[Icom] RE: KK7TV 781 Mods (Bruce Marton)

Sat, 15 Mar 2003 20:18:16 -0500

FB Adam,
As always, you come up with a very reasonable observation as to why
things work. Krassy was positive in his stand that the 781 would not
harm the 2000A, as you have outlined.

I have not put many hours on the 2000A over the past year; however I
will state that the Acom has functioned without a burp using the 781 as
an exciter. I have been very happy with the "new station".  At least the
amp is new.  

The audio reports have been excellent, and I will say that I am running
the stock filtering.  The distortion from the amp is very low, all
antennas are cut for the band used and the ground system is currently up
to snuff.  

Hope all is well with you, I am sure you are looking at new pals at this
point.  I feel your pain from your recent loss.

Best 73's,
Bruce

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Adam Farson
Sent: Saturday, March 15, 2003 6:49 PM
To: [email protected]
Subject: RE: [Icom] RE: KK7TV 781 Mods (Bruce Marton)

Hi Bruce,

The Acom 2000A uses two Svetlana 4CX800A's in a grid-driven
(grounded-cathode) configuration with a 50-ohm swamping resistor in the
grid
circuit. The swamping resistor will absorb the initial power spike from
the
IC-781, and the tubes' grid dissipation will not exceed the maximum
rating
(2W per tube).

A problem can arise when the IC-781 drives a grounded-grid 3CX800A7
or3CX400A7/8874 amplifier. The 3CX800A7 has 4W max. grid dissipation,
and
the 3CX400A7 has 5W. The spike can cause grid over-dissipation and
damage in
these tubes.

Cheers for now, 73,
Adam VA7OJ/AB4OJ