----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <amps@contesting.com>
Sent: Sunday, September 24, 2006 2:02 PM
Subject: [Amps] Titan 425 Reflected Power
> I'm encountering a strange problem with my Titan. If I follow
> standard practice and achieve a 1:1 match with the exciter, then
> take the amp out of standby, the amp typically shows mismatch on
> the order of 1.5 or 2:1.
>
> If I then re-tune the antenna for minimum reflected power on the
> Titan, the VSWR at the tuner, and also at a Bird inserted right
> after the Titan, shows a mismatch of 1.5 - 2:1. The mismatch seems
> to be greater at higher frequencies.
>
> The bridge in the tuner and the Bird are consistent -- that is,
> when the bridge in the tuner shows a match, so does the Bird, 12
> inches of RG-8 from the Titan's output connector.
>
> I have three different tuners -- a Ten Tec 229B, a Ten Tec 238A,
> and Drake MN2000. It happens with all of them. I've traded cables,
> made them as short as possible, changed the switch that switches
> between the tuners (from a B&W to an Alpha Delta). The exciter is
> going through the Titan to get to the antenna for tuneup. I've
> traded Birds. I've eliminated the Bird. All of the cables are RG-8,
> all are short.
>
> So you say, go with the bridges. BUT: Grid current in the Titan
> tends to be much lower when the Titan bridge indicates zero
> reflected power, and rises rapidly when I don't. And grid current
> is the big NO-NO in 3CX800A7's, the output tubes in the Titan. I am
> careful to keep grid current down to an indicated 30 mA for the
> pair (120 mA is absolute max). I can't get close rated power
> without excessive grid current unless I tune the antenna for zero
> reflected indicated on the Titan.
>
> The SWR indicator in the exciter (TS850) also agrees with the Bird
> and the bridge in the tuner -- IF I bypass the amp's RF circuitry
> and connect the radio direct to the tuner (or through the
> switches). This points to the RF path within the Titan between the
> SO-239 output connector and the final output stage. Has anyone
> encountered this before? Any fixes?
>
> 73,
>
> Jim Brown K9YC
>
Jim,
Sounds like you may have a load impedance that varies with
applied power. Try the following experiment - put the the Titan
in operate mode at full power, make the adjustment with the
external tuner for minimum VSWR (as you describe above) and
then slowly start backing off the exciter drive to see if the VSWR
changes slowly or abruptly as the output power of the Titan is
lowered. IOW, right now you have two data points - exciter
only and amplifier at full power. Try plotting the points in
between.
73, Mike W4EF.............
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