[Amps] Real danger to use the ORANGE button

[Jim Thomson]

Date: Sun, 24 May 2015 04:17:19 +0000
From: Charles Henry <k4vud at hotmail.com>
To: Jeff AC0C <keepwalking188 at ac0c.com>, "amps at contesting.com"
<amps at contesting.com>, Contest Internet <cq-contest at contesting.com>
Subject: [Amps] Real danger to use the ORANGE button

URGENT to all Yaesu radio owners with the orange Class A button.
ABSOLUTELY DO NOT RUN THOSE RADIOS AT LESS THAN FULL POWER OUTPUT SETTING.
I ran a FT-1000Mark 5 into an Alpha 76pac with Class A turned on and at about 50 watts of drive.
That adjustment resulted is burning up the Yaesu radio so badly no one could fix it;  not even Yaesu England.  Do not run Class A at any setting below full RF power with power knob fully clockwise!
 
The FT-5000/9000 can adjust somewhere between AB and full Class A.  I do not know about that situation.
 
Otherwise, lets hear some crowing about Yaesu Class A transceivers to go with the crowing K3 Lovers Club, eh?  73, Charly

##  I concur.... BUT..this ONLY pertains to the yaesu FT-1000 MK-V  transceivers.   I own two of the MK-V xcvrs..and they run blazing hot if used in Class A mode. 
As I noted b4... the mk-V  sucks a full 10A of idle current, keyed, and  no audio into mic...and zero watts out.  It also sucks 10A, with audio into mic.. regardless of
whether Po is set to 10 watts pep out...or the full 75 watts pep out.   On the newer FT-5000 radios and also the yaesu 9000, the cooling is vastly improved.  On both the newer
rigs, the bias is fully adjustable from the front panel.   The big problem on the MK-V rigs is the cooling.  There is virtually NO air going past heat sinks 1+2..... only 3+4. 

##  I have used class A  for brief test purposes..into a dummy load...while listening off freq..with a 2nd  MK-V.   When toggling the MK-V  between  Class A..and Class AB.... both running
the same 75 watts pep out....the difference between  the two modes is astounding.    The imd crap way off freq, drops  from real loud to zero, its that big a difference.   However the cooling on the 
MK-V  negates using it in Class A  on ssb.     However the bias can be tweaked for each transistor individually, for both the pair of driver transistors and also the pair of high power final transistors.
So FOUR bias adjustments to tweak for class A....and FOUR more bias adjustments  for class AB..... 8 x bias adjustments in all......and all 8 pots are INTERNAL. 

##  the best you can do with the MK-V  is to either lower the idle currents a whole bunch when in Class A mode.... OR  increase the idle currents when in Class  AB mode.   In normal class AB mode,
EACH high power transistor sucks 1A.....2 A for the pair..     In normal Class A mode,  EACH  high power transistor sucks 5A...10A  in total.    In normal  class AB  operation, the MK-V....when run at 60-120 watts 
pep output..is very clean vs running it at 200w pep out. 

##  When time permits, I will try experimenting with the 8 x internal bias controls.   2A  of idle is not enough..and  10A is way too much.   The idea here was to tweak the bias in Class AB mode, and increase the idle
current from 2A...up to 3-5A.   Listening off freq, on a 2nd transceiver  is a dead giveaway. 

##  The K3,...,while  having  superb zero key clix on CW.... is  nothing to brag about in SSB mode.  -29db pep for IMD3 is worse than a sweep tube amp from the 70’s.   However, a buddy found that by backing off
the power to just 30 watts pep out, the IMD drops like a rock, and hits a sweet spot.   The 30 watts pep out from the K3, is then used to drive the hb ARRL  2 VRF-151G, 250w pep output amp.   A 6 db pad is used on the
input of the arrl amp.  That combo works extremely well when run at 200w pep out..with real low IMD.    Next up is to change out the base resistors on the arrl  2 x VRF-151G  amp...and increase the NFB by 6db.   Then the gain of the amp will reduce
by 6db.  Then the 6db attenuator can be removed from the input of the amp.   On paper, the imd will improve by aprx 5.5 db. 

##  Increasing NFB on any amp is not all its cracked up to be.  Adding 3 db of NFB to say a SB-220  results in a 3db redux in gain. The same SB-220  now requires DOUBLE the drive power.   Adding 6db of NFB  would require
quadrupling the drive power.   Adding NFB is feasible on super high gain amps...like ur typ SS amps..and perhaps some high gain grid driven tetrode tube amps. 

##  it takes aprx 11 db of NFB  to reduce IMD by 10 db.....its almost a 1:1  correlation.   A lot of the older existing xcvrs can have their imd improved by increasing their idle current by 30-100% .  This doesn’t always work..since most older rigs,
like my FT-1000D  only have one bias adjustment pot..for the final pa section...and no provisions to tweak the bias for the driver transistors.    The entire chain has to work as a systems approach.    A method to get around the ALC induced imd, is
to use external 0 to –7 vdc  alc volatge, then feed it to thre xcvrs  rear alc input.   That way you can dial up any PO you desire, and not have to develop alc voltage..after the fact.  A simple setup with a 9 vdc battery feeding the outer ends
of a  50-100k pot  works great.    The wiper and one end is the V divider  supplies the 0 to –9vdc to the alc jack.   if you want say 65 watts pep.... just set the PO control on the xcvr to max.....then adj the alc voltage to 65 watts pep.  Done deal. 
Scream all you want, with mic gain cranked up..and wattmeter never goes higher than 65 w pep.....or what ever you set it for. 

later.... Jim   VE7RF