[Amps] Amp Conversion - 3CX1500D7

Paul Christensen w9ac at arrl.net
Fri Dec 10 06:07:02 PST 2010

I would like to get some feedback about a proposed Alpha 77Dx amp 
conversion.  What I'm considering will be considered sacrilege by many Alpha 
users -- but I've already added several improvements to my 77Dx,  all of 
which are easily reversible.  Here goes:  In looking at the 3CX1500D7, it 
appears as if it would retrofit nicely into my 77Dx and have the benefit of 
instant-on capabilities.

A bit of background information:  Last year, I modified by 77Dx by adding a 
filament choke to get the 10A of filament return current off the chassis. 
That modification has worked very well, and a side benefit is that the 8877 
now sees 5.0V rather than 5.5V. A few issues to consider:

1) The D7 appears to be about 1.25 inch taller than the 8877.  Clearance 
with the existing exhaust manifold will be close.

2) D7 availability.  I don't see many suppliers.  Is this still a tube in 
production by CPI/Eimac?  K8RA discusses this tube in the 2006-2010 ARRL 
handbook.  I really like what he's done with his homebrew amp.

3) Filament current.  The D7 draws 30A.  The 8877 is 10A.  Anyone know the 
filament specs on the 77Dx Hypersil transformer?  My transformer is the 
version capable of 77Sx (2 x 8877) service.  I would hate to outboard a 
filament transformer in my attempt to keep the amp modifications completely 
reversible as I have so far.

My Alpha 70A uses a 3CV1500A7 (electrically equivalent to a 3CX1000A7). 
This tube has a directly-heated cathode like the D7 and also draws 30A from 
a Hypersil transformer that's considerably smaller than that used in the 
77Dx.  Is it possible that the 77Dx transformer has a similar high-current 
filament structure to 30A, owing to the prospective use of a second 8877. 
In the 77Dx configuration, 20A of filament is required from the transformer. 
Seems like this is a real possibility.  I have absolutely no intention of 
ever adding a second tube to the amp.

4) Linearity & drive power.  It appears linearity may be compromised with 
the D7 when compared to the 8877 -- and, required drive power will increase 
although the latter is not a concern, provided I can reach 1500W on all 
bands with 80W drive or less.  Above this drive level with many current 
100W-class transceivers, IMD products from solid-state devices generally 
increases at an increasing rate.


Paul, W9AC 

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