To Jim's point, I forgot that I had also changed the plate choke in my 77Dx
to a B&W 802. Actually, Dick Byrd, N4UQ rewound the old #800. At one time,
he offered a trade-in service where he would keep #802 chokes on the shelf
and as #800 chokes came in he would strip them to the ceramic core and
rewind to the #802 standard.
Of all the changes I've made to my 77Dx, getting the filament voltage back
within spec has been most important. So far, no 8877 replacements. Before,
I was replacing them way too often when more than 5.5V was on the filament.
Utility voltage runs between 250V-255V here, averaging 252V. I suppose I
can ask the utility company to try and re-tap that down by 10V but I'm not
sure of the feasibility with ground-mounted residential utility
transformers. Anyone have experience with these?
Open filaments and filament-to-cathode shorts were common and expensive
occurrences. At least now with the filament choke, if the cathode shorts to
the filament, I won't loose drive and the amp will probably keep limping
along until that structure finally fails. If I had to go back in and take
it a step further, I think I would add a separate filament transformer and
add a variable control on the rear panel along with a pair of insulated test
jacks (through 1K resistors and chokes) for filament voltage monitoring with
a high-Z DMM.
----- Original Message -----
From: "Jim Thomson" <firstname.lastname@example.org>
Sent: Monday, April 04, 2011 7:40 AM
Subject: [Amps] Alpha vs Henry
> Date: Sun, 3 Apr 2011 10:35:01 -0400
> From: "Paul Christensen" <email@example.com>
> Subject: Re: [Amps] Alpha vs Henry
> If 160m is of interest to you, may want to re-evaluate Henry amps,
> 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
> 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
> 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
> 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
> 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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