[Amps] 200-ohm tank and "un-un" in HF amp design?

[Jim Thomson]

Date: Tue, 22 Oct 2013 18:11:26 +0200
From: "Peter Voelpel" <dj7ww at t-online.de>
To: <amps at contesting.com>
Subject: Re: [Amps] 200-ohm tank and "un-un" in HF amp design?


&&&& so what ?   The point is no single 8877 is gonna  require
a whopping 3 uh   b4 the main 10m Pi-net  to get it to work on 10m. 
Maybe .3  to .5 uh.   BTW.. there is one helluva lot of current in that coil b4 the main
pi net..and also the main 10m coil.   Both need to be made of an appropriate
sized tubing...or strap. 

&&  For folks that want to parallel 2 or more bandswitch wafers to handle more
current....esp for the upper bands, there is a right..and wrong way  to do it. 
Aside from paralleling the COMS..and each bandswitch contact... you have to strap
the input + output correctly.  Wafer closest to the coils does all the various taps.
The COM on the wafer that’s furthest away from the coils  is the output..and goes off to the
load cap.   IE:  In on wafer #1...and out on the COM of last wafer.  Done that way, the current
will divide equally between  all wafers.  IF instead you tap on wafer #1  and exit on the COM of wafer
#1... you will end up with 95 %  of the current  flowing through wafer #1...cooking it. 

&&  You would be surprised how many get that wrong.  Same principle is also applied to paralleling 
anything to handle more current, like  diodes, caps, etc.  If you draw out the above multiple wafer
setup described above..the principle is to have a  total  combined input + output  length between 
each device  to be paralleled.   IE:  1 inch  to the input of  diode #1....and a 4 inch output  lead to diode #1.   
Diode #10   gets a 4 inch input lead....and a 1 inch output lead.  

&&  If you want more eff on upper bands.. on a GG amp... install a 100 pf NPO cap  between
chassis  and cathode.   This increases the cathode to grid C  by 100 pf.   That will require 
a slight tweak on the C2 cap of the tuned input.  The improvement in eff is well worth it.

Jim   VE7RF   



Who talked about a YC-156?
There are tubes with less capacitance around.

The coil value depends on the tube capacitance.

73
Peter

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson

##  No way in hell is 3 uh gonna work for the  lead inductance.  That
circuit is commonly called a  L-PI.    Typ coil values are .3 to .7 uh. 
And no, it doesn't make the amp unstable.   A buddy recently completed
a 80-10m YC-156 GG amp.   Now that tube is 35 pf  from anode to grid..
and rises to 50 pf when the grid flange is bolted to the chassis.