[Amps] Ten-Tec Titan 10 Meter Input SWR

[Jim Thomson]

Date: Sun, 11 Jun 2017 16:33:19 -0600
From: Larry Benko <xxw0qe at comcast.net>
To: amps at contesting.com
Subject: Re: [Amps] Ten-Tec Titan 10 Meter Input SWR

<Gerald,

I have done this exact problem this many times for a high power ISM 
application at 13.56MHz.

1.) For example put an "L" network between the amp and the exciter.
2.) Adjust the "L" network for a 1:1 match at the exciter at full amp power.
3.) Assume the amp impedance at full power is 40-j10 (but you don't know 
this value)
4.) For a low pass "L" network with the shunt capacitor at the 
transmitter side a match occurs with L=340nH and C=112.5pF (you do not 
need to measure the component values)
5.) Now take the "L" network, reverse it, and measure the impedance with 
any VNA with the "L" network terminated in 50+j0.
6.) You will measure 40+j10 with the analyzer.
7.) The amp input impedance is the complex conjugate of the measured 
value or 40-j10.

The Smith Chart is not useless to solve this problem.

73,
Larry, W0QE

##  I used a slightly less high tech approach.
I put a hb  PI net with adjustable arco compression caps,or 
broadcast variables for the C1 + C2..and used the GM3SEK PI spread sheet
to calculate the values of C1 + C2  + the coil  for XXX  Q.
Most tubes will list the typ input Z anyway, so you can get pretty close on the 1st pass. 
Tweak tuned input PI Net  for min swr when amp driven full bore. 
Disconnected  output of PI net  from cathode, and substituted a  0-500 ohm pot. 
Install MFJ on input of PI net.  Tweak 0-500 pot for 1:1 swr on MFJ. 
Disconnect pot, and measure its  resistance. 
Then plug in the dc resistance value just measured into the GM3SEK spreadsheet,
but this time input the desired Q.  
Spreadsheet  spits out C1, C2, L  values.  
Measure the tube C  from grid to cathode with lcr meter.... it will be the same as what Eimac
sez it is on their spec sheet for any given GG triode. 
Since the grid to cathode C  is directly in parallel with the  C2 cap, the tube C is then subtracted 
from the spreadsheet C2 value. 
Tweak both caps +  coil with my  B+K 875-B  digital LCR meter to above values. 
Apply drive, then measure input swr.  This will typ result in being 75-90% in the ballpark for swr.
Then tweak both caps for 1:1  swr.
You are now done.   Well not quite.  Process is repeated for each ham band. 
If the tweaked up C1 + C2 values dont jive exactly with the spreadsheet, too bad, so sad.  Its now functional.

On a side note,  I installed  bird + CD watt meters  on both sides of my hb tuned input, that uses a pair of
4 x section, 18-540 pf per section broadcast caps...with all 4 x sections of each cap strapped in parallel. 
8 ga solid wire used for the 4 uh tapped coil.   Both caps padded on 160M only. 
Since the tube I used has a 50 ohm input Z...I  fed the output of the wattmeter into a 50 ohm dummy load. 
With 200 watts into 1st wattmeter, and C1 + C2 tweaked for  1:1   swr into dummy load, the output wattmeter
would also read 200 w.    Except on 15M.     On 15M only, with 200 watts  in... it only read 160 W out....and coil
would get  a bit warm...and a 1:1  swr.     Increased  uh on the 15M  tap by just 1/4  of a turn.....and a slight reduction in the value of the C1
+ C2 caps.  Now its  200w in....and  195 watts  out... and  1:1  swr. . 

I wonder how many folks have measured the power output of their tuned inputs ?  Point here is, you dont want the Q  too high on the upper bands. 
If I had not have measured the PO  of the tuned input, I would have ended up with less drive to the tube, cuz of tuned input losses.   This tuned input 
was for a 160-15m amp... no 10+12m.   I used a small 6 position switch for the coil taps.  And tapped for  160-80-40-20-17-15 
I should have used  a  10 position switch and added extra taps, to fine tweak Q on the low bands.   No big deal.  I like to keep the Q a bit on the higher
side if possible, even if that doesnt result  in covering the entire  160-80-40 band.  So one setting for CW..and one for SSB.   Kw  tune + load caps have to
be tweaked anyway  for  cw + ssb.   So both the manually adjusted tuned input....and manually adjusted kw tune + load caps..and  both bandswitches 
are all tweaked in one go.    I do not gang the tuned input band switch  with the  kw PI net band switch.   I want them separate. 

Jim   VE7RF