The AM Forum

I was looking at the schematic of my KW-1, and also the schematic of a Johnson Valiant... and I noticed that the neutralization cap connects directly to the plate terminals of the rf output tubes.

When I looked at the schematic of a Johnson 500, the neutralization cap as connected to the tank side of the plate blocking capacitor (which seems like a better design from the standpoint of the peak voltage across the neutralization cap).

Is there a subtle reason why the designers of the KW-1 and the Johnson Valiant would have chosen to use the approach they used?

Stu

The Gates BC1-T puts the neut capacitor on the tank side.  The reason I can see for running it directly to the plate is the possibility of rf phase shift through the blocking capacitor.  This would more likely be a problem on the lower frequency bands where capacitive reactance would be the highest.  The Valiant and KW-1 both cover 160, but the Johnson 500 is one of those "all-band, 80-10m" rigs common in the 50s.

My homebrew rigs each use two neut caps and each one  goes directly to the plate of one tube in the push-pull circuit, since I don't use a blocking cap; the +HV is series fed to the plate through the tank coil.  I believe the unmodified T-368 uses series feed and no blocking cap, so the neutralising cap would go directly to the plate.

Phase shift is indeed an issue and Im rather surprised the Johnson 500 did it that way. It would be interesting to put one on a SA and see how much the cancellation holds across the bands.
It works well for a BCB TX thats set and forget but it tends to be a problem in multiband rigs to find the sweet spot. Of course the lower cost just "may" have been a factor.

Carl