[TenTec] Ig drift on Titan 425
Dr. Gerald N. Johnson
geraldj at storm.weather.net
Fri Feb 6 19:15:20 EST 2009
On Fri, 2009-02-06 at 14:01 -0800, Jim Brown K9YC wrote:
> On Fri, 06 Feb 2009 15:45:23 -0500, Gary Smith wrote:
> >I seem to have a bit of Ig drift on the low bands, not so much if at
> >all on the upper bands.
> There are two potential issues. First, temperature sensitive components,
> especially capacitors, in the amplifier and antenna system can cause the
> tuning to change after you've been transmitting for a while. That happens
> no matter the condition of the tubes. When the Titan is mis-tuned, grid
> current increases, often a lot.
> Second, a symptom of tubes nearing the end of their useful life is grid
> current that drifts upward.
> I keep an SWR meter on the output of my Titans and tune for maximum smoke
> on the meter. That always corresponds to minimum grid current.
> 3CX800A7s are a class of tube that doesn't like excessive grid current,
> even for short periods of time. They're rated for 60mA each, absolute
> max. I run mine very conservatively, never intentionally exceeding 30mA
> for the pair. That leaves some headroom when something goes wrong, or for
> heating, or for when you want to QSY without retuning.
> Jim K9YC
Grid current metering components can drift when heated by larger than
planned grid currents.
Tuning for maximum output is the best way to tune a linear, especially a
grounded grid linear. It is easy to tune by currents along and to fry
the tubes from not getting much RF out while running full input power.
73, Jerry, K0CQ
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