George,
If 160m is of interest to you, may want to re-evaluate Henry amps, including
the 5K.
Concerning mods for the 77Dx/Sx: few mods have been published and either
version of the 77 is fine as it is. I own a 77Dx and have avoided a
conversion to the Sx for several reasons:
1) Added cost of an 8877 during replacement;
2) Slightly added difficulty in isolating trouble with a pair of tubes. A
minor issue;
3) The added class of power really requires changing the RF output connector
to a DIN type;
4) Fuse holders should be upgraded;
5) Relatively poor Ep regulation of the Sx version unless an outboard
Hypersil transformer is used. Some folks have reported better regulation
with a direct- replacement Dahl type although it's a snug fit;
6) The Dx already has way more reserve than needed for most operating modes
in American-licensed service.
Added together, I just can't justify the change. However, I did make the
following changes to my 77Dx:
1) Isolated neutral from chassis ground. As it stands, these Alpha amps are
not UL compliant. Although not compliant with the NEC, the NEC is factually
a premise wiring standard -- not a product wiring standard. The mod is
super-easy since it involves only cutting a jumper on the Cinch-Jones power
plug. However, 4-wire service and 4-wire power cord is required. If you
have not already added 4-wire 240V service, consider it if you plan on using
older Alpha and Henry amps;
2) Elevated the filament return off the chassis. I detest the chassis being
used for high-current filament returns -- even more of a concern with the Sx
as 20A of current flows in the chassis. It's a personal bias of mine; many
Alpha amps have run for decades without issues;
3) Added a filament choke. A side benefit is that there's just enough of an
IR drop through the choke as a result of my 250VAC mains to ensure that the
filaments run at 5.0V. Before adding the choke, my filament voltage ran
above Eimac's specification and I was replacing tubes way too often because
of filament issues. High filament voltage combined with many intermittent
on/off cycles leads to these failures. Also, after inclusion of the choke,
I did not tie the filament to the cathode (e.g., AL-1500) although that
could be done if I were convinced of the necessity. I've seen arguments
both ways (primarily fault related) from respected amp designers as well as
former Eimac engineers. It stays unconnected for now;
4) Added gas discharge tubes to the filament and cathode terminals, and
added bypass caps to the filaments connections.
Just my $0.02, and I'm sure others will disagree with me on some of these
changes. Good thing I'm not writing article for QST!
Paul, W9AC
----- Original Message -----
From: "George Steube" <at6c@bellsouth.net>
To: <Amps@contesting.com>
Sent: Sunday, April 03, 2011 7:58 AM
Subject: [Amps] Alpha vs Henry
> Looking at purchasing an Alpha SX/DX or Henry 5K for my amp collection.
> Any
> advice/comments on these amps would be appreciated. My biggest concern
> with
> the Henry would be future tube replacement. Any recommend mods for
> either
> amp? Thanks.
> George
> W2GS
>
>
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