[ETO_Alpha] Alpha 78 on 17m and 12m?

[Tom Hammond NØSS]

Fellas

George Guerin, K8GG, wrote to Duane Calvin, AC5AA:
>Hi Duane
>
>The Alpha 78 works fine on 17, but not on 12 meters - the plate choke is
>resonant there, and the broad band 10 meter plate tank circuit does not go
>low enough.  It is a very difficult modification to get it on 12.
>
>By the way, does work on 30.
>
>For more information try Dick Byrd, N4UQ.  Home page is:
>http://pweb.jps.net/~n4uq/index.htm.  Someone did a very expensive one time
>mod to get the 78 on 12.  Lots of time and an RFParts RFC plate choke.
>
>I can find that somewhere if you want to spend lots of time and energy
>and some money.

For what little it may be worth, I have a PDF which I created a while back, 
documenting my installation of the RF Parts plate choke, to allow me to use 
my Alpha 76PA on 12M. The installation is not THAT difficult, but... you 
will most probably find that the choke WILL be resonant close enough to 10M 
(30M) that if you try to use the amp on that band (as I did, at 200W, being 
driven by my Elecraft K2) the choke will resonate and blow!

The RF Parts choke is manufactured by/for Ameritron, for their amps and it 
was darn near impossible to get any info from them about the problem. It 
appears that Tom Rauch, W8JI, is their engineer (for their amps), but all I 
ever received from Tom was a couple of terse, non-helpful, replies to my 
queries, so I gave up. Subsequent to my queries, a friend forwarded a note 
from Tom to an Ameritron owner who was asking the same question. THIS reply 
DID give the info I was asking in the first place. Maybe I should have 
represented myself an an Ameritron OWNER...

It appears that the RF Parts choke is designed to place the closest 30M 
resonance about 2MHz or so away from 10MHz (near 12MHz), but when the choke 
is installed in the Alphas, the proximity of the shield tends to lower the 
resonant frequency a bit... too close to 10MHz.

Anyway, my 76 now works great on ALL bands BUT 30M. And since we're limited 
to 200W on 30M, and the fact that I now have a 100W amp for my K2, I no 
longer need to worry about using the 76 on 30M anyway.

I would be happy to provide the docs for installing the RF Parts choke in a 
76 ot 78 if anyone wishes to receive them. The PDF file is about 120kB in 
size and includes pics of the mod.

Oh yeah, my mod was NOT EXPENSIVE in the least! Basically, the cost of the 
choke (c. $30, if I recall).

73,

Tom Hammond   N0SS