[TenTec] Centurion on AM or other carrier modes

[Robert & Linda McGraw K4TAX]

Rob:

First I have no problem in making my comments open to the reflector and I am 
doing so with this communication.  Originally, I elected a private e-mail 
due to the original subject matter.

As to running the Centurion at the 250 watt power level, it should be 
understood that my modulation in the AM mode, due to external asymmetrical 
processing, on positive peaks exceeds 100%.  Thus it would not be correct to 
run at the 300 or 375 watt carrier level based on a PEP rated output of 1.2 
KW.  This is my reason for running in the 200 to 250 watt power level.  I do 
monitor my modulation with a scope, SB-610, and depending on processing, I 
do adjust the drive to attain a carrier level which allows full positive 
modulation without peak compression regardless of the actual unmodulated 
carrier value.

In fact, 250 watts with 125% positive modulation is more effective in 
communications than 350 watts at 100% modulation.  And 250 watts lets the 
amp run a bit cooler as the duty cycle of the carrier is less (21%) as a 
percentage of rated output.

Regarding tuning, here is a quote from my manual:   "11] Many operators 
prefer to "rag chew" at reduced power levels (500 watts, or so, output) but 
want the option to abruptly increase power to 1200 watts without re-tuning. 
To do this you must first tune the Centurion for 1200 watts output.  You can 
then reduce power to the desired level by reducing the drive from your 
exciter.  The grid current will go down as you reduce drive and that is OK. 
The amplifier will not be quite as efficient at reduced power levels using 
this method but the 3-500Z tubes will not be damaged.  The efficiency of the 
amplifier at 500 watts can be improved by retuning but remember, if you then 
increase drive you MUST re-tune the amplifier."

Keep in mind that with AM operation, I am concerned with the PEP value, and 
thus not to operate such that peaks are compressed hence the maximum PEP 
value is 1200 watts.  In using this value, I must then tune the amp for 1200 
watts PEP output while the unmodulated carrier is only in the 200 to 300 
watt range.

I think we are looking at the operation slightly different.  I agree that 
plate efficiency can be improved by tuning at a specific power level.  Doing 
this, however, clearly abates the definition of "linear amplifier".  In true 
terms, a linear amplifier should produce a given output for a given input 
based on gain of the stage up to the maximum rated output.  i.e.  using a 10 
dB gain stage, then 1 watt input produces 10 watts output, 10 watts input 
produces 100 watts output, and etc.  Should the amplifier stage not produce 
the required output based on input vs. gain then the amplifier is into 
compression and is no longer linear in power function.

Of the two amplifiers that I have, the gain runs about 11 to 12 dB depending 
on band, with 20 meters having more gain due to the characteristics of the 
input network.

Recent measurements on one of my Centurion amps using a Bird 43P and Bird 
50.5 ohm load determined the following:

Input 4.5 watts {10 watt element}
Output 65 watts {100 watt element}
Gain = 11.9 dB

Input 10 watts {10 watt element}
Output 145 watts {250 watt element}
Gain = 11.6 dB

Input 50 watts {100 watt element}
Output 800 watts {1000 watt element}
Gain = 12.0 dB

Input 100 watts {100 watt element}
Output 1500 watts {2500 watt element}
Gain = 11.8 db

I conclude the slight variations in gain as stated above is due to the FS 
scale range of the Bird slug used and the accuracy therein.  At the same 
time I accept the 0.4 dB different between  the power levels and 
calculations as being normal.  Also, I would point out that this amplifier 
does produce 1500 watts output which is slightly higher than stated in the 
manual and the published specifications.  I attribute this to operation on 
20M, which seems to have a wee bit more gain, and I have a slightly elevated 
line voltage at the amplifier.

I still stand solid on the position that the Centurion can be safely 
operated at the 250 to 350 watt carrier level on AM, if properly tuned and 
staying within the duty cycle limits of 50% and 10 minute transmission 
limit.   I've been running the amp since 1991 with original tubes and fan 
and only one failure which was caused by a flash over of the plate tuning 
capacitor when it was being operated on 10 meters.

In my case, if it fries, then Tentec gets to sell some parts to K4TAX.

73
Bob, K4TAX


snip


>
> I enjoy your statements and find them educational and thank you for them. 
> There is nothing there to disagree with, however I believe in your 
> original comments you created the false impression that the Centurion may 
> be operated in AM service with a 375 watt carrier.  You wrote in part:
>
> <<<As to AM it works nice, just adjust the power
> in AM mode to 25 to 30 watts output, LSB, and run the mike gain where you
> would for SSB.  This power level is just about correct for driving a
> Centurion to legal AM limit of 375 watts carrier.  A scope on the output 
> is
> a big help in getting modulation set correctly. >>>
>
> I disputed you on that, and I notice now that you are stating to me 
> privately that you normally operate your centurion at 250 to 300 w. cxr in 
> observance of the 1.2 kw pep limit.   Regardless of the technicalities, i 
> believe you are obliged to make public a revision of your earlier 
> statement, as to not do so, will mislead potential Centurion purchasers, 
> and do them a disservice.    With a new Centurion at an eye-popping $2700 
> I think this is kind of important.
>
> I would also like to point out that in the Centurion manual it is advised 
> that the operator tune for maximum output for the given drive, i.e. back 
> off on the drive for 600 w. and tune for max. output there.  TT gives 
> tuning for 1200 w. then reducing drive for ragchewing as an option, but 
> recommends tuning for whatever power level is desired as optimal.   This 
> could very well be a recent addition to the manual.
>
> Merry Christmas and 73,
>
> Rob / k5uj