[Amps] Linear amplifier tuning

[Jim Thomson]

Date: Tue, 20 Dec 2011 19:20:48 -0600
From: Edwin Karl <edk0kl at centurytel.net>
Subject: [Amps] Linear amplifier tuning

The discussion about tuning would not happen if we all still used 
o'scopes with a
trapezoid pattern. On such a pattern you can clearly see "flat topping" 
when the output
loading cap is too tight, a slight reduction in capacity results in a 
clear, sharp point
at the end of the triangle.

Additional tuning just upsets the whole thing. While an out put meter 
might again
see a small increase in power, the additional 10-20 watts won't make a 
difference,
but the splatter from your flat topping amplifier will.

The aforementioned 5% change in output loading is a good rule of thumb, 
lacking
a 'scope

Merry Christmas to all!

ed K0KL

##  You can easily achieve the same effect with any good pep wattmeter, like either
a Coaxial dynamics or an array solutions  'power master'.   Just alternate between the
tune and load cap  a few times, till u have max po one the wattmeter.  Then if the
load control is cranked a bit more UN-meshed, till the po  drops 2%, ur done. 
All u are doing by UN meshing the load cap is  you are setting it where it would be , IF peaked for
even more PO.  

## another method, that amounts to the same thing..is to just use slightly more drive, then
peak the amp up for say 1.6 kw pep out, instead of 1.5 kw pep out.   Then switch to 
ssb/cw, etc... and reduce the po from the xcvr slightly, such that pep out on the pep meter
is  now 1.5 kw pep.  Then it's linear, plus it will now handle any overshoots from the xcvr.

##  i just pulse tune my amps anyway... with 60 wpm dots.  On the  yaesu MK-V, in the
menu, you can shorten/lengthen  both dots and dashs, individually.  Default is dots= 10
and dashes = 30.   For normal CW use, I use  dots=10... and dashes =36. 

##  For pulse tuning on CW,   I reduce the duration of the dots...down to dots = 7
With dots = 10.... duty cycle is 50%  on the plate current meter.   By varying the
dot duration,  from  1-10 in the menu,   the  duty cycle can be varied from  10-50%.
At dots= 7, it's about 30%..which is just fine. 

## a trapezoid pattern involves sampling 1st the xcvr's output... and also the amps output,
and that will work too.  But if just sampling the amps output only [xmas tree pattern on ssb],
ur wasting ur time, since it won't show flat topping unless it's really gross flat topping. 

##  If u have flat topping, u don't have the amp tune dup for max po  for XXX  amount of 
drive anyway. 

## carl is correct. depending on loaded tank Q  on a band per band basis, you may well
have to tweak the TUNE control dead last.   It's  Ok  to have the LOAD control slightly
more UN-meshed, but it's a disaster if the  TUNE control is off resonance.  On my drake L4B amps,
on 80m, the tune control is tweaked for max po with just 25 w of drive.  After that, the tune control
doesn't get touched, just the load control...and that's at 800 ma, and more drive of course.  On most amps
the TUNE control needs tweaking as drive levels increase. 

##  It makes life a lot easier if you replace the knobs etc,  on your  tune + load control..and just replace
em both with jackson bro's  6:1 redux  ball drives.  Then you have big diam skirts, which are calibrated
from 0-100  across  180 degs.   Then it's silky smooth tune and load adjustments.  Then just log the
tune + load settings  for  cw/ssb  for  each band.   After that, it's  just ..  "dial it up by the numbers"  

##  with vac tune and load caps, it gets even better, since most are like 14-24 full revolutions from
min to max C.   With the old  Groth type  turns counters, they not only count turns, they also  calibrate
the 3"  diam skirt  from 0-100.  A 3"  diam skirt has a 9+ inch circumference.  A 500 pf tune cap is aprx
18 turns from min to max....or aprx   28-30 pf per knob revolution.  OK, now divide 28-30 pf  by 100
x small increments on the 3"  skirt..amd each increment is now just .3 PF  !!   The same groth can also be used
on any roller coil.  Most other's  will  split each rev on their turns counter down to tenths.  The frontier engineering
turns counters are digital readout, and also count down to 1/10ths of a rev. http://www.freng.com/   [$120.00 each]

## as long as the SWR hasn't changed much,  dialing up.."by the numbers"  makes life a lot easier..whether air caps
or vac caps are used.  BTW, jackson bro's in the uk has just informed us that they will again be making their various
6:1 ball drives, and plane 6:1 drives only...after xmas.   They are things of beauty. http://www.mainlinegroup.co.uk/jacksonbrothers/index.htm

later...... Jim   VE7RF