[AMRadio] Re: Max output off dip

[Bill Smith]

Did you ever have anything to do with the BTA-250M?

(I have one, and like it very much).

73 de Bill, AB6MT
billsmith at ispwest.com

----- Original Message -----
From: <RABRUNER at aol.com>
To: <amradio at mailman.qth.net>; <W2ZR at aol.com>
Sent: Saturday, June 29, 2002 8:14 PM
Subject: [AMRadio] Re: Max output off dip


>
> W2ZR writes
> >>I have a plate modulated AM transmitter that has max output at minimum
> plate
> current. A DX100. If the neutralization is off then it indeed does perform
as
> you
> describe but max output can be on either side of plate dip depending on
which
> way
> the neutralization is off.
> I will guess that all of the transmitters that you mention above did not
have
> neutralization?>>
>
> I will agree that the phenomenon of getting more plate efficiency on the
> capacitive side of dip seems much less pronounced on small transmitters
like
> the DX-100 and on 6146 transmitters in general.  I am not sure why this is
> so.  But turning the final amplifier off dip on plate modulated AM
broadcast
> iron was SOP among any engineers that I ever knew.  It is a phenomenon
that
> you can easily verify.  If you look into AM era ARRL handbooks, you will
find
> it referenced there, and as I say, many broadcast transmitters tune-up
> instructions include tuning off dip for more output.  And yes, all the
Gates
> transmitters were triode transmitters with neutralization circuitry and
they
> were properly neutralized.  4-400s are tetrodes and don't usually require
> neutralization at AM broadcast frequencies.  They certainly don't require
> critical neutralization.  Unless an amplifier is inherently unstable,
being
> slightly out of neutralization would not cause the output to rise because
the
> tuning was 'off dip.'  In fact, the most sensitive indicator of
> neutralization is the grid current.  If the amplifier is neutralized, when
> you monitor the grid current, it will stay steady or fall off slightly as
you
> tune the plate through dip.  The usual explanation for the improvement in
> efficiency has to do with power factor in the final amplifier and has
nothing
> to do with neutralization. In commercial operation proper neutralization
> would be assumed.
>     Heavily loaded VHF transmitters, that is, transmitters running very
close
> to the design limit of the final tube(s) don't improve efficiency very
much
> if any off dip.  For instance FM broadcast transmitters, that require no
> headroom in the final for modulation, were run pretty close to specs on
the
> final tube.  I engineered a number of these too, including Gates FM-1,
FM-5,
> and FM-20, as well as RCA BTF-20, and they usually achieve maximum
efficiency
> at or very close to dip.  This could be what your are seeing with your
6146
> transmitter.  A pair of 6146s is pretty heavily loaded making 100 watts of
AM
> modulated RF.  Why don't you try experimenting a little and see if you
don't
> find running the transmitter so that you mesh the plate tuning capacitor a
> little past dip doesn't give you a least a little extra output?  It might
be
> very subtle on a low power transmitter, and maybe you won't see much or
any
> improvement in that case.
> Bob Bruner
> WB4TAJ/9
>
>
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