[AMRadio] Push-pull Triode HF PA layout question

John Coleman ARS WA5BXO wa5bxo2005 at pctechref.com
Mon Feb 6 22:20:41 EST 2006

Construction of a push pull circuit is a real difficult project if
started wrong.  It has been my experience to find that the plate tank
and grid tank do need to be as far apart as possible if plug in coils
are to be used and they need to be at right angles to each other.  Some
folks choose to use a band switched coil arrangement where the grid
coils and switch assembly can be in a completely shielded case of its
own or below the chassis that is sealed up for RF.  I have actually used
a Johnson Match Box 500 as a grid drive circuit years ago.  It sat on
the desk and I used that 300 ohm shielded cable that Radio Shack, sold
years ago, to feed it to the grids.  I put two RF chokes in the Match
Box to attach the grid leak resistor and bias supply to.  It worked quit
well.  I remember I could adjust the length of the twin lead cable to
get better results on some of the higher frequency bands.

I prefer the switched coil assembly in a shielded box and mounted under
the chassis myself.

You may find more ideas at


Good Luck 
John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of ne1s
Sent: Monday, February 06, 2006 2:31 PM
To: AM Radio Reflector; glowbugs at piobaire.mines.uidaho.edu
Subject: [AMRadio] Push-pull Triode HF PA layout question

Hello all, 

I humbly request the experience-based advice of this esteemed group. 

I am working on a project, part of which involves completion of 1930s 
Push-pull trode PA deck for 80M - 10M. Notice I said "completion," not 
"construction." This deck already has the plate tank components, tube 
sockets, and neutralizing capacitors mounted on a beautifull
chassis & panel. The axis bisecting the holes for the tubes is parallel
the front panel, the plate tank air variable (split stator) is
to the front panel, and the plate tank coil (plug-in jack-bar style - 
actually I'll be using BC-610 coils, link-coupled to the antenna) is 
centered directly behind the plate tank capacitor, with its axis
parallel to 
the front panel. What this deck lacks is a grid tank circuit, which I
to add. The tubes will be 812s or T-55s, and will be plate-modulated for
 - 300W output. 

What is the worse sin: lack of physical symmetry, or proximity of the 
gazinta to the gizouta? It seems that I'll have to compromize one or the

other in adding the grid tank circuit, based on the present layout. The
tank will be link-coupled to the exciter. 

Thanks for your assistance, 

AMRadio mailing list
Home: http://mailman.qth.net/mailman/listinfo/amradio
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:AMRadio at mailman.qth.net
AMfone Website: http://www.amfone.net
AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb

More information about the AMRadio mailing list

This page last updated 16 Dec 2017.