Seneca problem fixed

Glen Zook gzook at YAHOO.COM
Mon Nov 21 15:41:51 EST 2005

Thanks to all of the persons who offered advice and
suggestions on the problem that I was having with low
plate current on 2 meters on my Heath VHF-1 Seneca. 
The problem turned out to be one of those "strange"
things that "just happen".

There are several feed through capacitors (0.001 mfd)
that go from the outside of the various cages to the
circuitry inside.  Somehow the feed through that goes
from the center tap of the grid coils through an r.f.
choke (for 2 meters) or a 100 ohm resistor (6 meters)
to the grid of the clamp tube isn't doing its job.  I
added a disc ceramic in parallel with the feed through
and things are acting much more like they should. 
However, I now have to reneutralize the finals since
there is a change in the grid circuitry of the 6146s. 
But, the rig is now putting out about 55 watts on 2
meters (about 47% efficiency) and 90 watts on 6 meters
(about 64% efficiency) which is fine for a pair of
6146 tubes.  The finals now show slight signs of
improper neutralization but that is easy to cure.

I believe that r.f. was upsetting the clamp tube
making it "think" that the grid was operating
correctly when it wasn't due to the improper

I finally found the problem when checking the voltage
on the grid of the clamp tube which comes directly
from the "cold" end of the r.f. choke from the grid of
the final amplifier tubes on 2 meters and the "cold"
end of the 100 ohm resistor that comes from the 6
meter grid coil.  On 6 meters the voltage was fine but
on 2 meters my DMM acted like it does when trying to
measure the voltage in a circuit where r.f. is

Don't have any idea as to what happened to the feed
through, but obviously something happened!

Again, thanks for those who tried to help.

Glen, K9STH

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