[Amps] 8877 input Z

[Jim Thomson]

Date: Fri, 28 Mar 2014 13:54:41 -0400 (EDT)
From: TexasRF at aol.com
To: n7rt at cox.net, amps at contesting.com
Subject: Re: [Amps] 8877 input Z

Hardy, the 54 ohms is a nominal figure that will vary with rf drive level.  
Certainly close enough for designing input networks. Higher drive levels 
will  result in a lower input impedance; lower drive levels will result in a 
higher  input impedance.
 
The 42 pF is the capacitance between the cathode and grid primarily. So,  
you will need to provide an inductance to cancel that out. The inductance 
will  be different for each band of course.
 
73,
Gerald K5GW

##  The function of the tuned input PI net is three fold.
1-provide a Z match between xcvr and cathode of GG tube. 
2-provide flywheel action, reduce imd, etc, since the tube only conducts for 210 degs of each cycle.
3- act as a LP filter..to provide a low Z path to all harmonics of the drive signal. This will increase eff. 
This has to be done at the cathode and not between the xcvr and input of amp.  

##  as u can see, a broadband tuned input wont work. On a HB amp, where you have some room to
play with,  I dumped the idea of using  6-9  bandswitched tuned inputs.  Instead, a pair of 4 section
2160 pf broadcast variables are used...padded on 160m...... and a tapped 4 uh coil.  Each of the
4 sections of each cap is 17-540 pf.  All  4 sections strapped in parallel.  Coil is either 6 ga
solid cu wire..or  .187 inch tubing..or  .25 inch tubing.   Coil is  17 turns on a 1.5 inch ID...
hanging in mid air between the two caps.   2 uh on 80m, 1 uh on 40m etc, etc. 

##  OK, now you can make the tuned input Q anything you want.  SWR is always dead flat from
1.8 to 30 mhz continuous.   However, on the upper bands  I measured  160w coming from the 
output of the tuned input and a 50 ohm dummy load.   200 watt was applied through a 2nd wattmeter. 
The fix was to reduce Q slightly, by increasing the coil by .25 of a turn.  Problem solved...and now
195 watts on the output side of the tuned input. 

##  A pair of 6:1  jackson bros verniers, with 0-100 marked over 180 deg of the 4 inch OD skirt  makes 
for easy pre-sets.   You can also get the jackson bros drives in 2 inch OD skirts.   You can easily stuff
1kw through this type of tuned input.   Works superb..with the caveat that it takes  up some room below
the chassis.  

Jim VE7RF