[TenTec] Ig drift on Titan 425

[Jim Brown K9YC]

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