>  II> AH YES!  I remember the BC-610!  Worked on them in the
 >  II> Army!  Wish I had one, but in the interest of avoiding moderator
 >  II> action, I say no more.
 >  JE> Got a pair of 250TH's here, in a homebrewed AM rig, that I'm 
urrently
 >  JE> in the  process of building a modulator for.
 > AH!  A medium-power AM loudenboomer!  :->
I wouldn't call 2500v @ 350mA quite MEDIUM power, in respect to Ham power 
limits ;-)
But, I won't be using that much - more like 2kV @ 200mA, for 400w input to 
the final.
 >  JE> Thought about using the 450TL's, but with a pair of 'em, it's
 >  JE> somewhere in the  area of 7.5v @ 25Amps, just to light up the
 >  JE> filaments!
 >  JE> Instead, I belive a more wise choice would be to use 4 811's in
 >  JE> push-pull  parallel.
 > Ok, remembering that 250 watts of carrier needs 125 watts of audio for
 > 100% modulation if you are plate modulating...
I've grown up around BC-610's, ART-13's, DX-100s, Viking Rangers Viking 1's 
and II's and other various AM tranmitters.  It all depends on how much 
clarity and quality you're looking for, out of an AM signal.
Since the absolute most power that can be run, on ANY mode is 1500w PEP 
OUTPUT (something I -never- agreed with.  How can it be properly measured?) 
in AM terms, that comes down to around 500w DC input to the final, or around 
375w carrier output.
Remember, that PEP out, at 100% moudlation, is 4 times the carrier.  1500w 
PEP out is 375 x 4.
 > are using grid modulation.  (I have forgotten how much
 > a 450TL can put out - seems to me it's around 200-250 watts).
The nice thing about Eimacs tube designators, is you could immediatly tell 
what the plate dissapation was, byt the number of the tube.  Hence, 450 (TH 
or TL) has 450w of plate dissapation.  250TH/L's are 250w of dissapation, and 
down the line.  4-65's, had 65w of dissapation, 4-125, 4-250, 4-400, 4-1000, 
were 125w, 250w, 400w and 1kw dissapation, respectivly.
A pair of 450TL's, according the 1957 ARRL Handbook, in class AB1, will run 
every bit of 1kW output.  Naturally, this much isn't needed, but does provide 
the necessary overhead, to increase (dramatically!) the QUALITY of audio 
comming from the speech-amp.
 > Personally, I would use a scope connected for a trapaziod
 > pattern as a modulation indicator on any such rig.
Sampling RF, wiht a 'loop' type of antenna, or possibly a coil, to view the 
carrier, and then the audio imposed on it is better, because you can 
instantly see when/if the amount of audio ever reaches/exceeds 100%.  Looking 
close enough to the audio on the 'scope, will also show how 'smooth' it is, 
instead of 'peaked', depending on what mic you're using.
 > Check the handbook, (at least the older ones), for details.
;-)
 >  JE> I have some other ideas for how a properly modulated AM rig should
 >  JE> work, and  definatly ways of increasing the fidelity!
 > Speech processing would be a good thing to have, at
 > least a compression amplifier, with about a 2.5-3 KHz l
 > owpass filter on the output.
Don't agree that processing is necesarily good - nor compression.  Perhaps a 
little compression, but only if you have a room full of hams, who you'd like 
to modulate the rig, who are a distance from the microphonium.  Other than 
that, you'll start picking up every car that drives by, the neighbors washing 
machine, or even the sprinkler running in the backyard.
Better energy and effort would be put into a passive filter as you said 
(around 3.1 kHz) at the output of the speechamp along with a little negative 
cycle loading, to prevent exceeding 100% modulation, on the negative cycles 
(near the baseline of the carrier)
 > Beyond that, a good-quality AF amplifier should be all you
 > need.
Using a Bogen PA system, highly modified.  Introduced another 20db of inverse 
feed-back, reduced the plate voltage to around 250v, and took the audio 
straight off the plates of the output tubes, through a .5uf cap, at 500v, to 
a hi-Z choke, straight to the bases of a pair of ECG 164's.  They were 
configured in an emitter follower circuit, and the emitter outputs are 
directly connected to the grids of the modulator.
 >  JE> Soon, I'll have a text file from John/WA5BXO on how to feed class B
 >  JE> modulators. Talked to him briefly last night on the telephone, and he
 >  JE> told me some of it - GREAT technical data, there!
 > Assuming you aren't severaly limited by finances, I
 > would suggest transformer coupling for the modulator
 > input and output.
Such conventional thinking, Ivy.  ;-)
Do you have internet access?  if so, check out the 'work in progress' at 
http://www.qsl.net/wa5bxo/driver1.html
 > Are you planning to use solid-state or hollow-state for the
 > audio up to the modulator?
The old Bogen PA gear, has nice seperate bass and treble controls, 2 seperate 
inputs, each with it's own 12AX7, to drive another 12AX7, configured as a 
phase inverter, to drive probaby a pair of 6AQ5's, who in turn, drive four 
7868's in push-pull, parallel.  At least that's how THIS version works.  The 
same ouput could be had by a 6L6 output speech-amp.
The idea is, transformer coupled audio to the grids, is 'lacking' in depth 
and presence because the secondary of the output transformer is near 
saturation, already.  The hysteresis loss in the transformer is such that the 
quality of audio that comes from the speech-amp, no where near matches that 
of the output of the speech-amp.
The solid-state, directly connected, Class B / AB1 driver circuit eliminates 
all that.
 > Please keep me informed on how it's going.
I've already used it before, on this rig, using a pair of 810's to modulate 
the 250TH's with.  However, the 810's just didn't produce the amount of audio 
that the 811's did, and I'm sure that with the audio driver circuit I'm 
describing here will more than be adequate for four 811's in push-pull, 
parallel, should I ever want to 'go for broke' 
                  };->
   73 = Best Regards
   -Jeff KA5THB
   ka5thb@bigfoot.com
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