Date: Thu, 27 Jun 2013 09:52:47 -0700
From: Vic K2VCO <k2vco.vic@gmail.com>
To: amps@contesting.com
Subject: Re: [Amps] Electronic bias circuit shematic
I don't like the idea of RF-operated EBS systems because it seems to me very
hard to avoid
distortion at the beginning of a CW character or syllable when the system first
opens.
What I have done in my last few projects is to drop the bias from cutoff to
operating
level along with the t/r switching of the QSK/VOX system. Since there must be a
few ms
delay built into this circuit before and after the start of RF, there is plenty
of time
for the bias to change.
I don't see how an RF operated system can have any advantage over this, and it
is much
simpler -- it can be done with a couple of transistors or a reed relay.
## His problem is his idle power is a whopping 800 watts on idle. On the
russian
4CX-800s... its 225 ma per tube ! 450ma for a pair..and an insane 675ma for
the
3 holer..and all at the same 2200-2500V. You do the maths.
Those tubes have lousy imd on ssb, so to get the imd way down, the idle
current is cranked way up. A tri state bias works better than a bi state bias
scheme.
## OK, on CW mode, you don’t require EBS..whether you use VOX or QSK CW.
On CW, you can increase the bias V..and decrease the idle current... like crank
the
idle current WAY down.
## On any GG triode, same deal, BUT you cant go much below 5-10 ma of idle
current
on CW mode. Biased to cutoff or beyond cutoff, the GG amp will generate key
clix.
## On GG triodes that don’t require sky high idle power, on SSB, the various
ebs schemes
don’t buy you anything when using VOX SSB. None of em are fast enough to cut
off between
syllables...or they will distort. A little power is saved between
words..barely.
The only real power savings is just after the last spoken word..and when the
vox drops out.
On PTT ssb, they do work.
Jim VE7RF
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