[AMPS] DAF amplifier

[Rich Measures]

>
>Gents,
>
>.......
>Finally a question, when talking IMD, enlighten a mere mortal as to where
>this distorsion is to be heard. Is it within the passband, outside of the
>passband or maybe both. 

€  For the DAF amplifiers I tested, with voice modulation, not much 
distortion could be heard in the fundamental passband window.  However, 
in the windows 3.6kHz below and 3.6kHz above, the level of IMD products 
was about 22db below the signal level measured on the fundamental.  // 
Convincing the owners of these amplifiers that the measurements were 
accurate took months.  One problem was glowing reports from pals who did 
not wish to offend their friend.  Another problem was denial of reality.  
.  No, Lou, there is really no such thing as a 300mpg carburettor for a 
1963 Cadillac.  

> I am not clear on this and the handbook I have
>(1984) does not explain this in great detail. Can we hear a difference of
>15 db IMD with our ears on the bands or is it academic ?
>
€  The splatter from an amplifier whose IMD products are 37db down sound 
very much like the splatter from an amplifier whose distortion products 
are 22db down.  The difference is relative signal level.   

>I do want to know what it is I am doing wrong when accepting a DAF
>amplifier as a rather nice afterburner... I'd gladly modify it if I am
>convinced that it is bad for my fellow hams.
>
€  Possible disadvantages of using a DAF amplifier.  
1.  You might run into someone on the air who can explain why multigrid 
amplifier tubes need regulated screen potential for linear amplification, 
and who knows what happens to linearity when the grid is driven into the 
positive region.  
2.  You want to produce relatively little splatter, even though a 
distortion level of minus 22db is perfectly legal in the Amateur Service 
Bands - provided it does not interfere with emergency communications.
3.  Having to eventually admit to other DAFers that you wrong.  
4.  Possibly being tarred and feathered at a DX convention.  

-  It is my opinion that builders/users of DAF amplifiers want to be 
known as smarter operators who have fabulously efficient linear 
amplifiers.  .  .  However, "You can fool some of the people some of the 
time, but you can't fool all of the people all of the time."  -  Abraham 
Lincoln. 

€  Converting a DAF configuration to something more linear is fairly 
easy.  
Regulate the screen.  Install an electronic switched bias supply in order 
to cut the tube off  whenever the transceiver is in RX mode (diagram on 
my Web site).  Increase the bias potential to the point where no grid 
current flows at max. drive.  Tune/Load the amplifier for the tube 
manufacturer's recommended value of screen current.  

-  cheers, Peter


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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