Date: Sun, 3 Apr 2011 10:35:01 -0400
From: "Paul Christensen" <w9ac@arrl.net>
Subject: Re: [Amps] Alpha vs Henry
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;
&& You also require 2 x tubes, 2 x sockets, 2 x parasitic suppressor`s etc.
The 8877 is a 50 ohm tube...and a pair = 25 ohms. The cheap and dirt fix
would
be a 50-25 ohm un-un.... between ouput of stock tuned input..and cathodeS.
&&& Your plate load Z will now be one half the original Z. That implies DOUBLE
the C1 + C2 used on each band. That also implies on half the stock uh be
used on each band.
IE: your stock tank taps are out to lunch with 2 x tubes. And ditto with the
L potion of the PI-L.
&& The stock plate choke is just a 90 uh, B+W 800 type. You need a bigger
choke...like a 802
or an Ameritron.
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;
&&& That stock 46 lb xfmr won`t handle 2 x 8877`s. It will barely handle
one tube as is.
The stock xfmr is only aprx a 2.8 kva CCS unit.
&&& I think the stock xfmr, when used with 2 x tubes... is good for aprx 3300
w out pep. You
don`t need a 7-16 din for that power level. Silver+ Teflon SO-239 + mating
silver-teflon males
are just fine below 6 kw.
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;
&&& If an outboard dahl is used, and more drive applied, you will destroy those
torroids used
in both the PI and the L section...poof. I think it was frontier
electonics in Colorado that custom
made bigger torroids for the 2 x 8877 alpha 77-sx..esp when an outboard xfmr is
used.
6) The Dx already has way more reserve than needed for most operating modes
in American-licensed service.
&&& I`d leave it alone....in the 1 x tube mode. You won`t get another 3db out
of it, unless an outboard
xfmr is used anyway. Then you still have to re-do all the taps, tuned input,
metering, new b+ supply, plate choke,
160m padding caps,etc.
&& If you require more db`s... a single YC-156 and a surplus 10 kva hypersil
pole pig will clean house...
and be a lot simpler. Stuff 200 w into a YC-156...and you will need 4 x
8877`s to get even get close.
Those 10 kva CCS surplus pole pigs are aprx slightly less than 1 cubic foot,
and weigh 120 lbs.
later... Jim VE7RF
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