Hi Scott,
It sounds like you are way overdriving the amp. When tuning up do not
increase the drive any more when you have obtained maximum output from the
amp. Never mind if the grid current is lower than expected, that's good.
If you have it tuned up for 450 watts output on CW and switch to ssb your
peak power will reach the 450 watt level and you will see little plate
current increase over the 100 ma idle current. The plate current may kick up
to around 150-200ma on voice peaks and you will see very little grid
current. That's normal! Any more and you are overdriving the amp. If you
have a scope lay the scope probe near the amp to get a reading of around 2/3
scale on the scope when in cw. When on ssb you should see the peaks reach
the same level as the cw did on the scope. That is reaching full power and
the meters will move very little.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of KB0NLY
> Sent: Friday, March 06, 2009 11:34 AM
> To: amps@contesting.com
> Subject: [Amps] A Symptom of Low Bias?
>
> Still fighting some small bugs in the new amp this morning, does this
> sound like a symptom of the bias being too low??
>
> When i key up with no drive the plate idles at about 90-100mA, and the
> grid goes slightly negative, just enough to pull the needle a hash mark
> width below the zero on the scale. As i apply drive the plate comes up to
> about 400mA and the grid starts to come above zero, but i have to
> literally YELL on SSB to get the grid to come up and see full output. I
> tried some CW on it last night and its making full output there. Also at
> the suggestion of some on here i tried applying drive on FM and the grid
> came up to 100mA and the plate stayed around 400mA yet.
>
> So from the info i can find online, in the handbooks, picking others
> brains, etc.. It would seem i still have a bias issue here as its only
> amplifying on the voice peaks on SSB. I dug through my drawers and didn't
> have the parts to do the mod for higher bias on the Triode Board so i
> ordered some zeners from Mouser today and i will get that done when they
> arrive and see if i can further adjust the bias and get the idle current
> down some more as well.
>
> What do you guys think?? Starting to realize once again why i like
> building stuff, learning is fun!
>
> 73,
>
> Scott
>
>
>
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