All this 77 talk reminded me that I have a NOS Dahl internal SX transformer
wasting shelf space.
Carl
KM1H
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <amps@contesting.com>
Sent: Monday, April 04, 2011 9:44 AM
Subject: Re: [Amps] Alpha vs Henry
> 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@telus.net>
> To: <amps@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@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
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|