The exciter/radio in question is my FT-847 that i have been using for many a
year. It worked great on HF this morning driving my SB-200. I can bypass
the amp on 6M and with the peak reading wattmeter i can see it hold about
100w output as i talk with a normal voice. Put the amp inline and with 20w
of drive i have dropoff between voice peaks, the needle on the meter doesn't
hold anywhere near steady at any level. It bounces around 200-500w. I
slowed the rate of decay for the meter reading and that didn't help a whole
lot, the fact of the matter is the amp is dropping off between voice peaks.
73,
Scott
----- Original Message -----
From: <gdaught6@stanford.edu>
To: "KB0NLY" <kb0nly@mchsi.com>
Sent: Friday, March 06, 2009 11:13 AM
Subject: Re: [Amps] A Symptom of Low Bias?
> Hi Scott,
>
>> 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!
>
> I'm thinking that if you increase the bias, thereby reducing idle current,
> it will take an
> even louder yell. Usually, the linearity is better if the idle current is
> higher. That is
> to say, the amplifier is getting closer to "class a" operation.
>
> Are you sure your exciter is doing the right thing? Maybe it's not
> operating correctly.
>
> Your CW and FM numbers seem to be pretty much OK.
>
> 73,
>
> George T Daughters, K6GT
> CU in the California QSO Party (CQP)
> October 3-4, 2009
>
>
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