Re: VTTC first sound :-)

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

Original poster: "Finn Hammer by way of Terry Fritz <twftesla-at-uswest-dot-net>" <f-hammer-at-post5.tele.dk>

David, Herwig, all!

Using a valve amplifier output transformer to modulate the tube coil
works well, it was what I used for the small plasma tweeter that Ulrich
Haumann conceived, and Duncan is now playing with too. 
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. 
One way to solve that problem is to use a transformer from a single
ended amplifier. This kind of amplifier has a DC bias current running
trough the plate winding all the time, so it is well suited for this
task.
The transformer needs to have a small airgap in the magnetic path, large
enough to absorb the DC Bias. However it also needs to have very close
magnetic coupling of the vindings to support the transfer at high
frequencies. Any primary inductance that is not coupled with the
secondary, will appear as a series inductance. In the tank of our
disruptive coils, this series inductance is seen as current limiting,
but in a circuit where a range of frequencies are to be transmitted, the
impedance of a series inductance will rize with frequency, and cutt off
the highs. Not good since it is with the highs, that tesla loudspeakers
excell. 
Audio guys tend to overestimate the value of their stuff, so be prepared
to pay.


Cheers, Finn Hammer

Tesla list wrote:
> 
> Original poster: "David Trimmell by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <davidt-at-pond-dot-net>
> 
> Hi Herwig,
> 
> I am asuming you placed the 230V side of the "Audio coupling" transformer
> in series with the grid to grid coil, and ran audio input into the 12V
> side? I have found that using transformers with a low voltage/low
> inductance (impedance) side as audio input you do get some nasty
> distortion. I have had best results with a 115V to 1000V Plate
> transformer, audio into the 115V side. Unfortunately, it is too large for
> a portable demo coil I am currently working on, about 20 to 25lbs. The
> sugestion by Jim Lux on using audio output transformers from a tube amp
> does sound appealing, but on some preliminary tests I have conducted with
> a small one from a 15 watt amp, they appear to suffer the same issues as
> other low impedance transformers... But really do need do some more
> testing. One thing I have done in the past is to place a large (>1000
> uF) cap in series with the hot audio lead into the audio coupling
> transformer, this assisted with matching, but does waste audio power.
> 
> Keep up the great work!
> 
> Regards,
> 
> David Trimmell
> 
> On Thu, 22 Feb 2001, Tesla list wrote:
> 
> > Original poster: "Herwig Roscher by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <herwig.roscher-at-gmx.de>
> >
> >
> > Hi fellow VTTC builders,
> >
> > Today I audiomodulated of my system for the first time. I used a 20
> > W car amplifier/equalizer driven by a signal generator. A 12/230 V
> > mains transformer was connected between grid coil and grid leak
> > combination. No other modifications were necessary.
> >
> > Connecting the scope across a 4-turn-pickup coil wound 3.5" below
> > the primary, I easily could adjust the amplitude of the signal
> > generator to a point, where the amplitude modulation was just 100
> > %. Deafening noise forced me to use ear protection. Sweeping the
> > modulation frequency I had to state, that the frequency response of
> > the mains xformer (like suspected) was dissapointing. So I'm
> > looking for an output tranny of a tube amplifier now.
> >
> > Still a lot of work to do, but a lot of fun as well.
> >
> > Regards,
> >
> > Herwig
> >
> >
> >
> >