[AMRadio] Neutralization Compromises - WD5JKO


Jim candela jcandela at prodigy.net
Sat Sep 24 14:14:31 EDT 2005



Hi All,

   On one of my QRO 20A projects I used a high Cgp Pentode, the EL-34. I had
to neutralize the final for any band above 160 meters. I could get a good
neutralization null when looking across my dummy load with my oscilloscope
where I could vary the scope vertical sensitivity as needed. I had to
disconnect and ground both the EL-34 screen and plate lines for this to
work. The problem with the 20a is that the grid circuit for the final varies
depending on the band. See hyperlink below. Therefore the neutralizing
circuit and adjustment was a compromise band to band. This is not ideal of
course, but a reality in a lot of vintage AM rigs. For example, the Gonset
G76 with a 6DQ5 final has varying neutralization settings band to band in
order to get a optimal null. In the case of the G76 if you really want to
work the upper bands, you should neutralize for that band. Sure as a
compromise, neutralize for the top band used, say 10 meters, and just live
with it on the lower bands.

  With the QRO 20a I was working on where the final amp runs AB2 linear
mode, I found that looking at the RF trapezoid waveform at 100% modulation,
and playing with the neutralization adjustment and final grid adjustment is
the best way to go. Lots of nasty things happen (like VHF parasitics) come
and go with the adjustments as well as non linearity in the Trapezoid
diagonal lines. A neon bulb insulated with a stick (near plate of final RF
amp) is handy since the color will shift to violet from orange when the VHF
stuff kicks in from a parasitic. For that 20a with the EL-34, getting a nice
clean trapezoid on 20 meters is the goal, then leave the neutralization
adjustment alone. Then go to 40 meters, and tune the final grid circuit for
the same. Then check 160, 80 meters where the grid circuit is not resonated,
just to see if all is OK. With this rig, I lost 15 and 10 meters with the
changes.

  My point here is that neutralization is often a compromise with a mult
band rig using a beam power, pentode, or tetrode tubes, and the variation
from optimal must be small, or undesirable things happen. Using the RF
Trapezoid monitor method with a clean sine wave drive is very useful for
many things including optimal neutralization. How many other Johnson,
Collins, WRL etc. out there have similar issues?

http://pages.prodigy.net/jcandela/CE20AQRO/RF_Schem.jpg

Regards,
Jim Candela
WD5JKO

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