Rick,
I also have an Alpha 76. My plate choke looks like it got hot down close to
the bottom which I think is the high voltage end. I want to replace mine too.
According to KK5DR in his webpage on the 76 http://www.kk5dr.com/OLD76.pdf
he says that RF Parts No. RFC-3 is a replacement. I am ordering one of these
and see
how it works out. It sells for $25.00 so pretty easy on the wallet, hihi. On
the caps, I would think the Alpha cap values would be the ones to use for the
coupling and bypass, but I'm no expert, hopefully one of the amp gurus can
shed some light on it.
Bob
K6UJ
On Apr 10, 2012, at 5:48 PM, rick darwicki wrote:
> Hi all,
>
> I replaced the plate choke in an Alpha 76 with an Ameritron that is much
> bigger. First time out it arced to the nut on the back of the load cap. This
> is an older 76 with the long cap so there isn't much room.
>
> I replaced the choke and put a piece of glass between it and the RFC and a
> 1/4" thick pice of plexiglass between the blower and the RFC. Well apparently
> the the Plexiglass lowered the resonate freq enough that it acred over to the
> blower on 20 M. I removed the plexiglass and put some corona dope on the
> choke and all seems well. I can tune to full power with 100 ma grid drive on
> all bands.
>
> The interesting thing is on 160/24/28 at 100 ma grid current I am showing
> about 900 ma plate current at max forward power, 1:1 SWR. On the other bands
> max power at 100 ma grid current is at about 600 ma plate current. It seems
> like there is a lot of RF on the key on 17 M but the cover is off.
>
> What I am wondering is if I should use the same coupling cap and RFC bypass
> as Ameritron uses in the AL-80B with their choke. They use 0.002 coupling
> and 0.001 bypass.
>
> I kept what was in the Alpha for RFC bypass and installed a big Russian 0.002
> coupling cap.
> At the bottom of the choke is a 0.006 (three 0.002's) to ground, a 15uh
> series choke and a 0.008 (four .002s) to ground that go to B+.
>
> The original Alpha arrangement was 0.004 coupling, 0.004 bypass, 15uh series,
> 0.004 bypass.
>
> I don't know what the best thing to do is. I wondering if the 15uh is
> necessary or screwing up the Ameritron.
>
> Any comment will be greatly appreiciated.
>
> Thanks,
>
> Rick N6PE
>
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> Rick, N6PE
> ==================================
> Talk is cheap... except when Congress does it.
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