Re: VTTC output

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Dr. Duncan Cadd by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>
> 
> Hi Herwig, All!
> 
> BTW thanks to Dave Trimmell for an excellent idea - I will definitely
> be digging out the old valve stuff and contributing to this worthy
> project!  Starting now . . .
> 
> >Original poster: "Herwig Roscher by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <herwig.roscher-at-gmx.de>
> 
> <snip>
> 
> >Is monitoring the
> >value of the current flowing into the base of the secondary a better
> >choice? Which methodes do you use?
> 
> The 'classical' way of tuning up a triode output stage is to look for
> the anode current dip, i.e. the plate current of a triode drops when
> the tank is in tune.  You tweak for minimum plate amps to tune it.
> Conversely, the more heavily it's loaded, the more juice it draws.
> 
> 1) Tweak the tuning for the dip - it's tuned.
> 2) Tweak the load coupling for more amps - more output.
> 
> You have a beam tetrode.  The best way of tweaking these is to monitor
> the screen grid current.  This peaks when the tuning is spot on.
> Also, the screen grid current drops as the load is coupled more
> closely.
> 
> 1) Tweak the tuning for the peak in screen grid current - it's in
> tune.
> 2) Tweak the loading for a drop in screen grid current - it's loaded
> more heavily.  More output.
> 
> This assumes independent controls for (1) tuning and (2) loading.  You
> can see that tuning and loading send the current in opposite
> directions, and usually the process of tuning and loading is an
> iterative one.

> The problem with taking this from ham radio to VTTCs is of course the
> only control you often have is changing a component/rewinding a coil.
> It might be useful to have a few taps on the coils to help with this,
> or perhaps use a variometer*, but I have yet to transfer my experience
> of tweaking valve transmitters to large scale valve TCs.  That's
> probably the next project, getting the 813s running.
> 
> Anode current is not always a reliable indicator with beam tetrodes
> because the dip is very shallow and broad (unless connected and
> operated as triodes) and as folks who have fried 4CX250s have
> discovered, some valves fry comparitively easily :-(   The plate
> current dip can be deceptive to the point that the tuning is tweaked
> until the net dc screen current goes negative - oops.  Operating
> continuously in that region of the valve characteristics is not
> recommended!
> 
> Dunckx

	Unfortunately, there is a bigger problem with VTTC's than all of the
above, which primarily applies to an amplifier coupled to a resistive
load.  The TC secondary is highly reactive, the load resistance and
capacitance are both non-linear, and vary with the power level.  So,
since in a TC both the primary and secondary are resonant, you have to
adjust either the primary or secondary frequency until both are the
same.  While you're doing that the load impedance is changing as the
streamer current varies, etc. etc.  Somewhere I have the manuscript of a
paper from UCLA covering the theory and practice of coupling a power
oscillator to a reactive load.  Work was done in connection with driving
the D's of a cyclotron.  One thing I remember is that there is a
possibility that, as you tune, your operation will jump instantaneously
from one unsatisfactory mode to another.

	Bottom line is that educated cut and try is probably the only practical
way to go, and that tuning/coupling/bias adjustments must be made at the
power level you want to use.  No comfort in this, but it's probably
correct.

Ed