[SCCC] SCCC Digest, Vol 49, Issue 19

[Art]

Rick
I converted my 76PA to vacuum relay output, reed input. Used a HC3 
relay. I believe they're available new for about $75, less if you find 
some on ebay. My amp had the old kerchunker relay in it so I had to make 
my own bracket for the QSK stuff.

This might not be exactly what you're looking for tubewise but it sure 
does work in my 76PA.

Plug up the innermost of the 3 holes on the RF deck, the one in the 
center. I just RTV'd a piece of perf board over it.

Plug in 2 each 3CX800A in the remaining two holes. Or 1, if you don't 
mind less power. Pinouts are an exact match. You maybe can still get new 
Si rubber chimneys from Alpha like I did, or make some. Assuming 
clearance between the larger plate cooling fins and the top cover might 
cause a problem, I used another flat piece of Si rubber between the 
tubes and top cover. A small square of fiberglas board would do equally 
well, if it is even necessary at all.

Install the Peter Dahl power transformer that fits in the existing 
holes, that he makes for 3CX800's. You need this for the filament 
voltage change.You'll get about 2400VDC key down in SSB mode. You'll get 
1500W out easily, much easier than the 8874's do. I tune it (hispeed 
dots) for max out at 1 amp plate current, yielding about 1500W out at 
500ma per tube. An added benefit is the 3CX800's have sturdier grids for 
that wrong-antenna situation. The xfmr is identified on Dahl's list 
"Alpha 76A-2 Power XFMR Special Heavy Duty 3CX800A7's Version". 
www.peterdahl.com and he advertises in QST. This hypersil transformer is 
smaller, lighter, at least as powerful as the original.

One cautionary note. The amp was built back when power supplies were 
built for 1KW rather than 1500W output. If you operate RTTY, use the CW 
position because you won't have sufficient airflow across the tubes for 
1500W CCS out. That runs 800-900W minimum. Or, do what I did, add a 
temperature controlled muffin fan for those tense moments that blows 
into the grille on the rear panel. My fan comes on at about 220 deg F 
air outlet temp, stopping the temp rise in its tracks.

I've owned the amp since about 1978, replacing the 8874's when they 
finally went south with the 3CX800's. It's a Good Thing! Costs a grand, 
you end up with contemporary power for about the same as replacing those 
sensitive 8874's. This only works economically for existing Alpha owners 
like you. Starting from zero I'd go buy a used TenTec amp with the pair 
of 3CX800's. Would be nice to have one of the autotuners but my wallet 
won't support it.

73 Art
>
> Message: 3
> Date: Mon, 22 Jan 2007 08:37:32 -0800
> From: "Rick D" <n6pe at pacbell.net>
> Subject: [SCCC] Alpha 78
> To: <sccc at contesting.com>
> Message-ID: <00f301c73e43$a4389110$210110ac at MAINBOX>
> Content-Type: text/plain;	charset="iso-8859-1"
>
> Hi all,
>
> My old original Alpha 78 will not QSK and I suspect the vacuum relay is shot. Is there a cheap replacement or a solid state mod floating around?
>
> It is a 3 hole and the 8874s are original.  I've seen some replacement mods, but the replacement mods are almost as much as Russian 8874s.  Is there a good mod using some easy to get cheap tubes. Even if it is less power it is OK with me. Seems like I could buy a new Ameritron for what it will cost to fix this one.
>
> Also, anyone have an easy fix for the gummy paint these things develop.
>
> Thanks
>
> Rick Darwicki  -= N6PE =-
> =================================================
> 17775 Elmhurst Circle
> Yorba Linda, CA 92886
>  
> http://www.qsl.net/n6pe/
>
>
>
>
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> End of SCCC Digest, Vol 49, Issue 19
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