[Heathkit] DX100 ANT

[Glen Zook]

4.5 mA was the original recommended grid current for
the 6146 and 6146A/8298.  BUT, RCA dropped the maximum
control grid rating to less than 3.5 mA per tube after
experience showed that the tube life was greatly
reduced.  It has been my experience that running no
more than 5 mA grid drive to a pair of 6146 or
6146A/8298 tubes definitely increases the tube life
and has no effect that can be noted on the
performance.

You can run up to 7 mA for a pair and not exceed the
ratings.  However, the difference in the life of the
pair of tubes is definitely worth running at the lower
grid drive.  There will be no difference in the output
power and I have never noted any difference in the
modulation, at least anything that can be noted on the
air.  Possibly if you are using some pretty good test
equipment to look at the signal, but nothing that the
other station is going to be able to tell.

I run my Heath TX-1 Apache all the time at 5 mA grid
drive.  The audio circuit has been slightly modified
(changing 3 capacitor values - the article was in
Electric Radio a while back and can be found on the
K9STH website, URL listed at the end of this message).
 Frankly, I get "rave" reviews from the group that
hangs out on 75 meters that is VERY critical of
transmitted audio.

There is a notation in the Johnson Viking II manual
about not running more than 6 mA grid drive to the
pair of 6146 tubes in the final amplifier.  I agree
with this caution.

The screen current should be around 12 mA per tube. 
When the "plate" meter is actually in the cathode you
have to subtract the screen current from the reading
to get the actual plate current.  On transmitters like
the DX-100 and Apache subtracting 25 mA is easy to do
mentally and gives a pretty good idea as to the plate
current.  Besides, the accuracy of the meter in
amateur transmitters isn't all that good and
subtracting 25 mA works pretty well.

Glen, K9STH


--- "Robert & Linda McGraw (K4TAX)"
<RMcGraw at Blomand.Net> wrote:

One other factor relating to reduced tube life is
power dissipation of Grid #2.  It is rated at 3 watts
up to 60 MHz. and is reduced to 2 watts at the tubes
upper operating frequency of 175 MHz.  With 250 volts
applied that is a screen current of some 12 ma.  A
class C amplifier incorrectly and lightly loaded may
exhibit higher than normal screen currents.  Heath
make no provision to monitor screen currents.
 
For a pair of tubes, 6146 or 6146A's, the recommended
control grid current is 8 ma MAX or 4 ma per tube. 
Experience has shown that drive levels, producing
upwards of,  but not exceeding 4 ma per tube, lends to
better modulation characterizes.  The safe and
conservative range is in the area of 3 to 3.75 ma per
tube or 6 to 7.5 ma total.  It is important to know
that the meter and associated meter shunt are such
that accurate current values are indicated.  For AM
operation, drive values of less than 3 ma per tube 
have been proven to cause modulation to suffer.  It is
quite common, although Heath doesn't incorporate the
concept, is to modulate the driver so as to attain
higher drive levels on positive modulation peaks. 
These short duration peak levels cause relative no
increase in average driving power but do significantly
increase the positive portion of the modulation
envelope.










Glen, K9STH

Website:  http://k9sth.com


 
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