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FW: Re: TopBand: MLA2500

To: <topband@contesting.com>
Subject: FW: Re: TopBand: MLA2500
From: measures@vc.net (R/S Measures)
Date: Wed, 20 Nov 96 14:36:56 -0700
I received the post that follows via relay from a friend.  I believe that 
some questionable information about pi-network tank circuit design was 
presented.  
 According to Mr. Tom Rauch, W8JI, the operating Q need be  no  higher 
than one plus 
the square root of the anode output-Z divided by the load-Z.  Since the 
8875s' output Z is on the order of 1200 ohms, for a load Z of 50 ohms, 
according to Mr. Rauch, the Q need be no greater than 1+(1200/50)^0.5 = 
5.9.  However, on page 35 in Eimac's *Care and Feeding of Power Grid 
Tubes*, Figure 21 shows  tank circuit Q versus relative harmonic output.  
With a Q of 6,  the third-harmonic content is over 1.5%.  This means that 
with 1000w output at 1.9MHz, the amplifier will be delivering 15w at 
5.7MHz.  This level of out-of-band emission is clearly unacceptable to 
the FCC.  
 Eimac recommends using a Q of 15 to obtain satisfactory suppression of 
harmonics.  According to Figure 21, lowering the tank Q from 15 to 10 
triples the second-harmonic level.  . 
 Any comments, Mr. Rauch?
thank you
Richard L. Measures, AG6K, 805-386-3734 
===========================================

>Subject:     FW: Re: TopBand: MLA2500 
>Sent:        11/20/96 1:22 PM
>Received:    11/20/96 10:06 AM
>From:        John Ferebee, w2cya@broadcast.net
>To:          measures@mail.vcnet.com
>
>Rich:  Don;t know if you saw this...
>
>
>---------------Original Message---------------
>In <961118225227_351477338@emout11.mail.aol.com>, on 11/18/96 
>   at 10:52 PM, W8JITom@aol.com said:
>
>>The MLA does NOT need more Q, once Q is larger than one plus 
>the sqrt of
>>Rplate/RLoad any further increase in Q lowers the 
>efficiency. The MLA
>>already has more Q than necessary.
>
>Tom, 
>
>Is a Q of 8 enough to provide sufficient harmonic 
>rejection/TVI
>supression, etc?  All of the "how to build a linear" articles 
>I've seen
>over the past 40 years of the magazines (I had a fair size 
>collection
>before I moved the last time) talk about a minimum Q of 10 
>and preferably
>12.  Even with the "high impedence" tubes, 1 + 
>(Rplate/Rload)^.5 gives a Q
>of less than 9 at amateur power levels. 
>
>Of course, keeping the Q down to 10 is a real trick at 10 
>meters with the
>output (plate to grid) capacitance of most tubes.  The MLA is 
>a real
>problem with it's 75pF minimum in the tuning cap!
>
>-- 
>
>73, 
>   ... Joe Subich, AD8I
>       <AD8I@ibm.net>
>       <subich@ibm.net>
>
>
>--
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>
>----------End of Original Message----------
>
>********************************************
>Name:    John Ferebee
>Radio:   Talk Radio on 1850 khz - W2CYA
>Company: NYNEX Corporation, New York
>E-Mail:  w2cya@broadcast.net
>Voice:   (212) 766-0411, (212)429-8882
>Fax:     (212) 577-7426
To: <topband@contesting.com>
>Date:    11/20/96  Time: 12:22:07
>********************************************
>
>


R/S Measures, [805]386-3734    


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