[Johnson] re: Valiant

[Glen Zook]

First of all, get the station grounded!  I have been
doing seminars, etc., on both lightning protection and
grounding for quite a number of years.  A synopsis of
both my lightning protection and r.f. grounding
presentations are on the two websites listed at the
end of this message.  What I recommend is not the
"only" way of doing things, but they have been proven
in many hundreds of commercial two-way, microwave, as
well as amateur installations.  Also, what I recommend
can be accomplished with the minimum of cash outlay
since it does not require and "special" manufacturers
devices.

Next, it sounds like the final amplifier tubes are
"taking off".  The spurious frequencies being
generated are causing the capacitors to overheat.

Check your neutralization very carefully.  There may
be a capacitor, or two, in that circuit that is
causing the problem.  Also, you might have to replace
the suppressor coils in the plate leads.  The wires
don't change, but the resistor that they are wound
upon could have been "cooked" and be of a much lower
value than they were originally.

Now, the 6146 is rated at only 67.5 watts INPUT when
used in AM service.  This is per RCA specifications. 
They are rated at 90 watts maximum input when used on
CW.  If you are running them at 300 mA, and the plate
voltage is in the neighborhood of 750 to 800 volts
(which many Valiants run), then you are running them
right at their maximum ratings.  The Valiant actually
measures cathode current and not true plate current. 
Therefore, you have to subtract about 12.5 mA from
this reading for each tube to allow for the screen
current.  That makes a little over 260 mA for the
plates.

At this level, you should be putting out around 120
watts from the 3 tubes.  This allows for 60%
efficiency.  With brand new tubes this could be a
little higher, especially on 80 meters.  Of course
this is carrier and not the peak that is 4 times the
carrier power for 100% modulated AM.

If you cut the power output back to about 100 watts,
the station at the other end isn't going to be able to
tell the difference.  However, your final amplifier
tubes will last between 5 and 10 times longer than
when run at maximum ratings.

Glen, K9STH


--- skip <k3cc@fast.net> wrote:

After abt a 10 to 15 min steady QSO  the caps from the
bandswitch start to get hot. Then after couple of more
transmissions the plate current goes to Full scale
from a max loading of 300 ma.

=====
Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco

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