----- Original Message -----
From: "Garey Barrell" <k4...@mindspring.com>
To: "Gary Winblad" <garywinb...@comcast.net>
Cc: "drakelist" <drakelist@zerobeat.net>
Sent: Friday, January 20, 2012 3:24 PM
Subject: Re: [Drakelist] AC-4 (bias mod)
Gary -
Obviously the bias would remain in the Class AB1 area for
SSB and AM.
Applying a keyed signal to a non-linear amplifier is a bad
idea. Every time the drive is started or stopped there is
the opportunity for additional distortion products to be
generated. Some SSB distortion is caused by biasing the
amplifier too high or too low, resulting in 'compression'
or limiting the waveform to less than it's peak. More
distortion.
Perhaps we have an amplifier expert on here who can
definitively respond to your question. There are no doubt
MANY reasons why all transmitters switched to linear
operation for CW, in spite of the 'power/efficiency
savings. If nothing else than it takes large amounts of
drive POWER, more trouble, including greatly increased
Grid dissipation.
To me, turning an input signal on and off to a Class C
amplifier is asking for trouble. A Class C amplifier is
essentially a switch, ON or OFF, and relies on the Q of
the tank circuits to 'smooth things out'. A lot of
potential headaches for a few percent efficiency increase.
There are LOTS of 'ideas' on the internet that 'sound'
good.... :-)
Let us know what you find out!!
73, Garey - K4OAH
Glen Allen, VA
There is a lot more than changing bias to converting a
linear amplifier of any class to a Class-C amplifier. For
one thing the plate impedance and thus tank requirements
will be different. Its not trivial. There is a lot of
information on designing Class-C amps in older handbooks,
maybe even on the web, although I have not looked. The
difference in output for a tube with given plate dissipation
can be considerable. A Class AB linear has less than 66%
efficiency and a Class-C amp around 78% (can be made higher
with special tank circuits).
Essentially, Class-C amplifiers are pulse integration
devices. The plate efficiency is partly a matter of the
pulse duration. I think if you want a transmitter primarily
for CW or FSK building one from scratch might be best.
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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