[ARC5] A Chicken Band "Leaner Applefryer" for Our Rigs

[Neil]

> Here's the circuit:
>
> http://home.netcom.com/~arc5/AVRAVT/newbias.jpg

I think in the original configuration, the transistors are being driven in
class B zero bias.
Normally the center tap would be fully grounded, with each half of the
secondary driving its respective base.

The "wimpy RFC" lifts the CT further and further above ground for
successively higher harmonics that may be present in the signal from
the driver stages. That is to say, the wimpy RFC reduces the drive
level for out of band signals and is actually a harmonic and out of band
suppressor.

By replacing it with a hefty 1mH choke you have actually placed the
CT way above ground, even for in-band frequencies, meaning that
the bases are now being driven by the entire winding. This has forced
the unnecessary step of using .004 uF capacitors to provide a ground
to work the opposite base against.

Your better option is to remove those capacitors and the other biasing
components, and if you're designing for 3.5 - 7 MHz, ground the CT
with your 1mH choke. It's 5 times the reactance of the wimpy RFC,
so it should be about right for those bands.

Then make a regulated 5V supply and feed it to each base via a pot
and an isolating resistor (a pot and resistor for each base). Leave the
resistors unbypassed at the bases. They can be quite low, 10 ohms
will actually aid stability.

With no drive, set the pots at lowest setting (zero volts at bases and
no collector current), then advance them until the transistors start
drawing (equal) small currents. You can experiment with pot settings
for best linearity. Do not advance the pots too far - keep an eye (or
rather a finger) on the transistor temperatures.

73 de Neil ZL1ANM