It could easily be weak tubes. It was the case on one of my first
SB-200. I have 2 now. Plus a couple Dentrons. Anyway, if your plate and grid
also seem to be running lower it would be one of the same things I was
seeing from the case of weak tubes plus a higher than normal SWR. New tubes
did bring the SWR back down. Please before ever buying the new tubes and
through the life of the new tubes do not refer back to a possible old tuning
habit (tune for max power and then check grid and plate). A new pair might
run the grid way higher or even higher than you think it would be possible.
Life is heck not knowing any other amp but the SB-200 then putting new tubes
in to only short one out during tune up. Don't go above 250 or so on the
grid which is the books max (half scale range) limit. At least it reads that
way. It will get there very quickly at a drastically lower drive power with
the new tubes. It might not seem fair for the tubes to want to keep going up
in power after reaching the max grid or plate but be fair to your self and
don't set yourself up to be working on repairing your bias. You might end up
being limited to only putting 60 watts max into the SB-200 after new tubes
even though the book says 100 is ok. Ask for description of explosion of 33
ohm resistors and no visible damage to bias circuit but a pegged plate
reading with any watts applied. This is what I have now after trying to back
down from 800+ watts with a grid of .600+. Don't go there it is certain
doom. Watch your grid, and plate readings as much as watts out during
incremental increases of exciter power. Any questions of just how stupid I
am can be addressed to 1st of the 487th FA 29th SIB Enhanced Brigade. Don't
act like an artilleryman in the infantry / All the way every day doesn't
work with amps. Air Mobile all the way to you Phil. Air Assault all the way.
Sorry about the cheesiness.
73 Jim AH6OY
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