[ETO_Alpha] Alpha 78 Tune/Load Settings

[Tom Hammond]

Hi Joe:

UNfortunately, I have an Alpha 76PA, so I probably can't give you 
exact numbers,
but...

Any time I have the amp on I monitor the RF envelope with an in-line
monitor scope (currently a Kenwood SM-220) to keep an eye on what I'm
actually transmitting.

I have noticed (on the scope) that (generally) when I'm drawing grid
current in excess of about 50 mA, the top and bottom of my RF envelope
has a slight 'ripple' (waviness) to it. For my 76, this generally
translates to undesired harmonic output(!) at some level...

When this occurs, I've found that cranking in just a bit more LOADing
will 1) reduce the grid current, and 2) virtually ELIMINATE the ripple
on the output waveform, and thus significantly reduce or eliminate any
harmonic output as well.

It doesn't take much reduction in the grid current to 'clean up' the
RF waveform... usually a drop of 1mA or less will do the trick.

It's NOT difficult to 'tune' harmonic output into your amp, but if you
can LOOK at the RF envelope, it just as easy to TUNE IT OUT as well.

I may lose a few watts of output by this method, but that's more
desirable than having RF in the wrong place.

I don't know where I'd be without my monitor scope.

73,

Tom Hammond    N0SS


At 12:00 02/19/2008, Joe Staples wrote:
>I would like to know what is being used as the recommended Tune & 
>Load settings for an Alpha 78 on 80 M.  For many years we have been 
>using a Tune setting around 20-24 and Load at 78-84 on three 
>different Alpha 78s.  Where the original numbers came from is a 
>mystery but they have apparently worked.
>
>However our old Alpha 78 manual lists Tune - 65 and Load - 30.  This 
>is nearly the reverse of what we've been using.
>
>Recently there have been reports of a 2nd harmonic on 40 M.  We've 
>checked to see that the amplifier is not being overdriven ... we 
>keep the grid current at 1/3 scale (50 ma) max. and seldom use more 
>than 1200 watts out on the Bird.  It's possible that the harmonic is 
>originating in the xcvr and just being amplified.
>
>Thanks for the help.
>
>Joe, W5ASP
>
>
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