[TenTec] Ig drift on Titan 425

Gary Smith Gary at doctorgary.net
Sat Feb 7 14:20:04 EST 2009


Hi,

First of all, thanks for the on & off list replies. There are some 
common thoughts that have been offered. More below.

> >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. 
 
This seems to be a recurrent theme on the replies. Another frequent 
reply relates to the same issue at the antenna feedpoint/matching 
network. As I have the same issue using either my 5KW Bird dummy load 
or the antenna and that the SWR readings do not change at the 
exciter, I'm excluding the termination as a variable and localize the 
problem to the amplifier.

> Second, a symptom of tubes nearing the end of their useful life is grid 
> current that drifts upward. 

I visualize from this that with bad tubes under load, the Ig readings 
on the meter is seen to increase, showing the grid current increasing 
while in a key down state.

I believe I have the opposite situation; In a key down state with 
high output, the IG drifts downward. After a brief moment of no key 
down, when I transmit, the Ig is so high that the meter will pin or 
if not pin, the overdrive light is bright. At lower input/output, IE; 
800W out, the drift is minimal.

Another comment I got related to this being an issue on the lower 
bands & padding capacitor issues. I rarely am on the higher bands as 
I've been a bottom feeder on 160 trying to earn 6 band DXCC and don't 
use the upper frequenciwes much at the moment.

So I tuned into the dummy load, 100W input and adjusted the amp to be 
at max output with a 30 Ig. The amp read around 2,100 W out on the 
analog meter. I waited some time and keyed with no issues with the 
Ig. Repeated waiting and trial keying and there was no problem of any 
kind so indeed this Ig issue appears to relate to the lower bands and 
not all bands.

I will need to figure out what to do regarding the padding cap so 
that this is not an issue. What I am concerned about is this drift 
issue may have been the reason I felt the original tubes were bad and 
perhaps they were damaged by unregulated drift as suggested by the 
2nd paragraph below.

> 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. 

I have been really observant about not abusing the tubes and only 
with the bad results from the drift when I transmit after tuning to 
frequency and then transmit again following a period of rest, do I 
get high Ig. Hopefully this second set of tubes has not been damaged 
by this.

I guess I need to either find a new single padder cap or a pair of 
them to distribute the current between them.

Any ideas where I might be able to find a good source for these caps 
other than Surplus Sales who I refuse to deal with ever again?

Thanks,

Gary
KA1J

> 73,
> 
> Jim K9YC 
> 
> 
> 
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