[AMPS] 3CX800s in MLA-2500

km1h@juno.com km1h@juno.com
Wed, 20 May 1998 11:42:25 EDT

On Wed, 20 May 1998 08:40:09 +0000 Tom Rauch
<10eesfams2mi@mass1-pop.pmm.mci.net> writes:
>> Date:          Tue, 19 May 1998 19:32:14 -0400
>> From:          "David A. Pruett" <k8cc@ix.netcom.com>
>Hi Dave,
>> I bought a MLA-2500 last winter just for contest expeditions.  
>> it has two apparently strong 8875s in it.  If they ever blow, my amp 
>> be a candidate for conversion to a pair of 3CX800s.

Wouldn't it be easier to consider 8874's as the first option?  They are
often available as commercial pulls for reasonable money....and Svetlana
is expected to produce them soon.  It would be nice if they took my
suggestion and produced a tube that could use a clamp-on radiator. Then
the 8873, 8874, 8875 would all be covered.

The 8874 uses the same size chimney as the 4CX250/350 series so parts
cost would be minimal. The only real work is in modifying the air flow.

>Another thing to consider is two tubes require less airflow than a 
>single tube, for a given output power in a given circuit.
>I doubt people are going to have enough static pressure to cool a 
>single tube at a full 800 watts dissipation, so the second tube 
>would add headroom with a given backpressure limit.
>One thing to watch in these tetrode mods is regulation of the screen 
>and overtaxing transformer windings. One mod had very poor screen 
>regulation and severely overtaxed the 120 volt transformer winding. 
>If you think tubes are hard to find, wait til you look for a 

Those tetrode conversions are a real chore to perform also. BTW, Peter
Dahl sells the MLA xfmr.
I have a dead MLA here and a bunch of 8874's. Some day I may find the
time to perform and write up a conversion.

73  Carl  KM1H

>By the way, a two tone test with a few hundred Hz or more 
>tone spacing indicates nothing about intermodulation distortion 
>caused by poor screen or anode dynamic regulation.
>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

You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com
Or call Juno at (800) 654-JUNO [654-5866]

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