[sdiy] Integral scope on VCO

[René Schmitz]

Hi all,

Theo wrote:
> On second thought, maybe put a small 220V transformer in reverse first
> (EE20=22x23mm).
> Its outside the specs but using 12V to get 530V from a 220V/5V transformer
> should work ok.

I doubt that. The transformer will saturate. The core can handle so and 
so much magnetic flux and is close to the maximum in normal operation 
anyway.

Oh, and a stepup with the villard circuit will only work for so many 
stages until all of the input AC is consumed by the diodes voltage 
drops. The AC is reduced by 0.6V for each stage. So the theoretical 
limit is 20 stages with 12V, after that the voltage will actually go down...

So use plenty of AC and a cascade with fewer steps. In my scope tube 
experiments I used a 6 stage cascade driven from 60V, which were made 
from a small transistor bridge and a ferrite core 1:2 stepup 
transformer, that I had laying arround. I ran it with some 25kHz (the 
bridge being small signal transistors driven from a 4013 to get equal 
mark space ratio), and that allowed relatively small caps to be used for 
filtering. An additional benefit is that you can tap the g2 (intensity) 
and g3,4.. (focus) voltages right of the cascade.

The trouble with integrating such things into a modular is that you need 
a seperate heater circuit. Usually the anode of the tube is grounded, 
and so the cathode/heater is at the negative acceleration voltage. 
(Either the heater is connected to the cathode internally, or you have 
to make that connection, because the heater/cathode insulation isn't 
made for high voltage differences.)

Using a seperate transformer is possible, but beware of any stray fields 
that it might generate, it will make the whole picture quite wobbly. I 
can't see how you could make it into a standard module space without the 
need for extensive mu metal shielding.

You might as well buy a small scope and put it into your modular. 
(Anybody who has seen the D*pfer Modular in the Schneider Booth at the 
Frankfurt Musikmesse will know what I mean. :-))

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159