Hi Jim,
If I am reading your post correctly it sounds to me that everything is near
normal.
I don't know the titan amp but assume it operates like most other amps.
The first thing you want to do with your antenna tuner that should be
between the amp and antenna, and your bird between the tuner and amp, is
adjust the antenna tuner for a 1:1 match on the bird with just the exciter
on or the amp on it doesn't matter. Then DON'T touch the antenna tuner
again!
Now tune up the amp in the normal manor, for greatest output with whatever
amount of drive it is supposed to have. If the amp is under loaded the grid
current will be higher with the same amount of drive. The grid current will
drop as you load the amp heavier.
See what the SWR is between the exciter and the amp when the amp is
operating at full (normal) power and normal drive and fully loaded. It
should be under 2:1.
I assume that the amp is grounded grid; if you are going to adjust the
matching coils on the input of the amp or another tuner between the amp and
exciter, that adjustment needs to be done at full (normal) drive and full
(normal) power out of the amp. If you try and adjust the input match at a
lower power level the impedance will change as the amp puts out more power
or as you tune the plate circuit of the amp. That is a characteristic of
grounded grid amplifiers.
The SWR between the amp and the antenna tuner on the antenna should never
change.
Now if you switch the amp to standby and the exciter just feeds through the
SWR as seen at the exciter may be different than that seen on the bird meter
at the input of the antenna tuner. This would be due to a not perfect 50 ohm
path through the amps coax change over relay.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Jim Brown
> Sent: Sunday, September 24, 2006 4:03 PM
> To: amps@contesting.com
> 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
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|