[Amps] Manual tuned inputs

[Jim Thomson]

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