[Johnson] Viking II question(s)

[Glen Zook]

Even with the 6293 tubes installed instead of the
original 6146 types, you are running way too much
input power!  The maximum power input for a pair of
6146 tubes in class C CW is 180 watts input which,
under normal circumstances, will give about 110 watts
output when the tubes are new and you are operating on
80 meters.  If you are running 300 mA at 750 volts,
then you are running a plate input of 225 watts and if
the plate voltage is 800 volts, then you are running
240 watts input.  This is in excess of the rated power
by a significant amount.

The output power will drop as the frequency increases.
 At 10 meters, a pair of 6146 tubes often are in the
85 watt output range on CW when running normal power
input.

You will get much longer tube life and the guy on the
other end won't know the difference if you run the
finals at about 150 watts input which will give
between 90 and 100 watts output on 80 meters when
running CW.  On AM phone the 6146 is rated at a
maximum power input of 67.5 watts which is 135 watts
input for a pair.  That will give about 80 watts
output.

The 6146B, which is a different tube, runs 33% more
input power than the 6146/6146A.  However, in most
cases, replacing the earlier versions with the 6146B
results in problems.  For an explanation of this, see
the article that was published in Electric Radio about
18 months ago.  It is reproduced on both of the
websites that are at the end of this message. 
Replacing the 6146 with the 6293 works quite well and
the 6293 will outlast the 6146 by from 5 to 10 times
the life when run at the same power levels.

The 6293 is rated at 1 KW maximum pulse input power. 
However, when used in place of the normal
6146/6146A/8298 it should be run at the same power for
maximum life.  Back in the late 1950s and into the
1960s we would almost "kill" to get our hands on a
pair of 6293 tubes for our DX-100s, Viking IIs, etc. 
Whenever I get a chance to acquire some at a
reasonable price I usually do so.

As for grid current, a pair of 6146 tubes is really
much happier at 5 mA instead of 6 mA grid current. 
The tubes will normally put out the same power and
will last significantly longer at the lower grid
current.  Grid currents as low as 1.75 mA per tube
(3.50 mA for a pair) won't harm the tube and they
usually put out full power even at that level.

The power dropping off on AM phone as you increase the
grid current is probably due to the fact that the
tubes are definitely overloaded.  Also, the very low
grid current will also be a factor in the downward
modulation problem.

By the way, the maximum ratings published on the
Viking II was 180 watts input CW and 135 watts input
on AM phone.  Thus, Johnson was keeping within the
ratings of the 6146 tubes.

If your plate voltage is 800 volts, then you should be
running more like 200 mA on CW and around 170 mA on
phone.  225 mA on CW at 800 volts will get you 180
watts input.  But, again, for maximum tube life you
should cut the CW power input back to around 150 watts
and the AM phone power back to around 120 watts.  As I
said before, the station at the other end won't know
the difference in your power and your tubes will last
several times as long as when pushed to the maximum. 
When you overload the tubes, the tube life is
considerably shortened, often by a factor of 10 or
more.  

I am amazed that the rig hasn't had more problems
since you are overloading the high voltage supply by
about 50 percent.  That may be one reason why your 5R4
blew.  Also, the strain on the power transformer will
eventually start to show.  Of course, tubes are much
more forgiving than transistors.  But, considering
that they are getting more and more expensive every
day, I like to cut back slightly on the power that I
run and get the much longer tube life that results.

Your modulation problems and the creeping plate
current might just go away when the tubes are run at
the rated power input levels.  Tubes tend to creep in
plate current when overloaded until they finally "run
away" and go into self destruction.

Glen, K9STH


--- Telegrapher@worldnet.att.net wrote:

Found one of the two HV rectifiers (5R4) blown. 

I noticed before the fuse blowing incident that my
plate current would drift upwards slowly and its still
doing that.  But a bigger problem is that i've never
had upward modulation indications on the Ip readings.>


One of the things that is constant tho is that when i
load up the transmitter, i can never see the
reccommended 6ma Ig.  If you watch the output on a
wattmeter, i can see 110W out on phone, but only with
a trace of grid current.

=====
Glen, K9STH

Web sites

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

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