DX-100 Output

[Glen Zook]

You can get that kind of power out from the DX-100.
However, you are pushing your final amplifier tubes to
the maximum, or even beyond.

Remember that in the DX-100 and Apache (along with
most transmitters of the era) you are actually reading
cathode current and not plate current.  Cathode
current also includes the screen current, which, in a
pair of 6146 tubes is about 25 mA.

If you run a "meter" reading of 250 mA and the plate
voltage is 800 volts (this is about where both my
DX-100 and Apache run), then the actual plate current
is 225 mA and the power input is 180 watts.  Remember
that the maximum input for a 6146 tube is 90 watts
(the 6146B, which is NOT really the same tube type is
120 watts).  To get 135 watts actual output from this
power input is an efficiency of 75%.  Although the
6146 is sometimes capable of 75% for a short period of
time when new, the actual efficiency of the average
6146 is about 65% which will give an output of 117
watts at 180 watts input.

If you run a "meter" reading of 225 mA and the plate
voltage is 800 volts, the actual plate current is 200
mA and the power input is 160 watts.  At 75%
efficiency this will give an output of 120 watts, and,
at the much more "normal" efficiency of 65%, this will
give an output of 104 watts.

In the "practical" world, 20 watts (or so) difference
will not make any difference in your signal strength
at the other end when running 100 watts, or more,
output.  Now, if you are running 5 watts, and add 20
watts, then there is definitely a difference.  But,
once you reach the 100 watt level, then adding an
additional 20 watts will have no apparent effect.  You
have to have at least 3 dB or more power in the vast
majority of situations to show any difference in the
received level at all.  This means at least another
100 watts.

Next, in terms of tube life:  If you run the 6146
tubes at their maximum CW ratings they will have a
certain "finite" life.  If you reduce this by about
10% (11% in the case between 180 watts and 160 watts),
you will increase the usable life of the tubes by a
factor of several integers, often as much as 4 or 5
times the life.

RCA recommended that for AM operation that the power
input to the 6146 be reduced to 70% of that of CW.
This would make for a power input maximum of 126 watts
for the DX-100.  In "round" figures this would make a
power output of 82 watts at 65% efficiency and 95
watts at 75% efficiency.  At 800 volts on the plate,
this would make for an actual plate current of just
under 160 mA.  Adding the 25 mA screen current, this
would make a meter reading of 185 mA.

Now, in the "real world", the 6146 in AM phone
operation is capable of running more than 70% of the
CW rating.  Running at levels up to around 85% of the
CW rating seem to have little, if any, effect on the
life of the tube.  Over that and the tube life is
reduced, sometimes considerably.

I run both my Apache and DX-100 primarily on AM these
days.  The "plate" current meter readings (actually
cathode current reading) are normally kept around 210
to 220 mA which gives an input of about 150 watts and
a power output in the neighborhood of 100 watts.  This
results in much longer tube life than running them at
the additional 30 watts input.  The signal reports are
the same.

Also, exactly what type of wattmeter are you using?
It has been my experience that the vast majority of
wattmeters produced for the amateur market definitely
read on the "high" side when compared with a
commercial type of wattmeter like the Bird 43.  This
difference can be as high as 20% (even more in some of
the less expensive units).  The manufacturers tend
towards the "high" side readings to make the amateur
radio operator "think" that he/she is getting more
power out of their transmitters.  After all, who would
want a wattmeter that read significantly lower than
the actual output.

A lot of amateurs want to get the maximum power out of
their rig, thinking that a few more watts will make
all of the difference in a "pile up".  But, "it just
ain't so!", A few watts doesn't make any difference at
the other end.  However, considering the price of
tubes these days, and the fact that they are becoming
scarcer and scarcer, I definitely vote for a slightly
reduced power input and a greatly increased tube life.

Glen, K9STH




--- Jordan Arndt <jordana at NUCLEUS.COM> wrote:

My DX-100 runs about 120 watts AM outpour and about
135 watts CW output... with new finals tubes, it runs
about 115 watts AM and 125 on CW even on 10


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