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[Amps] Reducing heat in tetrode amps with EBS

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Subject: [Amps] Reducing heat in tetrode amps with EBS
From: "Jim Thomson" <>
Date: Fri, 31 Oct 2014 18:57:06 -0700
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Date: Fri, 31 Oct 2014 11:28:56 -0400
From: "Paul Christensen" <>
To: <>
Subject: Re: [Amps] Reducing heat in tetrode amps with EBS

##  Paul,  you are not going from full class C cut off to normal operating
ssb bias.
That?s the old alpha bi-state ebs scheme.   With tri-state bias, its more
like toggling
between 20 ma to 180ma on a 8877.   Its only ever cut-off bias when in RX.


To your last point -- in the case of VOX operation, I think most tri-state
EBS systems rapidly go from class C in STBY, to class B on PTT key line
activation, then to class AB upon RF sense - and back again during the VOX
T=>R sequence.  Under normal hand/foot PTT operation, the amp is only
switching between the last two states and not three like it is under VOX.

In the case of the Alpha 77Dx, a 50V supply supplies cathode bias and sets
the amp in class C during STBY.  When the amp key line goes low, bias goes
to 20V and produces a slight amount of Ip.  This state/voltage is adjustable
via a bias control rheostat.  Finally, upon detection of 100 mW of input RF,
the amp becomes AB biased and cathode bias goes to 8.1V via a switched Zener
diode.  I have my 77Dx biased at stage two to produce about 30 mA of current
which is a bit more Ip idle current than the stock factory setting.  So, the
77Sx/Dx does indeed use a tri-state EBS system.

Paul, W9AC

##  I stand corrected. I checked my old notes..and it was the ARRL  QST article 
40 years ago that used the 2 state bias scheme, early 70s.  It was used for qsk 
cw operation, and to
cut off the plate current between dots and dashes.  It was all the rage at the 
time.  IMO, for
cw use, you are better off to simply increase the bias V  with a larger value 
zener etc, and 
reduce the idle current to a lower value... like 10-30ma.  On any GG amp, you 
cant cut off the
plate current and run it in class C..... you will get key clix.  10-30ma of 
idle will  result in no key clix
and next to nothing for idle power + heat. 

##  a local buddy back in the early 80s, had a 77-SX, 2 x 8877.   The problem 
with the 100 mw
input threshold on ssb was..... the xcvr he used, combined with some rf 
clipping, would result in the
100mw threshold being exceeded most of the time, rendering the ebs semi useless 
on ssb.  It did
work good on cw though, both qsk and vox cw.  Audio noise gates were not in 
vogue at the time.
The fix was to increase the threshold to .5 watt or higher. 

Jim  VE7RF

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