tube prototyping safety

[Magnus Danielson]

From: Doug Tymofichuk <dougt at cancerboard.ab.ca>
Subject: Re: tube prototyping safety
Date: Sun, 2 Jan 2000 15:46:46 -0700 (Mountain Standard Time)

Hi folks!

> On Sun, 2 Jan 2000 20:53:31 +0100 jhaible 
> <jhaible at debitel.net> wrote:
> 
> > Hi,
> > 
> > I was wondering about safety in building tube circuits,
> > especially about grounding.
> > 
> > I think there is no doubt that on the *finished* device 
> > there should be protective earth (PE) attached to the metal 
> > enclosure, (and probably to the signal ground as well).
> 
> Funny you should ask, I just did myself. Definitely the 
> metal enclosure or chassis needs to be earth grounded, but 
> I have heard differing opinions on signal ground.
>  
> > But what are you doing while prototyping, when you're
> > working on the open circuit ? My thought would be *not* to 
> > attach protective earth anywhere and keep the high DC 
> > voltage floating, so you'd only run into danger when you're 
> > touching both the high voltage and the signal gnd at the 
> > same time. (With parts of the circuit connected to PE you'd 
> > be in danger just touching *one* point of the circuit.)
> > 
> > Is this the way to go ? Or am I very wrong here ?
> 
> I agree with this method, I have been prototyping this way 
> myself without problems. AFAIK this is also the correct way 
> to service line powered devices like televisions, where the 
> entire piece of equipment is isolated via bench transformer 
> so that all points are floating with respect to earth.

Right. When you operate on live current like this, you would like to have
the box floating so that only if you touch both ends in the box you would be
in danger, where as the rest of your world is not conducting well with this
source. It is a standard procedure and for a good safety reason. One has to be
aware that this floatingness is relative, hook up any non-floating device to
the box and you got a ground reference right there. So, one has to really
think carefully about using this floatingness, since it is easilly defeated and
thus can give you a false security. If you do it, do it all the way or you may
better avoid it since it may not help you one bit, it migth even be the reason
you do get electrecuted!

In a way it is nice to be down at sensible voltages, you don't have to care
very much other than frying your curcuit, which is more of a material thing
than onself (to me at least).

Hmm... I _ALSO_ want to do something with tubes... maybe a small 32 bit RISC
processor or something...

Cheer,
Magnus