[AMRadio] Class C modulation

[Jay Bromley]

Hi John and all,
I had some 4-250s in the final of my old KW-1, then later switched them out
to 4-400s.  While I liked the looks of the slightly orange plates of the
4-250s and under modulation the 4-250s getting brighter in color, the 4-400
seemed to work better with the 810s modulators.  I can't show any proof, but
it just seem to modulate and have more headroom.  Was this in my head??  All
I had to go by was my monitor scope.

73 de jay/w5jay..


> Yes and some run multiple 833s for a 1KW output.  I have asked many times,
> why such and over kill in plate dissipation. Most of the answers I have
> received say that the BC rigs were not finely hone and engineered for high
> efficiency and they were made so that some mistuning would not cause a
> meltdown.  I can't see why an amateur XMTR that is tuned well an monitored
> would require the extra expense of overkill, not to mention the extra
power
> required for filaments, screen, and drive. I have never run 4-400s but it
> seems to me that a single 4-400 could easily run the old legal limit of
1kw
> input and deliver 750-800 watts output with 125-150% modulation applied.
I
> did run a pair of 813s (plate disipation of 150W each intermitant and I
> think I remember 100 watts CCS for each tube).  I ran 2500 Volts @ 400ma
for
> many years with that rig the modulators were the ceramic 4cx300s from a
> common supply. The screen supply was separate and passed through screen
> windings of the mod XFMR to acheive about 40% mod on the screens when the
> plate was at 100% the screen voltage was 400V and the screen current was
40
> ma.  The tubes were operated in push pull with plug in tanks. The grid
drive
> from came from a 6146 and the grid was 30 ma at 150 volts. the rig
delivered
> 790 watts to the dummy load and showed no color on the 813s except a
little
> at full sustained modulation.
>
> I don't really know a lot about the 4-400 but it seems like a lot of over
> kill to use 2 of them.  I later ran a push pull pair of 304tl's.  Very
High
> plate efficiency but it took 250 watts to lite em up and 500 volts bias at
> 250 ma grid drive. It took an 812 with 1500 volts to drive em.  I now run
a
> pair of 250ths  push pull and I can't tell any diferrence with 1 or 2 ( I
> was without  one for a while and Jeff WA5THB gave me the other.  He said
it
> just didn't look right with only one. HIHI
>
> GL, John, WA5BXO
>
> -----Original Message-----
> From: amradio-admin at mailman.qth.net
> [mailto:amradio-admin at mailman.qth.net]On Behalf Of Shirli Sieb
> Sent: Tuesday, March 19, 2002 1:13 PM
> To: amradio at mailman.qth.net
> Cc: jec at pctechref.com
> Subject: RE: [AMRadio] Class C modulation
>
>
> From: Ed Sieb, VA3ES:
>
>
>
> On Tuesday, March 19, 2002 12:53 PM, John Coleman wrote:
>
> > I can't really understand why any one would want to run a pair of
> > 4-400s as  finals in class C.
> > Maybe Class B or AB but not Class C.
> > I think they would make a much better AB modulator for a single
> > 833.
>
> Why do you say that, John?  Many 1KW BC rigs used 4-400 pairs for the
> class C stage.
>
> Just curious what your reasoning is.
>
> 73
> Ed,  VA3ES
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Ed Sieb, VA3ES  va3es at rac.ca
> P. O. Box 8377, Ottawa Terminal, Ottawa, Ontario,  K1G 3H8
> 613-738-8924  (H)
> AMI #529  Canadian Division Director.
> Member - Radio Club of America,  QCWA,  AWA
> Net Manager - Canadian Boat Anchor Net (3745 Kcs)
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
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