Re: VTTC first sound :-)

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

Original poster: "David Trimmell by way of Terry Fritz <twftesla-at-uswest-dot-net>" <davidt-at-pond-dot-net>

Hey all, I ran some tests this weekend, and thanks too Duncan, I found a 
circuit that works using a ~15 watt output transformer from a old tube amp 
as the audio coupling transformer. More work needs to be done yet, but this 
is showing some hope       for "standardization". Component values need to 
be tweaked, but here's a rough (T1 has audio into 8 ohm output side):
http://216.36.6.9/webdoc2.htg/Tube_coil/Audio/New_Audio1.jpg

Regards,

David Trimmell


At 03:19 PM 2/23/01, you wrote:
>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