 4th February 2019, 08:07 PM   #35
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by PRR View Post
...stood-off on an RC "ground" network which reduced leakage to a few mA, which was allowed in those days.
A while ago I found an online discussion about today's switching power supplies / wall warts with 2-pin AC plugs. The (DC) output is electrically isolated from the incoming AC, but in order to avoid the output ground floating to some arbitrary voltage, it seems a pair of small (few nano farads) capacitors typically connect the DC ground to both incoming AC mains wires - live and neutral.

This leaves the DC ground floating at nominally 60 volts AC, but with a very high impedance (hundreds of kilo ohms.) Presumably this means a hundred micro amps or so of leakage currents will flow through you if you touch the DC output plug. Not enough to cause any kind of shock hazard, and presumably, allowed by contemporary safety standards.
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Originally Posted by PRR View Post
Put your lips on the mike while holding an axe on some of these amps and it will knock you down.
Among the extras on a Pink Floyd concert DVD I have, there is some footage of casual conversation between bandmates. Gilmour says after many shocks at poorly wired temporary venues, the original Pink Floyd members had gotten into the habit of slinging their guitars over their shoulders without touching any metal part, then standing next to the mic, and gently swinging the guitar so that the guitar strings contacted the mic stand. They did this before touching the mic stand, as a crude safety check.

On this particular occasion, Roger Waters (bassist) swung his bass guitar to touch his mic stand. There was an eruption of sparks, and one of the bass guitar strings burned right through.


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