[Amps] Manual tuned inputs

[K8MLM at aol.com]

I built my 4-1000 amp with a manual input.  Pi network used a 4uh  variable 
inductor and fixed switched input and output caps for each  band.  Adjusted 
the 10m position with the cap right at the base of the tube  and for the 
lower frequency bands,  just switched in more  capacitance.  It works great... 
switch the input band switch and tweak  the variable inductor.  1:1 input 
SWR every time.  Great power output  from the 4-1000.  Normally need no more 
than 85 watts drive.
 
Bob
K8MLM   
 
 
In a message dated 4/9/2010 8:59:17 A.M. Eastern Daylight Time,  
Jim.thom at telus.net writes:

I  think  Larry, W7IUV remarked about not liking manually tuned.. tuned  
inputs.... as having to have yet
one more thing to tweak, when changing  bands etc. 

Using BOTH hands, it takes 45-50  seconds to  switch  from 15m.. to 160m  
[worse case]  on a 
YU-148  amp... and that's including the KW  big  band switch, BOTH  vac  
tune + vac load caps..
and also the tuned input  band switch.. and  also  BOTH  broadcast air 
variables used for the tuned  input.

Both vac caps  use  turns counters.. and each   individual revolution  is 
split into 100 x increments [ typ 3"   skirt
has  100 x increments]    The 2 x  broadcast  variables  both use a large 
4"  skirted  Jackson bros  infamous
6:1  reduction drive.    The  JB ball drive  skirts are calibrated 0-100.. 
over a 180 deg arc. 

Having the   PI  net  tuned input  totally independent [ not  ganged]   
from the hi power PI  net, allows you to 
do cool  things.    Like, if you want to run 1/2 power.. 1/2 drive, etc.... 
 normally the loaded Q would double. 
If on 40m,.. simply throw the hi power  band switch to 80m.. and  loaded Q 
is back to normal.   Meanwhile
the  tuned input is not affected at all. 

Some  variations of the manually tuned.. tuned input include using a small  
4  uh  roller  coil.   Other
configs  I use include a  tapped  4 uh coil  [6-8 ga wire... 17 turns on a 
1.5" ID]..  suspended between
2 x  2240 pf  broadcast caps.  [ 4 x  section, 17-540 pf  per section, all  
4 x sections strapped in  parallel]

I found if the loaded Q was too high..[like  5-6] ..esp  on the high 
bands.. that the tapped  4 uh coil would
run warm on   17 +15+10m.   Since the GG tube  is very close to 50 ohms... 
we  used coax between output
of tuned input... and cathode.   For  testing, we put bird line sections on 
each side of the tuned  input....then
into a heath cantenna  dummy load.    With 200  w cxr...  we were only 
getting 150- 160 w into the dummy load...
on  the high bands!     By increasing the coil uh... by JUST   1/4 turn... 
power into dummy load shot up to 195 watts. 

This of  course, makes a helluva difference when driving any GG amp.  Now  
the  drive to the cathode is way up, more
grid current for XXX   drive, and more power output, etc. 

With the manually tuned input, once  tweaked for each band.... you just  
'dial by the numbers'  after  that... it's  that
simple.  With the low Q.. +    6:1  JB  ball drives  and  0-100 scale  
across a  4"  skirt  [  I also use the smaller 2"  skirts]
it's  repeatable every time.  Flip band switch, tweak caps to pre-sets.. 
bam,  dead flat input swr. 

One other major advantage.... the length of  coax  from xcvr to  input of 
amp is now a moot point.  We tried  everything from 
2'  up  to 85'.. and  in-between.. makes no  difference.   A dead flat 
input swr can always be obtained.  [  not like
my L4B's.. where > 3'.. and swr  rises] .   

The original idea  came from Ron VE7XR.... who  got it   from  Orr's very 
last book.  Orr  had used a manual  tuned
input.. but  only for  160 + 80m  monoband amps.   Ron tried  the  2 x 
broadcast caps  [ fair radio.. $16.00]  and
a tapped coil on his  2 x 4-1000  amp. [80-10m]  It  worked superb.  It 
also works good on any other GG  tube.  

The  small band switch I use.. is usually  either a 2 x  wafer.. or a 3 x 
wafer unit  [1.5 to 2"  diam, small ceramic,  SSON]
2nd wafer  just pads the broadcast cap on 160M.   160m  position of 1st 
wafer  pads the  2nd cap.   1st wafer does  the 
coil taps. [160m not tapped.. the entire coil is used.. freeing up the  
160m position for the padder]    The 3rd wafer
activates the  plate choke relay.  I use a  2 x piece 135-180  +   45-70uh  
plate choke set up.   I used 4 x 500 pf
HT50/58  caps  to pad each air cap on 160m. 

Just for a laugh..... [with  the dummy load set up and 2 x line sections]  
we rammed  800 w  cxr  through the tuned input
on EACH band  for 10 mins... and it  runs just fine.. no heating. Moot 
point anyway.. since in normal operation,  the tuned 
input is subjected to XXX  cfm  from the  blower.   It's a simple setup  VS 
 9 x tuned inputs... and  also  cover's  1.8 to 30 mhz
continuously.  My patience ran  out a long time ago, with trying to build  
9 x tuned inputs.    The  2 x caps + tapped coil
do take up space though.. which was not an  issue on the hb amps. 

later........... Jim   VE7RF
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