Fun oscilloscope (filament transformer) was: Re: tube synthsizer

[Rick Jansen]

In message <199910280822.KAA05072 at window.intermetall.de> you write:

> Wouldn't be such a bad idea to have a simple scope in a modular,
> I remember there was a kit, but this is years ago. Perhaps better
> to buy a cheap chinese or russian type, and fit it mechanically into
> a new enclosure, or something like that.

I have such a Russian scope, that I use in the Formant for this purpose!
The big scope tube is too big to fit in a 6HE 19" rack even, it's very
long too, about 50 cm. 

> Since the deflection is electrostatic instead of electromagnetic (like
> in TV) I'd expect no moise/shielding problems...

Not against noise, but there are very high voltages that need shielding.
  
> btw.: Some tube circuits use DC heating, NS have a special regulator for 
> 6.3V for this very purpose. Other people say that this will shorten 
> the lifetime of the filament (electromigration?). Any experiences?
 
Tube filaments are usually very long lived. I think that more important to
a filament's life is how it is switched on (and off), the usual story with
filaments like in incandescant lamps. I can imagine if a regulator is used
that protects the filament against peaks, or even switches it on slowly
that the life is even prolonged.

Also important is to protect tubes with a glowing filament against mechanical
shock. A hot filament is very brittle. NEVER MOVE A TELEVISION OR MONITOR THAT 
IS ON OR WAS ON A MOMENT AGO!

Another important thing is to make sure there is no difference in potential
between the filament and the encapsulating metal tube. In many oscilloscope
tubes the cathode is heated indirectly: the filament heats a metal tube, which
is set to the right potential. If this results in a potential difference 
between filament and actual cathode, wear is the result. 

In oscilloscopes you need to accellerate the electrons quite a bit. This means
the cathode must be negative, the screen positive. If you really make the cathode
negative and no voltage difference may exist between filament and actual cathode,
you must connect the filament and cathode, so the secundary of the filament 
transformer is negative too. That is why you need a VERY VERY well insulated 
transformer here.
The other option would be to have the potential of the cathode (near) ground,
so you don't need a very well insulated transformer. In that case you need
positive accelleration voltages. I don't remember now why that isn't done
in oscilloscopes, in television sets they do use a positive accelleration 
voltage, don't they?

Rick Jansen
__
rick at bpa.nl rja at euronet.nl   http://www.euronet.nl/~rja/
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