Off subject

[Glen Zook]

If you want to fry your final tubes, tune this way!

The proper method for tuning tube type equipment is
the old advance the antenna loading and then "dipping"
the plate current using the plate tuning capacitor.
You do this until the input to the final amplifier is
a whatever power level that is recommended by the
manufacturer, or, less if you want to run less power.

The grid current for a pair of 6146 tubes is a maximum
of 6 mA (3 mA per tube), but they will last much
longer if you don't run more than 5 mA (2.5 mA per
tube).

It is possible to overload the final tubes and get
more power output than what they are rated for.  Also,
if the neutralization is not "right on", maximum
output and minimum plate current will not be at the
same point, resulting in more power being dissipated
by the plates of the tubes, which can result in things
from shortened tube life to immediate destruction!

A pair of 6146/6146A/8298 tubes has a maximum input on
CW of 180 watts, on SSB 170 watts per RCA
specifications.  On AM phone 135 watts input is the
maximum.  The 6146B/8298A have approximately 33
percent more power input.  However, in most equipment
designed for the 6146/6146A/8298 substitution of the
6146B/8298A results in problems.  You can see the
article in Electric Radio a couple of years ago or
else go to http://home.attbi.com/~k9sth or
http://home.attbi.com/~zcomco to read this article for
free.

The same thing with other final amplifier tubes.  Each
has a maximum power input rating and a maximum plate
dissipation rating.  For best tube life, you do not
want to exceed between 80 and 90 percent of this.  You
can go to 100 percent, even more, and many amateurs
who use the old "boat anchor" equipment like the
Apache, DX-100, etc., think that running 180 watts
input on AM phone is great.  However, the difference
in power output and the signal quality received on the
other end is, for all practical purposes, nil between
180 watts input and 130 watts input (which puts the
6146 tubes back in the proper operating parameters for
AM operation).  The life of the tubes will be between
5 and 10 times as long with the lower power.

What you have to do is to multiply the plate current
on the meter by the plate voltage.  However, most, but
not all, of the commercial boat anchor transmitters do
NOT actually read plate current, but read cathode
current.  This includes the screen current which must
be subtracted from the cathode current to get the
plate current.  In the case of a pair of 6146 tubes
this is normally between 20 and 25 mA.  You need to
look at the schematic of your transmitter to see if it
reads plate or cathode current.

For example, if a DX-100 transmitter running a pair of
6146 tubes is properly "dipped" at 225 mA and the
plate voltage is 750 volts, then you subtract 25 mA
from the reading.  This gives a "true" plate current
of 200 mA.  Multiplying this by 750 volts (0.2 X 750)
give a power input (not output!) of 150 watts.
considering the normal efficiency of a pair of 6146
tubes, this will result in a power output of about 85
to 90 watts on 80 meters and drop to about 70 to 75
watts on 10 meters.  Sometimes, with a good pair of
new tubes, you will get 5 to 10 watts more power.
But, remember that the guy on the other end won't be
able to discern if you are running 85 watts out or 95
watts out.

Yes, you can load the transmitter to get 110 watts out
on 80 meters.  But, if you look at the plate input
power you will definitely be beyond the recommended
power input for the tube.  It will work, for a while!
Then, depending on the individual tube(s), they will
go "soft" in from a matter of minutes to weeks.  It
just depends.

There are a few transmitters (the Hallicrafters HT-37
comes to mind) that you do tune for maximum output
(there is no provision on the meter to read plate
current).  However, the voltages in the rig are lower
than many other rigs and thus you can't exceed the
maximum power input for the tubes.  But, these rigs
are a very small percentage of the boat anchors out
there.  If there is a plate meter, you "dip" the final
to tune it properly.  If maximum power does not occur
with minimum plate current, then the finals need to be
neutralized.  However, you still tune for minimum
plate current if you don't want to "fry" your tubes.

Glen, K9STH

--- Dave Edwards <kd2e at COMCAST.NET> wrote:

Also same as near everything before 1975 or so...just
tune for maximum output.


=====
Glen, K9STH

Web sites

http://home.attbi.com/~k9sth
http://home.attbi.com/~zcomco

__________________________________________________
Do you Yahoo!?
Yahoo! Mail Plus - Powerful. Affordable. Sign up now.
http://mailplus.yahoo.com

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.asp?list=HEATH
To post - HEATH at LISTSERV.TEMPE.GOV
Archives - http://interactive.tempe.gov/archives/HEATH.html