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Re: VTTC output
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
>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
1) Tweak the tuning for the peak in screen grid current - it's in
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
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
* A variometer consists of two coaxial coils which may be connected in
series, one of which is free to rotate so that its axis assumes an
angle with that of the other coil which can be anything from 0 degrees
(in line) to 90 degrees or possibly to 180 degrees (in line the other
way round). This varies the mutual inductance between them and hence
the coupling/loading/tuning as appropriate and depending on whether
they are connected in series or not. Connecting in series varies
total self-inductance and hence tuning. Using the coils independently
varies coupling/loading. The usual variometer has one coil housed
entirely within the other, but this is not essential for many purposes
and makes construction more difficult.