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Re: [Amps] 3CX1200 tubes

To: amps <>
Subject: Re: [Amps] 3CX1200 tubes
From: Larry K4AB <>
Date: Sun, 22 Mar 2015 10:53:32 -0500
List-post: <">>
GM Randy,

Its interesting for you to bring up this question at this time.
I just finished operating a DX contest this weekend, and I was
thinking along the same lines as you.

I have (2) AL-1200 amps. An old one and a new one.
The old one I purchased used some time ago, and I'm guessing
it must be 20-25 years old or longer.  It uses the 3CX-1200A7

The new one was purchased very recently and uses the 3CX-1200Z7.

I've always used the old amp for my "run" station, which means it
gets A LOT of constant CQing sometimes for up to 48 hours non-stop.

The new amp is in my "mult" position and is only occasionally keyed.

That is until one of the filter caps in the old amp went South. At that
time I swapped the 2 amps and gave the new (Z7) amp a couple of
contest workouts while I repaired the caps in the old (A7) amp.

Now, I'm no engineer, just a user of these 2 amps. Here are some of
my observations:

-The A7 amp is much easier to drive at full power, requiring less drive
to achieve 1.5 kW.  Bear in mind this is a 20+ year old tube vs. a brand
new one.  B+ is identical on both amps, and the key-down voltage drop
is also the same. Same antennas, same exciter, etc.

-The Z7 amp will occasionally "spit" and arc for no apparent reason.
Nothing major, no signs of charred components inside the amp, just
enough to grab my attention and cause concern.  I've never had the old
A7 amp even whisper an arc. Even when accidentally transmitting
into a wrong antenna.

-When looking at both amps side by side, they look like different models.
The new Z7 tube requires a redesigned RF compartment, socket, chimney,
anode connector and blower configuration among other things.
It also does not have the neutralization tab which is present in the old A7 amp.
I'm surprised that its still called an AL-1200 at all.  Perhaps an AL-1210
is more suited.

-The new Z7 amp obviously does not have an original Peter Dahl transformer.
I don't know the manufacturer of the new one.

I've since replaced the filer caps and resistors in the old A7 and have swapped
them back into their original "run" vs "mult" position.  I used the old A7
amp this weekend for 20+ hours of mostly CQing.  It performed flawlessly
as expected. The newer Z7 worked well for its lighter duty uses.

Now, I'm not one of those MFJ bashers. But the fact remains, that as
I see it, the older 3CX1200A7 AL-1200 is the preferred choice over the
newer 3CX-1200Z7 version.

Just my opinion.

Larry K4AB

On 3/21/15, Randy <> wrote:
> Aside from the physical and filament differences is there much difference
> in
> RF performance between the 3CX1200A7, D7, and Z7 tubes?  Is one more suited
> for HF SSB service (lower IMD, etc.)?  If you had all three types available
> and wanted to build an HF SSB amp project from scratch, which of these
> would
> be the best choice?
> Thanks.
> Randy, W7HR
> Port Orchard, WA
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