[sdiy] Xfig users

[fmg]

On Sun, Sep 6, 2009 at 10:21 AM, Ingo Debus <igg.debus at t-online.de> wrote:
>
> That's amazing! What does the VFD do here, is it a buffer?
>

It also offers a bit of amplification too.
You can see a VFD as a 'multiode', a multiple-grid multiple-plate triode,
but unlike multiple grids devices (tetrodes, heptodes, etc) in wich each grid
occupies a 'specific plane in the field' (according to the travel of
electron) and
each of these 'plane of fields' have individual properties (and are
biased and used
for diferent purposes) in a VFD, due to machanical construction all grids are
at same potential and are seen equally (almost) by the electron eyes.

Above is not completely true due to filament voltage. One end is at different
voltage than the another across the display, and being the filament also the
emitting electrode (cathode) it plays against this 'equally grids' distribution.
This unbalancing leads in practice to get different signal headrooms for
each grid before severe distortion occurs.
Displays manufactures generally feed the filament with a hi frecuency a.c.
current and forget the problem but for audio I had to make a constant current
source and take care about the voltages at filament ends for best results.

If you take a look at the schematics (bottom-right corner) you'll see
Mix2 entering grid 1, at this point the useful headroom is about 1.5Vpp
before severe distortion. The higher Eq band is entering grid 8 with a poor
300mV signal headroom. Grid 10 is unusable

The heart of this VFD thing is not their amplifying characteristics but their
sound (a friend of mine said it was the crappiest eq/mixer ever seen).
Playing with bias and overdriving gives their sound, from a normal nothing
fancy just a little bit tube resembling sound to a "tube starved need 100V more
on plate + tape compression distortion your ALC just blown up" sound.
That was the function of sw1, I think I simply labeled Norm/Dist :)

Fabio
.