[AMPS] Tuning the PA

[Tom Rauch (W8JI)]

Bill Turner wrote:
-
> One of the basic concepts in tuning up an amplifier is one I had to
> figure out the hard way.  Here it is:
> 
> The amplifier should be tuned up for the maximum output that you
> intend to use at that time.  For example, if you want to put out 1000
> watts PEP on SSB, you have to tune it up with a continuous 1000 watt
> CW output.  THEN you can drive it to 1000 watts on peaks without
> clipping.  If you tune it up for 900 watts output and then try to
> drive it to 1000, you will splatter all over the place.  Actually, to
> avoid splatter, I would typically tune up and then reduce the drive
> about 5-10%.  Works like a charm.
> 
> This is such a simple concept, but for some reason it doesn't come
> across to a lot of people.
> 
> 73, Bill W7TI
> wrt@eskimo.com

 Hi Bill,

This is the REASON the SB-1000 manual tells people to load to tube at
full drive, not to exceed 500mA (some later manuals 600 mA) of plate
current.

The manual then says to reduce drive to a specific key down current of
400 mA on CW, and not to exceed 75 Ma grid current on SSB while making
sure the peak output never reaches the full value acheived at the higher
tune up current.

Following the tune up instructions will result in a clean SSB signal,
because, JUST like you suggested, the amplifier will be overcoupled.

Undercoupling is probably the single biggest cause of grid damage,
splatter, and arcing.

If any error is made, it is best to err in favor of overcoupling. You
are absolutely correct.

73 Tom



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