>>> Part 2 of 2...
 
It all depends on how high you set the gain.  Compression is good to the
extent of leveling your audio when you move from 1 inch to 6 inches from
the mike, however I definately would NOT have it so high that it picks
up the coo-coo clock in the next room, the TV/radio 2 rooms away, the
kids screaming in the yard or grandfather's belching.   :-}
And the term "audio processing" can mean a lot of things!
 
 JE> Better energy and effort would be put into a passive filter as you
 JE> said (around 3.1 kHz) at the output of the speechamp along with a
 JE> little negative cycle  loading, to prevent exceeding 100% modulation,
 JE> on the negative cycles (near the baseline of the carrier)
 
Agreed.
 
 II> Beyond that, a good-quality AF amplifier should be all you need.
 JE> Using a Bogen PA system, highly modified.  Introduced another 20db of
 JE> inverse  feed-back, reduced the plate voltage to around 250v, and took
 JE> the audio  straight off the plates of the output tubes, through a .5uf
 JE> cap, at 500v, to a  hi-Z choke, straight to the bases of a pair of ECG
 JE> 164's.  They were configured in an emitter follower circuit, and the
 JE> emitter outputs are directly connected  to the grids of the modulator.
 
 II> Assuming you aren't severaly limited by finances, I
 II> would suggest transformer coupling for the modulator
 II> input and output.
 JE> Such conventional thinking, Ivy.  ;-)
 
Perhaps, but how do you think AM BC stations do it, and they HAVE to be
flat with under 1% THD up to 20 KHz at all modulation levels up to 100%?
One of the jobs I have had is transmitter engineer for a 50 KW AM BC
station, (KXEL), and they used a pair of HUGE air-cooled tubes, (finned
plates at least 2 1/2 Ft. diameter), push pull for a modulator, driving
another pair of the same type tube as a final.  The modulation
transformer, (in the back room), was at least 6 Ft. X 6 Ft. X 8 Ft. high!
25 KW of PERFECT audio... I bet the rock concert promoters would LOVE
it!   }:->
 
 JE> Do you have internet access?  if so, check out the 'work in progress'
 JE> at  http://www.qsl.net/wa5bxo/driver1.html
 
Sorry, no got.   :-<
 
 II> Are you planning to use solid-state or hollow-state for the
 II> audio up to the modulator?
 JE> The old Bogen PA gear, has nice seperate bass and treble controls, 2
 JE> seperate  inputs, each with it's own 12AX7...
...
 JE> The idea is, transformer coupled audio to the grids, is 'lacking' in
 JE> depth and  presence because the secondary of the output transformer is
 JE> near saturation,  already.  The hysteresis loss in the transformer is
 JE> such that the quality of  audio that comes from the speech-amp, no
 JE> where near matches that of the output  of the speech-amp.
 
Guess that's why KXEL had such a big transformer... LOTS of iron - takes
a LOT to saturate it!  Though I would imagine that since the modulator
is push-pull, that should reduce the saturation problem if not eliminate
it alltogether.
 
 JE> The solid-state, directly connected, Class B / AB1 driver circuit
 JE> eliminates  all that.
 
I suppose so.
 
 II> Please keep me informed on how it's going.
 JE> 
 JE> I've already used it before, on this rig, using a pair of 810's to
 JE> modulate the 250TH's with ... and I'm sure that with the audio
 JE> driver circuit I'm describing  here will more than be adequate for four
 JE> 811's in push-pull, parallel, should I ever want to 'go for broke' 
 JE> };->
 
Heh heh... 10-4, good buddy!   :-]
 
73 DE KB9QPM
   Ivy
 
 
 
... Keep the shack warm: Run radios with tubes!
 
          
 
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