[AMPS] Amps in Sept QST

[km1h @ juno.com]

On Tue, 26 Aug 1997 21:38:37 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> To:            amps@contesting.com
>> Date:          Tue, 26 Aug 97 19:28:51 +0000
>
>> The Product Reviews of the Alpha 91b and the QRO 2500DX merit 
>reading
>> since these are the first with the Svetlana 4CX800/GU-74B tetrodes.
>> To be honest, I am amazed at the IMD performance of both amps; the 
>Alpha
>> is 45/49 dB and the QRO is 43/40 db for 3rd/5th orders. This sure 
>beats
>> the old 25/28 dB typical of 4CX250's.
>
>Not by as much as you think.
>
>All Eimac and commercial specs use dB below ONE TONE while the 
>ARRL uses dB below PEP.
>
>So the numbers are really:
>
>AL800H -43  dB third order, -49 dB fifth, -53 seventh
>Alpha 91 -39  dB third order, -43 dB fifth, -49 seventh
>QRO       -37 dB third order, -34 dB fifth, -54 seventh   
>4CX250's  -25 dB third order, -28 dB fifth (if your numbers are 
>right)

Tnx for reminding me about that little ARRL anamoly.
The 4CX250 runs anywhere from 25 to 28 3rd depending on what spec sheet
you read and was not necessarily best case.


>The old 4CX250 is really only seven dB worse than the QRO, but 18 dB 
>or more worse than an AB2  triode!
>
>I don't have my copy of QST. What was the failure of all three amps? 
>Does anyone know? So far as I heard, the AL-800 never failed but did 
>lose some IMD performance from an earlier test when it was new, but 
>not enough to get unacceptable.

The wording in QST was: "Late in our review process, we discovered that
our AL-800H had developed a problem that caused IMD products that did not
seem reasonable for an amplifier of this design".  That is all they said
about it. Damaged toroid maybe?

>
>Did they say what the failures were for all three amps?  


Alpha had a suspected problem that was a fluke during early testing;
whatever that means. It also had a screen supply failure during a
contest. 

QRO also had spectral purity problems that were attributed to the 4:1
output toroid. 

Makes me wonder how many amps using toroids develop IMD/harmonic problems
without the owner being aware. 

Of course for $7.50 a pop the ARRL will allow paid up members to see the
expanded review.  $12.50 a pop for non-believers. 

73....Carl   KM1H

>73, Tom W8JI
>
>--
>FAQ on WWW:               http://www.contesting.com/ampfaq.html
>Submissions:              amps@contesting.com
>Administrative requests:  amps-REQUEST@contesting.com
>Problems:                 owner-amps@contesting.com
>Search:                   http://www.contesting.com/km9p/search.htm
>

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm