[Boatanchors] Need suggestions and question on antenna for DX-100

[Glen Zook]

Looking directly at the Heathkit advertisement for the
DX-100 it states as follows:

100 watts RF on phone - 120 watts CW - parallel 6146
final.

This is from page 53 of the Catalog Section in the
1956 ARRL Radio Amateur's Handbook.  

100 watts output at 135 watts input represents an
efficiency of 74%.  120 watts output at 180 watts
input represents an efficiency of 66.67% which is more
realistic but even then it is optimistic.  A 6146 tube
is generally in the 60% efficiency range.  A brand new
one can do around 65% but not for that long.

On page 52 of the DX-100 manual they say to dip the
"plate" meter at 240 mA on phone operation.  Now the
240 mA is actually cathode current and you have to
subtract an average of 25 mA for the screen current of
the pair of 6146 tubes to get true plate current. 
This represents 215 mA of actual plate current.  With
a nominal plate voltage of 750 volts this represents a
plate power input of 161.25 watts which is 26.25 watts
over the maximum input ratings of a pair of 6146 tubes
running in AM.

Now if you get 100 watts output with 160 watts input
then the efficiency is 62.5% which is reasonable for a
pair of new tubes.  Unfortunately, with a pair of 6146
tubes this is exceeding the ratings by almost exactly
20% where AM operation is concerned.  This can easily
reduce the life of the tubes by well over a times 10
factor.

The 750 volts is "nominal" plate voltage with 115 VAC
input from the power mains.  Considering that the
"target" line voltage by most power companies in the
United States today is 125 volts +/- 2 volts (with
many running at the "+" 2 giving 127 volts during the
summer months) then the "nominal" plate voltage (under
load) is going to run between just over 800 volts with
123 volts AC applied and about 830 volts with 127
volts applied.  

As such, you need to cut the "plate" current to 250 mA
maximum when running CW if the line voltage is 123
volts (which represents a true plate current of 225
mA) and to about 240 mA (representing a plate current
of 215 mA) if the line voltage is 127 volts.  Of
course this is maximum and for best tube life you need
to reduce this by at least 10% to 15%.

For AM operation to stay withing the maximum ratings
of the 6146 tubes you need to cut the plate current to
205 mA on the meter (180 mA true plate current) if the
line voltage is 115 VAC.  If the line voltage is 123
volts then you need to reduce the plate current meter
to just under 195 mA (just under 170 mA true plate
current) to get 135 watts input and if the line
voltage is 127 volts then you need to reduce the meter
current to around 185 mA (true plate current of 160
mA).

Of course you need to adjust the plate current due to
the absolute plate voltage.

Now the Johnson Viking II, which also uses a pair of
6146 tubes in the final, is rated at a maximum INPUT
of 135 watts when operating using AM.  This is inline
with 
the maximum RCA ratings of the 6146 tube.  Frankly,
Heath was grossly overrating their equipment (the
Apache ratings are just as bad as those of the DX-100
series) where AM operation was concerned.  The result
is greatly reduced tube life.

Anyway, if you keep the maximum input to the DX-100
under 150 watts on CW and 100 watts on AM the life of
the final amplifier tubes will definitely be increased
several times and the station on the other end will
never know the difference.

Glen, K9STH


--- Rodger <WQ9E at dtnspeed.net> wrote:

I agree that 125 is a bit optimistic but that is
straight from Heathkit's specifications for the
DX-100.  In any case, the point remains that be
careful using smaller modern antenna tuners with these
high level AM rigs.
 
Of course if you reduce the input to increase tube
life (not a bad thing to do!) be sure and also reduce
modulator power accordingly









Glen, K9STH

Website:  http://k9sth.com



 
____________________________________________________________________________________
Need Mail bonding?
Go to the Yahoo! Mail Q&A for great tips from Yahoo! Answers users.
http://answers.yahoo.com/dir/?link=list&sid=396546091