On Tue, 22 May 2007, Jack Schmidling wrote:
I am building an exciter for my 8000 rig. 6AG7 VFO, 6CL6 buffer and 807
final.
It would help to get more details from you. What frequency is all of this
for? Link-tank output? Or pi-network? Are the coils of the tank/pi-net
according to specification (from some plans)? Do you know that the VFO is
on frequency? Are you doubling in the 6CL6?
I am plagued with the problem that resonance in the final does not agree with
peak power out.
Can you removed B+ from the VFO and twist your driver and final tune knobs
and get dips in plate, grid current (signs of self-oscillation)? Do you
generate grid bias in the final from a grid power supply or rectified
drive power?
I have had some experience with matching networks between the final and
the load having slight disagreements between peak output power and maximum
dip in plate current. Not to worry if the difference is slight.
Most important: find out if the final is self-oscillating (remove drive
and rotate final tuning knob to see if there is any wiggle in plate
current at any point).
The power peak (watt meter or light bulb) is a considerable twist south of
the plate current dip.
What is a considerable twist? five degrees of arc? or 45 degrees of arc?
What is your load? Dummy load?
Looking at the output on a scope shows nothing strange about the wave shape
as the amplitude increases beyond the dip.
All sine waves should look like sine waves.
I have tried neutralizing with coupled links between the input and output
tanks but it makes no difference nor does anything else I have tried.
This can be very difficult to do. If your frequency is HF or low HF you
shouldn't have to do this. Most old commercial HF tube rigs did not have
neutralization and they worked fine.
Any ideas would be welcome.
You do have the screen grids grounded through good RF bypass capacitors,
right? Once I forgot to add these to a 6146 amp and it self-oscillated at
160 meters (it should not do that) until I put in a good screen to ground
bypass capacitor.
Another area where you have to look out for a mistake is if you have the
coil/inductor at the grid input of the final located where it can pick up
the magnetic field from the final plate output coil/inductor, which can
contribute to making the final just a self-excited oscillator.
So, the next question is: if you get a dip in the final plate current at
some position of the plate tuning capacitor, then does that final plate
current change if you also tune the grid input capacitor? If yes, then
your final has a configuration to make itself a self-excited oscillator,
and the next thing is to look for how feedback can be contributing to
that.
Jack K9ACT
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