[AMRadio] Re: Push-pull Triode HF PA layout question


ne1s ne1s at neandertech.com
Tue Feb 7 13:57:15 EST 2006


Thanks, John. I'll make a posting after I've implemented and tested what I 
conclude will be the best comrpomise. 

73,
 -Larry/NE1S 

John Coleman ARS WA5BXO writes: 

> 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 
> 
> http://wa5bxo.shacknet.nu/wa5bxo.qsl/pptriodes/pptriodes.htm 
> 
> 
> 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.  
> 
> Background:
> 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
> black-wrinkle 
> chassis & panel. The axis bisecting the holes for the tubes is parallel
> to 
> the front panel, the plate tank air variable (split stator) is
> perpendicular 
> 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
> need 
> to add. The tubes will be 812s or T-55s, and will be plate-modulated for
> 250 
>  - 300W output.  
> 
> Question:
> 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
> grid 
> tank will be link-coupled to the exciter.  
> 
> Thanks for your assistance,  
> 
>  -Larry/NE1S
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