Rob Atkinson ranchorobbo at gmail.com
Sat Jan 9 17:07:20 PST 2010
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I am a Centurion owner. I assume you plan to operate typical SSB and
CW. It is okay for those modes. It has good metering and the tank
circuit and plate h.v. choke seem good. I get 1 KW on 160, and 900 w.
on 17 meters. Getting full rated output on some bands may require a
bit more than 100 w. drive. There was an issue with the stock plate
supply transformer but I take it your rewound transformer addressed
that.
### I did a repair job on a Centurion yrs ago, for a local buddy. We could
get well over 1 kw on it.. on any band... BUT the xcvrwas a FT-1000-D...
with loads of drive. On 160m. the load cap did not max outeven down on 1801
khz... into a dummy load. However, the load cap tuned ... 'sluggish'The load
cap did not hit a definite sharp peak... but was broad in it's tuning. It
appears looking at the schematic, that ten tec used too small a tune cap.. and
insteadincreased the tank coil value. IE; the loaded Q is less than optimum.
With too small a tunecap.. the load cap also ends up smaller than optimum. It
all works... but with a real low loaded Qon 160m, the load cap is sluggish
during tuning. Wide excursions on the load cap resulted in wattmeter dropping
off slowly on either side. I agree with Bill here, I'd use the biggest NPO
5 kv ceramicdoorknob cap HEC makes .. which I think is a 160-170 pf unit.
In this case, you would require 2-3 of emin parall
el. Allen bond has loads of the 120 pf variety, NPO.. cheap. I wouldn't
use ceramic discs.. and I doubt youcan get em in NPO..... but probably a lot
worse Temp Co-efficient. The last thing you want is a padder, that
drifts.Alpha tried that stunt on their 77-DX /SX.. and used a 1000 pf 5 kv
ceramic doorknob as a 160m loading padderand it drifted badly. [ the 1000 pf
doorknob is only good for 1 A CCS on 160m anyway] In the alpha case, the
fix was to replace with 2 x 500 pf 5 kv doorknobs. Each 500 pf doorknob is
good for 3 A on 160m... so 6Ain total - zero heat. Even though the TC of
the 500 pf caps is better than the 1000 PF unit... it's still not as good as a
100-170pf NPO cap. That's not an issue, since the pair of 500 pf caps
won't heat up to start with. There must have been several configs of
Centurions..on 160m. Obviously, if the load cap max's out below 1950
khzsomebody at the factory didn't do a real test. Throw in some SWR, and y
ou may well want to throw in 500-1000 pf of extrapadding C. You may well
not have a padder drift problem on ssb / cw. RTTY is another matter.
later......... Jim VE7RF The only improvement I know of you might want to
consider is adding
250 pF of capacitance to the loading output pi network to get a tad
more output and load the amp better on 160.
There is actually an unused lug on the band switch directly opposite
the Tune air variable that is only switched in when the band switch is
in the 160 m. position. All you have to do is solder a wire around 6
inches long (I used no. 14 stranded insulated house wire) to the lug
and drop it down between the bandswitch and Tune cap to the bottom of
the cabinet where the other loading padder fixed value caps are and
put your cap in there between the end of your wire and the ground lug.
You'll see what I mean if you get the amp and take the cover off and
look around in there. I used a CRL doorknob because I had it on
hand. You could use a high voltage disk ceramic too for less $$$ I
guess.
Without the extra cap the load cap on 160 maxes out before you get to
the sweet spot from which you back off a bit. If you only operate up
in the top 50 KHz of 160 you may not need it.
73
Rob
K5UJ
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