[ARC5] A Chicken Band "Leaner Applefryer" for Our Rigs

[Jay Coward]

Dave and All,
 There was an article published in Electronic Design magazine about 20+ years ago for an HF amp useing those MRF 455's.There was a pcb and kit of parts available but no heatsink or enclosure. I think it needed around 20A @12V for 160W out.
Jay



-----Original Message-----
From: David Stinson <arc5 at ix.netcom.com>
To: milsurplus <milsurplus at mailman.qth.net>; ARC-5 List <arc5 at mailman.qth.net>
Sent: Sun, Dec 16, 2012 10:31 am
Subject: [ARC5] A Chicken Band "Leaner Applefryer" for Our Rigs


The little RCA AVR/AVT rig is running well 
or hours at a time and I've been using it 
or QSOs and nets, so I've set the vibrator problem 
side for now and am concentrating on something
that will help with this and several other rigs.
 will get back to the vibrator... honest (crossed fingers).
Getting these SCR-183, SCR-288 and Light Aircraft HF sets 
orking is a lot of fun and I've made many QSOs 
ith them "as-is," but there are a lot of times when 3-8 Watts
in't gonna "cut it."  So I got hold of a Chicken-Band so-called 
150 Watt, two-pill Leaner" and decided to make 
 "foot-warmer."  The trucker won't miss it.... I hope....
Very common bipolor P-P amp with a pair of MRF455s.
his thing is a real "piece of work"   (eye-roll).
o bias- it ran hard Class-C.
o base stabilization.  No output low pass filtering.
ny signal so unfortunate as to find itself forced 
o drive this train-wreck came out 
ounding like a water buffalo gargling Drano.
The first step was to stabilize the bases and bias the thing linear.
 used .004 at the bases.  Yes, I know that's alot.
'm trading output and drive ease for stability and that settled
he thing down just fine.
Next, biasing the thing linear.  
irst thing that had to go was the wienie RFC in the 
ase circuit.  It was like 220 uH and, at 3.9 MC, 
ight as well have been a short.  I replaced that with
 manly, non-potted, non-ferrited 1 mH CHOKE
s the Good Lord intended!
(Everything following bypassed for RF, of course)
sed a voltage divider down from the +12 V supply, 
3 Ohms 10 W  and 5 Ohms in series, the 5 Ohm 
onnection point clamped with a power diode to ground
t one diode drop, about .7 volts and the bias picked-off
t this point.  This worked, after a fashion.  The amp was
airly linear and the transistors, being mostly always on,
ot just a touch warm.   There sure was a lot of  power
etting wasted as heat in that divider.
Next I tried an adjustable voltage regulator through
eries diodes.  Amp looked best at about .9 volts bias,
ut the transistors were "on " harder and got a little
armer.  No "scary" warm but still.
So I tried another divider, but this time with 2-ohms
efore the clamping diode to provide just a touch more
orward bias than one diode drop.  That divider still
ot warm.  Until I touched it during a test and it was cold.
urned-out my lead that provided the +12 to the divider
ad fallen-off and the base circuit was self-biasing
t about 1 volt full drive.  Transistors run a little warm
ut not alarming and the linearity is pretty good- acceptable.
nd no heat/wasted power in a divider.
Here's the circuit:
http://home.netcom.com/~arc5/AVRAVT/newbias.jpg
OK, Smart People:  Why is this working?
My antenna tuner is being my LPF for now.
 "real" filter set is next.
73 DE Dave AB5S

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