[AMRadio] Class C modulation

[John Coleman]

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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