Hi guys...
FYI, at ETO/ALPHA we did come up maybe 7 or 8 years ago (?) with a solution
to the 12M enablement of legacy ALPHAs. The only one that we were
sufficiently satisfied with to offer as a factory mod was essentially
similar to what Bill describes below.
We judged it too critical to offer as a kit, as it involved one or more of
the techniques mentioned by John W0UN in his recent postings - non-trivial
ways of determining when the amp was on 12M, plus use of a tapped choke and
a vacuum relay to switch it.
I personally spent many hours trying to find a way to build a single choke
that would fit in the 76 & 76A/78 series amps and handle everything from
160 thru 10, including 12M. John (W0UN) and I have not talked about this
issue at all, but it's my judgment (based on my own unsatisfactory efforts)
that placing the first series resonance between 15 and 12M OR between 12
and 10M isn't satisfactory even if it's located "perfectly."
My guess is that the choke's unloaded Q at those frequencies just isn't
high enough to avoid tank detuning and/or excessive RF choke power
absorption on one or both adjacent bands. IMHO, on bands above 17M or so
the best place for the plate choke resonance is 35 MHz or higher.
If anyone out there has a repeatable choke design that will present 2k or
more Z on all hf bands except 160 & 10, and at least 1.5k there, I'd love
to know the details. Of course to be useful it has to do this while located
in the available chassis space.
If we'd known that 12M would be a ham band some day we'd have adopted
something like the ALPHA 86/87/89 band switched rf choke approach way back
in 1970. BTW, it's not a new or weird idea. A quick look at various Collins
and other classic books on general (not just pre-WARC ham band) transmitter
design reveals that bandswitched rf chokes apparently were standard
practice at least as far back as WWII.
73, Dick W0ID
-----Original Message-----
From: William Gode [SMTP:wiley@interaccess.com]
Sent: Wednesday, October 21, 1998 6:07 AM
To: amps@contesting.com
Subject: Re: [AMPS] Re: ETO/Alpha 12M mods
At the risk of repeating old information, remember that N4UQ offers
upgrades for older Alphas, including a "WARC Band Fix" for the 76, 78
and 374A models. This is a rewound, relay-switched plate choke, but it
is NOT available in "kit form" for those who want to do their own work.
I didn't purchase any of his modifications, but Dick was very helpful
in furnishing information when I restored my 76PA last year.
Check out his services at: http://www.mindspring.com/~n4uq/
73,
Bill, W9NHQ
>May be Alpha/Power Inc can get into the or even better, offer a retrofit
>Plate choke for a reasonable cost to all of the "Older" Alpha owners.
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