Uwe wrote:

With the help of some folks here I did troubleshoot my AC-1 tube transmitter
(using a 6V6) and got it working somehow. Since I passed my code test and
had my first QSO using the transmitter and boy was that exciting.

But questions remain.

I monitor the output signal on my scope and notice that right after key down
the waveform contract just a bit and the tone changes pitch. I guess this is
called chirp. I normally use B+ 200V.
If I increase the voltage lets say to 300V this effect becomes much more
pronounced.
I still use an external bench supply capable of much higher currents and I
don't think it is a power supply weakness. In fact putting a VOM on the
supply line shows no sag in my supply voltage.

How can I minimize this and especially keep it from becoming more severe at
higher outputs.


Also, the circuit diagram for the tranmitter did not state the coil diameter
of the pi network. The pi network still has me scratching my head. Coils
with slight variations in diameter give dramatically different results.

Also changing the air cap with one of an identical range can have a vast
effect, which surprised me. Is this the "real world components" versus the
theory???

Uwe


I don't have the schematic, but if the screen voltage comes from the
usual dropping resistor & bypass cap then it is probably dropping a
little bit under load. This will cause the plate current & output power
to fall a bit, and change the operating characteristics enough to chirp
a little bit.

As mentioned by another poster, this is the nature of a single-tube
transmitter (or single-anything power oscillator, for that matter). No
matter what you do the amplifier characteristics will be different for
different RF outputs, so the frequency _will_ change. You can minimize
it but you can't make it go away.

---------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com