mwhouston wrote:
I have been toying with the idea of using direct mains to provide HT. I have tried it and it doesnt work. In fact you could take out the rest of your gear if used.
I haven't tried tube rectification with direct mains yet. Until I do I'm staying with isolation methods for generating HT.
I hate to say this but direct mains is just asking for a fatality.
It was used in old gear and that is also very dangerous.
OK its a cost/weight saving but you end up with a floating chassis on mains equipment.
There should not be any connection between Earth and one side of a non isolated supply<<and it also makes RCD's fail.
So all the control knobs etc are potential death traps.
In old equipment the chassis was inside a wood enclosure with Bakelite knobs to isolate the control pot shafts and switches.
But it didn't take into account the women with wet hands just turning it up while washing the dishes.
With only two wire mains cord and plugs that could be rotated or wired up incorrectly the chance of a live to chassis connection is inevitable. With series heater chains if one heater goes open circuit the whole lot rises to mains potential.
Direct mains gives no limitation of fault current either.
Also if you do it you are directly liable for anyone who may come into contact with it, even if it's accidental.
You have to remember that mains voltage rectified or voltage doubled has no limitation on current (A fuse doesn't limit current)
You may if lucky survive contact with HT on an isolation transformer (capacitance storage dependant)
But the chance of survival on stepped up mains is much lower.
If you earth the chassis and use a separate electronic ground that is even worse, because you can be in contact with earth as you try to disconnect a phono plug which is at electronic ground!
In industry its very common to see floating electronic ground even up at thousands of volts but its a very specialised field and people are trained to deal with it. Years ago it was sort of custom and practice to have the earth removed on a scope so you didn't blow the equipment up with a floating electronic ground but its frowned on now and you would probably loose your job.
Enter the double insulation..phenomena.
Regards
M. Gregg