[Amps] Electronic bias circuit shematic

[Jim Thomson]

Date: Thu, 27 Jun 2013 09:52:47 -0700
From: Vic K2VCO <k2vco.vic at gmail.com>
To: amps at 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