Bob,
I thank you for your correction and i think you have now set the record
straight.
All the best,
Rob / k5uj
>From: Robert & Linda McGraw K4TAX <RMcGraw@Blomand.Net>
>To: Rob Atkinson K5UJ <k5uj@hotmail.com>
>CC: Tentec <tentec@contesting.com>
>Subject: Centurion on AM or other carrier modes
>Date: Mon, 19 Dec 2005 18:58:08 -0600
>
>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
>
>
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