[Amps] Alpha 374A add a 3d 3CX-800?

Roger (sub1) sub1 at rogerhalstead.com
Wed Aug 17 11:32:03 PDT 2011

On 8/17/2011 3:26 AM, chas wrote:
> Have purchased a 374A which is NOT at my QTH or even in this area of the
> state but...  it has been converted to a pair of 3CX-800s along with a mess
> of other mods. Done about 3 yrs ago.
> The owner cannot handle the fan noise and is going to buy an ACOM 1000
> instead.  I have known him for about 60 yrs and we are attached at the hip so
> I feel that this is just another example of stuff we swap back and forth and
> eventually, forget to whom it belongs at any time.  Point to that is, he has
> gone thru it with a Cobra Sr and the common MARS matrix for this state and
> has tuned for best power production.  It does talk but it seems to me that
> the PA needs to have a txfrmr upgraded and maybe add a another bottle to it.
>   it did not have a vacuum relay mod done since it will be a SSB only in usage.
> Not sure if this was a 3 holer to begin with or not.  However, I am wondering
> what it would gain me, if it could even be done - to add a 3d Cx800?

The early Alpha series amps such as the 76 and 374 series are very 
compact amps with little room to spare.  They will take, or I should say 
that some will take 3 8874's (3CX400A7s) which is a total of 1200 watts 
plate dissipation.  A pair of 3CX800A7's has 1600 watts dissipation, but 
a pair of them are easily as large or larger than 3 of the 8874's.   
Transformer size is also limited due to space.

The pair of 3CX800A7's and 3CPX800A7s are popular conversions even 
though the KVA rating of the transformer may be  a bit anemic they'll 
put out plenty of PEP power.  Plate transformers are available with the 
proper filament winding and higher voltage for the 3CPX800A7's for a 
neat conversion.  I think Carl has a good deal of experience with the 
Alpha conversions.

But back to 3 3CX800A7s.  There is no space in there for 3 of them and 
the transformer doesn't have enough of a KVA rating to take advantage of 
them if you could get them to fit.


Roger (K8RI)
>    The
> weakest tuning freq so far is around 5.2mcy which can only crank out ca 800W.
>   The rest of the thing will do 1.2 to 2kw as I understand it.  I am asking
> the owner to write down the list of mods done by RL in Abilene and to send
> that along with the amp.
>   isfaiK, it has a low power mod, the txfrmr is not a new Hypersil and I am
> wondering if it could be upgraded to a PDahl model made for a 7n series??
> I probably have not given enough info to make an informed decision or to
> generate worthwhile advice but maybe I can get some questions to provide
> answers to when I get the PA here.
> thanks es 73
> Chas  k5dam

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