[Elecraft] K2/HF Packer Amp incompatability?

[Pat Byers]

Hi Don and Geoffrey,

The manual doesn't discuss biasing the amp for Class C, just for 
linear operation. This is a 2nd Gen amp with only a single-turn pot 
for bias adjustment which makes the adjustment tricky but doable (3rd 
Gen has a ten-turn pot) but I'm pretty sure I set the bias correctly 
for linear operation. I'll recheck it next time I get a chance to put 
the amp on the bench.

I should also mention that although I started with a 2W drive level 
(measured by an external Watt meter into a 50 Ohm load before I 
connected the K2 to the amp) I experimented with other drive levels 
both above and below the 2W setting while the K2 was connected to the 
amp. My crude listening tests didn't detect any difference in output 
quality at different drive levels. I should also mention that the 
K2`s SWR meter didn`t indicate an abnormal match between the radio and the amp.

I recognize that I need to make more definitive measurements and 
that`s why I plan to run a sine wave into the mike input of the K2 
and observe the amp`s output with an oscilloscope. House renos 
prevent me from doing that at the moment.

If there is a solution to this problem, hopefully it will be as 
simple as adding attenuation between the K2 and the input to the amp.

Again, thanks for all the help.

Pat     VE3EUR

At 11:58 AM 14/09/2010, Geoffrey Mackenzie-Kennedy wrote:
>Don,
>
>Shortly after I sent my comment earlier today I received an email telling me
>that the HF Packer amp was intended to be used for CW (Class C?) not SSB,
>and that a modification was necessary for its use as a linear amplifier for
>SSB - presumably at least a change in the amplifier's standing current by
>changing its bias.
>
>However Pat said that the K2/ HF Packer signal was distorted whereas the
>IC-703/ HF Packer signal was not distorted. This suggests that the HF Packer
>amplifer he tested is biased for linear operation, and is not running in
>Class C.
>
>Nevertheless as you suggest the results of a gain vs drive level test would
>remove any uncertainty.
>
>73,
>Geoff
>GM4ESD
>
>
>Don Wilhelm wrote on Tuesday, September 14, 2010 3:48 PM
>
>
> >  That sounds like the amp is not running linear.  First make sure the
> > bias is set correctly.
> > Then do a test in CW mode -
> > Put a small amount of drive to the amp (measure it), and measure the
> > output.  Compute the gain.
> > Increase the drive by a small amount, and again compute the gain.
> > Continue in small steps until you have reached the maximum output of the
> > amp.
> > The gain should be the same if the amplifier is really linear.
> >
> > If the input impedance of the amplifier is changing with drive level,
> > that can be cured with an attenuator at the input - the driver will then
> > see a more constant impedance.
> >
> > 73,
> > Don W3FPR
>
>
>
>
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