[TenTec] Ig drift on Titan 425

[Dr. Gerald N. Johnson]

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. 
> 
> 73,
> 
> 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