----- Original Message -----
From: "iain macdonnell - N6ML" <ar@dseven.org>
To: "Bill Turner" <dezrat1242@yahoo.com>
Cc: "AMPS" <amps@contesting.com>
Sent: Tuesday, July 03, 2012 2:31 PM
Subject: Re: [Amps] AL-1200 plate choke go bang
On Mon, Jul 2, 2012 at 8:25 PM, Bill Turner <dezrat1242@yahoo.com> wrote:
> ORIGINAL MESSAGE:
> On Mon, 2 Jul 2012 19:39:18 -0700, you wrote:
>
>>
>>In a fit of haste/stupidity/DX-angst/all-of-the-above, I popped the
>>plate choke in my AL-1200. It made a nice *crack* when the stray wire
>>hit the casing of the blower ;)
>>
>>I see that Ameritron have the replacement choke listed at $20, and RF
>>Parts have one for $24.95. Is the RF parts one any better?
>
> REPLY:
> Any idea what blew the choke? An arc? Broken wire? Something else?
I was trying to tune up at the time (after not having used the amp in
a couple of months). I didn't seem to be getting as much out as I
should have, then it went BANG. I (stupidly) wasn't watching the Ip
meter, and am fairly sure it blew due to excessive current... but I
may find some other problem when I get the new choke installed.
> I have used the RF Parts chokes in several homebrew 1500 watt amps
> with no problems. It really does have no resonances in any ham band
> from 160-10 including WARC.
>
> One thing I would advise: Don't mount the choke where capacitance
> coupling to the tube anode might be a problem. Either place a shield
> between tube and choke (preferred), mount it far away (not
> recommended) or orient it 90 degrees to the tube. Placing it where
> there is significant capacity coupling between anode and choke creates
> a secondary circuit with some potentially bizarre characteristics.
There is such a shield in the AL-1200. You can see it here:
https://secure.mfjenterprise.com/pictures/AL-1200t.jpg
73,
~iain / N6ML
The original 3CX1200A7 amp was especially hard to tame and resulted in the
rather different shield and parasitic suppressor arrangement plus some
neutralization added. The early amps had several arrangements as they were
testing on the fly.
That suppressor was "borrowed" from a description I gave back in the late
80's to the YCCC that I used to tame a couple of members amps and later
described on the original version of AMPS.
The latest version uses the same setup as the AL-1500
http://www.pic2fly.com/AL-1200.html
Carl
KM1H
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