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Subject: [AMPS] IMD
From: (2)
Date: Tue, 18 Sep 2001 05:02:10 -0700
>>>>Rich Measures said:
>> > >
>> > //  Few AB1-grid driven. or AB2-GG amplifiers have as much distortion as 
>> > modern transceivers.  Many of the dirty signals that I hear seemingly 
>> > involve operator ignorance and/or over-exuberance.  
>> > 
>Steven Katz responded tongue in cheek:
>>      ::You mean the, "damn the torpedoes, all knobs to right!" theory of
>> operation is not the correct one?  Darn...-WB2WIK/6>...
>Ian White responded:
>>Good point. It emphasizes that the performance of an amplifier is a
>>combination of several things: 
>>* The tube(s)
>>* The circuit configuration
>>* The power supply 
>>* The operator who tunes, loads and drives it.
>I have a question. I tune for max power out with min grid I 

//   Electons emitted by the cathode mostly either end up going to  the 
grid, heating the anode, or going to  the antenna.  However, a compromise 
must be reached during tuning/loading since less grid current does not 
equal more power out.  When relative power out is maximum, grid-current 
can not be excessive because electrons would be diverted from the 
antenna, and relative RF power out would drop.  Thus, tuning for max out 
with max drive is the best compromise for a G-G amplifier.   In other 
words,, grid current will be right if one tunes/loads for relative max 
PEP out with max drive.  However, the fly in the ointment is that very 
few SSB amateur radio amplifiers have been built with 2500W power AØ 
(NØN, "hammer down") anode supplies that maintain a more or less constant 
anode potential with AØ.  Thus, the best way to tune up a typical 
ham-type amplifier for SSB is with a tuning pulser or a CW-keyer sending 
50-wpm dits.  

>and plate I no 
>greater than ICAS spec. I seem to remember someone recently stating that 
>adjusting the load just to the point of the power dropping off sharply 
>increases IMD performance with only a slight loss of power out.
>The three posters suggest there is a method of tuning which improves IMD, but 
>fail to state it. Can someone tell me how and more importantly why for 
>someone who does not know (a.k.a. me!!)?

//   True enough.  Slightly heavier loading (less C-load) than necessary 
(for max power out) slightly reduces IMD,  but the limiting factor on IMD 
is typically determined by the radio, so making the amplifier a bit 
cleaner at the price of a bit less efficiency does not buy anything 
-  I discussed IMD with a solid-state designer.  He said that it would be 
easy to design a solid-state RF power amplifier that was substantially 
cleaner than the current crop.  The tradeoff is that a bit more drive is 
needed to compensate for the use of RF-negative feedback resistors in the 
output transistors.  However, on the ham bands lower IMD would not matter 
all that much.  
>I run with my TS-850 set with ALC as prescribed in the manual and 
>compression peaking at 10%. The exciter power varies with the efficiency of 
the amp 
>on a particular band (I keep plate current to spec as I don't care to lose a 
>transformer to overheating again!!!).
//   On voice-SSB, the current seen on a Ip meter is c. 30% of the real 
current, so the indicated current must be multiplied by a bit over 3x to 
get a handle on what's really going on. 
-  Alas-   Mo' anode current is not mo' betta, good buddies.  

cheers, Ron

-  R. L. Measures, 805.386.3734, AG6K,  

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