[Amps] Available Class A in Yaesu

[Jim Thomson]

From: Charles Harpole 
Sent: Saturday, August 18, 2012 4:22 AM
To: jim.thom at telus.net ; amps at contesting.com 
Subject: Available Class A in Yaesu

Correction:  xcvrs with choice to xmit Class A are, at least, the following:  FT-1000MP Mark V, FT-9000Contest, FT-9000d, and FT-9000MP.  The latter three also have a bias adjustment on the front panel to slide Class A downward away from pure Class A in SSB or CW modes.  I burned up a Mark V by running it full Class A (only choice on that rig) and at about 40 watts... great techs tried to fix it and it blew two more sets of finals before I lost track of it.  Morale, do not run a Mark V in Class A.
 
 
Today, I own K3f, FT-9000d, and IC-7800, as well as KWM-2A, and several other rigs.  Very fun to compare them.
GL 73

 

Charles Harpole k4vud at hotmail.com 

### And none of em  (except the 400w  version of the 9000)  is capable of 100 w pep put in class A !     I own TWO  MK-V’s...  and alek is correct,   in class A,  you gotta  run it full bore at 75 w pep out.
Here’s  why.  In  Class A, the MK-V  sucks  a constant 10 A of current @ 30 vdc = 300 w dc input.     300-75= 225 watts  of CCS  diss !!!    Which is a blast furnace.    If you decide to run the mk v in Class A,
but say at only 40 w pep output,   now it’s   300-40= 260 watts diss.   The heatsink design + toilet roll tube blower is a fubar design in the MK-V.   There is NO air  going down between heat sinks 1 +2 at all,
due to the fact the motor on the end of the squirell cage blower is blocking off the airflow.   My fix  was to use some thin teflon sheeting, and tuck it in behind the blower, front panel side of blower. 
Then tape off the 3 x huge gaps between each of the 4 x top mounted heatsinks, sorta like tunnel rams.   Ok,  now you get a whole bunch more  air down all 4 x heatsinks.   The teflon sheeting is curved towards the rear
and is secured to the top lid.   The stock, upwards firing loudspeaker is removed, and tossed.   The teflon sheeting stops a ton of air being misdirected towards the front panel.   The blower has No housing around it,
like a normal squirel cage blower, rendering it semi useless at best.  

##  there is no point at all  in using Class A  on CW.   It buys you nothing.  There is NO imd problem on CW-FM-RTTY, etc.    What can be done to improve iMD, yet not cook the xcvr is to reduce the idle current
on the pair of final PA transistors, and also the pair in the driver stage.. in class A.    If you read the service manual, you will see there is FOUR individual  bias adjustments  for those 4 x transistor’s , 4 in total for Class A, then 
4 more adjustments for class  AB.   Eight in total.    You have 2 x choices.  Either DECREASE the idle current of all 4 of em a bit, when in Class A mode.... or  B,  INCREASE the idle current of all 4 of em, when in class AB. 

##  Then you can obtain excellent IMD.... that is midway  between the oem Class A  and class AB modes.  Class A on the MK-V  is overkill for imd.    Class AB isn’t good enough.  Class A idling current is a whopping 10A.
Class AB idling current is 2A.     Some where  between  2-10 A  of idling current  you will find  a good compromise between  max power output,  IMD, and heat..and eff.     On the newer yaesu xcvr’s, the idle current can be tweaked
via the front panel.   No point in having –60 IMD3..... when your table top kw amp is only capable of  -40 IMD3.     I’d  tweak the idle current  on the mk-v, such that the IMD is  SLIGHTLY better than the amp you are driving,
and no more.    Then you end up with the best imd that you can possibly obtain from the  COMBO  xcvr-amp.   Total system imd will always be the lesser of the xcvr or amp. 

Later....... Jim  VE7RF