[AMRadio] AM Transmitter Advice??


W7QHO at aol.com W7QHO at aol.com
Mon Jan 30 17:00:50 EST 2006


Linears for AM -- here we go again for at least the third time in the past 12 
months.

Basic considerations:

1.   Under carrier only conditions a correctly designed and adjusted linear 
amplifier will be operating at about 33% efficiency.   So, with a pair of 813's 
the math works out to 125 watts of carrier, obviously not worth the effort if 
you're starting off with a 100 W carrier exciter.   

2.   The linear must be initially tuned up at the peak RF output value which, 
in turn, requires that the exciter (or some other source) must be capable of 
providing the input necessary to do this.   Typically, his would be 4 times 
the carrier value but expect to hear a lot more on this from the "asymmetrical 
speech waveform" crowd.

3.   "Real AM" can only come from a plate modulated class C PA in the view of 
certain members of this community.   At the same time, big mod iron is 
expensive and hard to find.   Also, for a legal max rig the wall plug efficiency of 
"high level" and "linear" is not that much different in the final analysis.

Bottom line -- If you're going to build a linear do it right and go for a 
pair of 4-400's, single 4-1000A, 3-1000Z or one of the big Russian tubes I've 
seen on eBay recently.   I use my HB 3-1000Z amp on both SSB and AM, BTW.   Works 
FB.

Good luck with the project.

Dennis D. W7QHO
Glendale, CA

*******
Original message dated 1/30/06 12:08:03 PM, kq4by at rocketmail.com writes (in 
part):

> My friend, KR1S (Jim Kearman) is designing an AM
> transceiver, for me.  We plan for this to be a
> solid-state unit that will deliver about 100 watts
> out. It will be single band (75 meters) and I have
> been considering using that to feed a homebrew linear.
> I have the makings of a 2 x 813 amp thanks to my and
> Jim's hamfesting efforts.
> 
> So far, we have been reading and kicking around ideas.
> It seems that with the derating of the amp to handle
> the continuous carrier, etc.. may be more trouble than
> it is worth. 
> 








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