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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: sci.electronics
Subject: Re: Power Supplies for Tube Circuits.
Message-ID: <8240@utzoo.UUCP>
Date: Fri, 3-Jul-87 12:58:01 EDT
Article-I.D.: utzoo.8240
Posted: Fri Jul  3 12:58:01 1987
Date-Received: Fri, 3-Jul-87 12:58:01 EDT
References: <12887@rose.UUCP>
Organization: U of Toronto Zoology
Lines: 22

> ... I could obtain the less than 330 volts needed by
> directly rectifying the line current and putting it through a voltage
> doubler, eliminating the power transformer.  This could reduce the cost
> of the thing quite a bit *iff it's a safe (for both me and my toys)
> thing to do*.  Are there any experts out there who'll expound on the
> wisdom (or lack there of) in eliminating the power transformer? ...

The trouble is that the negative end of the power supply is not isolated
from the AC line and hence can pretend to be "ground" only if the whole
gadget is self-contained and well-insulated, and you avoid connecting
things like oscilloscopes to it.  I believe some TV sets do their power 
supplies this way, much to the irritation of TV service people who need to
haul around an isolation transformer if they want to connect test gear.

If the thing were self-contained, e.g. a TV set, I would rate this dubious
but acceptable given precautions.  For a pre-amp, you are presumably going
to be connecting it to other equipment.  Isolating those connections is
going to be hard, probably too hard to be worthwhile.  Accordingly, I would
give this a big black NOT ACCEPTABLE.  Use a transformer.
-- 
Mars must wait -- we have un-         Henry Spencer @ U of Toronto Zoology
finished business on the Moon.     {allegra,ihnp4,decvax,pyramid}!utzoo!henry