[Amps] Alpha vs Henry

[Paul Christensen]

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.

Paul, W9AC


----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
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" <w9ac at 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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