[TenTec] Jupiter selectivity

[Rob Atkinson, K5UJ]

hi bob,

<<<If it is not up to 375 watts on AM then how does one justify the RTTY an
SSTV limit of 650 watt limit, 50% duty cycle, 10 minute transmission limit?
After all RTTY  and SSTV is a carrier mode with 100% duty cycle.>>>

Rtty is like a continuous dead carrier if FSK is being used.  of course 
there is sideband energy from freq. shifting but not as much as with voice 
modulated carrier.  these modes are 100% d.c. not 50%.   you seem to think 
the duty cycle is 50% if you transmit for 10 minutes and rx for 10 minutes.  
i guess that's one definition, but usually d.c. refers to the percentage of 
peak power transmitted averaged over the actual time of transmission (10 
minutes), or % of time peak power is transmitted over the time of 
transmission (if on and off keying such as telegraph code is used).

another problem with AM v. RTTY is that in the case of RTTY, the amp is, or 
should be, tuned so that it maxes out at 650 w. if that is the power to be 
used, for the necessary drive.  With AM, the amp is tuned for 4 x whatever 
cxr you are using, if 375 w. then 1.5 kw, but then the amp is run with the 
375 w. cxr and two sidebands at an efficiency much lower than the 650 w. 
RTTY mode.  this is why it may be okay to run rtty key down at 650 w. for 10 
min. (but i have my doubts about that) but it is not okay to run 375 w. cxr 
AM if it can be done with this amp, for the same amount of time.  in AM the 
efficiency is maybe 40% -- the heat that has to be thrown off and dissipated 
is huge and i very much doubt it has the stock cooling to handle the job.

<<<As to your 160M operation, I suggest you look at your tuning very 
carefully.
My Centurion is a 1991 vintage, original tubes and the Bird 43P indicates
1200 watts plus with an Ip of about 700 ma and Ig of about 120 ma on 160M -
20M.  The tubes show very little color except when in a tune condition with
100% steady state carrier at the 1200 watt output level.  Then they warm up
nicely.  According to recent discussions, this is the opportunity for the
"getter" to de-gas the tubes. >>>

it sounds  as if there are differences between our two units.   mine uses 
the chinese 3-500zgs, which i think were used in the later models (mine is 
about 10 years newer than yours).   on 160 i have both my tune and load at 
6.9.   however i notice that the load cap has very little affect through its 
arc on 160 (a very broad output peak).   maybe it needs more C added to it.


<<<My experience is that there is NO PROBLEM running the amp at the 300 watt
carrier output level on AM... >>>

It just occurred to me that you may be running your exciter at a lower 
modulation level than 100% maximum bob; that would reduce your t.p.o. and 
help your situation somewhat.
I know that some hams prefer a strong carrier and less modulation.
I am amazed that you are able to get 1.5 kw out of your Centurion.   I know 
of no other Centurion owner who has done this unless they have done some 
modifications to the power supply.   Well, perhaps I have been too easy on 
mine.

QST ran an article about 50 years ago on how to run an outboard linear amp 
in AM service:
Technical Topics:  Linear Amplifiers for AM.  QST Feb. 1956 p. 39-41.

if you are an arrl member you can look at it online by going here:

http://www.arrl.org/tis/info/am.html

and scrolling down to the link for the pdf version.

73,

rob / k5uj

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