[AMRadio] Re: Max output off dip

[RABRUNER at aol.com]

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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