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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