Re: VTTC first sound :-)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Finn, Herwig, David, All!


>But if there is any current going to the grid that handles the
>modulating, this current will have a DC component, which will tend to
>saturate the transformer core. And this in turn produces awfully
>sounding clipping of the audio signal.


Yes, it sounds _terrible_.  One way round this which I've used with a
crystal set running a loudspeaker via a mini valve output transformer
is to block the transformer to dc with a smallish (in this case 1uF
but YMMV) capacitor and then to bypass that winding to ground for dc
purposes with a suitable resistor, in this case 30k.  And yes, it
makes a huge difference even with a crystal set :-)

For modulating a valve grid, you'd just use the normal grid resistor.
A small blocking cap between grid and transformer output (or between
ground and the earthy end of the transformer output as hopefully shown
in ASCII art below) will automatically provide bass cut.  In the
circuits as shown you might want to protect the transformer with an rf
choke.  Depends on how much rf there is on the grid you are
modulating.  Circuit 1 of course means the transformer secondary is
floating WRT ground which may not be ideal.  Alternatively, the cap
goes between the grid and the trannie output, as per the second
diagram.


1)

grid---------transformer out (hot)
Z                              )
Z                              (
Zgrid res        transformer out (earthy)
Z                               ¦
Z                              =blocking cap
Z                               ¦
ground--------------ground


2)
                blocking
                cap
grid---------¦¦--------transformer out (hot)
Z                                     (
Z                                     )
Zresistor              transformer out (earthy)
Z                                     )
ground--------------------ground


Dunckx