[Amps] MLA-2500B Question

[Tony King - W4ZT]

daniel hearn wrote:

>Tony, I think your understanding of the tuned input pro's and con's is
>better than most of the comments posted to date. 
>
Be careful Dan, that might put you on the wrong side of the fence. 
Sometimes I don't feel like I know so much. I do love asking questions 
and I do love learning about how others have solved these problems and 
learning what worked for them.  In my opinion, it's one of the greatest 
benefits of a group like this one.

>A look at my 2001 ARRL HB
>(p 13.19)gives a good explanation of the multiple benefits of a tuned input.
>While the HB material is copyrighted, I doubt if they will object to me
>quoting their exact explanation rather than paraphrasing it.
>"The input impedance of a grounded-grid 8877 is typically on the order of
>50-55 ohms shunted by input capacitance of about 38pf. While this average
>impedance is close enough to 50 ohms to provide negligible input swr, the
>instantaneous value varies greatly over the drive cycle- that is, it is non
>linear. This nonlinear impedance is reflected back as a non linear load
>impedance at the exciter output, resulting in increased intermodulation
>distortion, reduced power output, and often meaningless exciter swr meter
>indications. In addition, the tubes parallel input capacitance, as well as
>parasitic circuit reactances often are significant enough at 28mhz to create
>significant swr. A tank circuit at the amplifier input can solve both of
>these problems"
>  There is more but I think this may answer your question. 73, Dan, N5AR
>  
>
My most recent handbook is considerably older but I've read similar 
stuff.  I "like" what I read there but I don't always trust what I read. 
I guess that's my reason for asking for opinions. Sometimes they will 
confirm those things I've read and sometimes they blow holes or create 
more doubt.  That starts the thought juices flowing and makes me want to 
know more.  We've got some very experienced and knowledgeable people on 
this list that  are very technically qualified. I really like hearing 
their opinions on things that are so magical. Sometimes it is just 
difficult to get them to come out and play  ;~)

Thanks for your comments Dan.

73, Tony W4ZT

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