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## Re: [Amps] AL1200 on AM - max carrier?

 To: Gary Schafer Re: [Amps] AL1200 on AM - max carrier? Robert Logan via Amps Robert Logan Fri, 31 Oct 2014 07:34:58 -0500 mailto:amps@contesting.com>
 ```So am I. Sent from my iPhone On Oct 30, 2014, at 11:46 PM, "Gary Schafer" wrote: The legal limit on AM is 1500 watts PEP, just as it is for all other modes. There is no 375 watt carrier "limit" set by the FCC. But it does work out to 375 watts of carrier when you modulate to 100% on positive peaks. You could run more carrier and modulate at a level less than 100% and still maintain PEP below or at 1500 watts and be legal. Or you could run less carrier and modulate over 100% on positive peaks and still maintain PEP at or below 1500 watts. A 100% modulated carrier is when the audio (side bands) voltage equals the carrier voltage. This doubles the composite signal voltage. Doubling the voltage gives you 4 times the power (PEP). Ohms law gives this to you E^2 / R = Power. So if your amp is tuned up for 1500 watts PEP you would drop your carrier power to 1/4 that value or 375 watts in this case. When you fully modulate to 100% your peaks will hit 1500 watts. In this case you do not want to modulate over 100% even if you were to have your negative peaks limited to less than 100% as you would overdrive the amplifier and cause distortion because of flat topping, just like you would if you were on SSB and overdrove the amp. With a plate modulated transmitter you can run over 100% positive peak modulation (it is common to run 120% positive modulation) with out distortion as long as the negative peaks do not exceed 100%. This increases the PEP along with the average audio power in the signal. This can be done without changing the maximum plate load as you would have to do with a linear amplifier. With a linear amp if you were to tune up to the 1500 watt level and then drop the carrier down to something less than 1/4 power you could then modulate over 100% as long as your peaks did not exceed where the amp is tuned at (1500 watts in this case). If using your SSB radio in the AM mode and you exceed 100% negative modulation it will not cause the same type of distortion (splatter) that a plate modulated rig would. Your signal will be distorted when this happens but no where as bad as cutting off the carrier in a plate modulated transmitter. With the SSB transmitter there is a phase reversal when you exceed 100% negative rather than a carrier cutoff that you get with a plate modulated rig which causes sever distortion and splatter when you exceed 100% negative modulation. All of the above operation is to have a clean signal and not just to stay within the 1500 watt PEP limit. You always want your peaks to be below where the amp is tuned no matter how much power you are running. Also remember DO NOT retune the amplifier once you drop the carrier level down for AM. It must remain tuned at the peak level that you will be hitting. Efficiency of the carrier in the AM mode will be about 1/2 of what full carrier (tune up) efficiency is. If your amp is 60% efficient at full power it should be around 30% at 1/4 power of the carrier. But this is really no different than when you operate SSB. At low speech levels on SSB the amplifier efficiency is much lower than at full peak power. With full modulation on AM the amplifier efficiency is 60% too, the same as it is on SSB. You should always tune your amp for maximum output power for whatever drive power you are using. If the power is going over what you want (1500 watts) then reduce the drive power. You never want to limit your tune up by not fully loading the amplifier! This holds for AM or SSB. Actually slightly "over tuning" or over loading a little heavier than maximum output so that the output drops slightly from maximum is ideal as the amp will not flat top as easily if the speech level is driven a little harder. When looking at your signal with a scope your carrier should be 1/2 the voltage that you see at full power of the amp. When you modulate the modulation peaks will hit the level that you tuned up at. Remember power is E^2 / R. so when the voltage doubles that is an increase of 4 times the power. Like wise when you reduce the carrier to 1/2 the voltage on the scope that is 1/4 the power. Remember that in the above with 100% AM modulation the voltage doubles which is what you see. Sorry this turned out so long. 73 Gary K4FMX > -----Original Message----- > From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Robert > Logan via Amps > Sent: Thursday, October 30, 2014 9:17 AM > To: Kimberly Elmore > Cc: Amps@contesting.com > Subject: Re: [Amps] AL1200 on AM - max carrier? > > Thought I might mention the legal limit for AM is 375 watts carrier. > Bob. NZ5A > > Sent from my iPhone > > On Oct 30, 2014, at 9:04 AM, Kimberly Elmore > wrote: > > Any amp will be inefficient when used as a linear amplifier for AM > because as drive is reduced, efficiency is reduced. They will all > generate a lot of heat in this application. However, the AL-1200 should > hold up well to the lock-and-talk culture. In AM, PEP is 4X the carrier, > Overall, you want the carrier to be at 1/4 of the amp's rated PEP > output. > > Assuming the driver generates 100 W output, tune the amp for max or > 1500 W output (whichever comes first) with the amp in SSB mode. You want > SSB mode because you want the best linearity. Whether or not you use SSB > or CW makes little difference in the efficiency for your application but > makes a big difference in linearity. > > Usually, when the driving rig is in AM mode, the carrier is reduced to > 1/4 of the max PEP SSB output. In such cases, you should see no more > than 375 W carrier power (375 x 4 = 1500). > > If you are trying to drive it with an older plate-modulated tube rig, > you ill have to be careful if you reduce the power output because the > modulation transformer is designed with a particular output impedance in > mind. If you reduce the drive to the PA, the impedance changes and you > will have to set the proper audio level using a scope or modulation > monitor.Start with audio gain at zero, bringing it up slowly. Otherwise, > you risk arcing in and destruction of the modulation transformer. > > Kim N5OP > > > On Thursday, October 30, 2014 8:05 AM, Gary Smith wrote: > > > > One thing for sure, run it at the CW > setting, use the lower KV setting and you > will run far cooler for a given KW out. > > 73, > Gary > KA1J > > >> I want to run my AL-1200 on AM for a short stint (2 hours operating >> time, 50% overall duty cycle). I have tested it at about 225 watts >> carrier output, and with moderately high duty cycle transmissions > (known >> in the AM crowd as 'Old Buzzarding'), it gets quite hot. >> >> What is the max recommended AM carrier power for long-winded > transmissions? >> >> Also, I'm assuming it should be tuned up for proper operation at > double >> the carrier power. >> >> I searched the list finding no real answers. >> >> Thanks, >> >> Tony, K1KP > _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
 Current Thread Re: [Amps] AL1200 on AM - max carrier?, (continued) Re: [Amps] AL1200 on AM - max carrier?, Robert Logan via Amps Re: [Amps] AL1200 on AM - max carrier?, Carl Re: [Amps] AL1200 on AM - max carrier?, Drax Felton Re: [Amps] AL1200 on AM - max carrier?, Glen Zook via Amps Re: [Amps] AL1200 on AM - max carrier?, Robert Logan via Amps Re: [Amps] AL1200 on AM - max carrier?, Paul Christensen Re: [Amps] AL1200 on AM - max carrier?, Roger (K8RI) Re: [Amps] AL1200 on AM - max carrier?, Bill Turner Re: [Amps] AL1200 on AM - max carrier?, Gary Schafer Re: [Amps] AL1200 on AM - max carrier?, Carl Re: [Amps] AL1200 on AM - max carrier?, Robert Logan via Amps <= Re: [Amps] AL1200 on AM - max carrier?, Carl [Amps] AL1200 on AM - max carrier?, Jim Thomson