[TenTec] Ig drift on Titan 425

[Bob McGraw - K4TAX]

Sounds like the amp may be overdriven at 100 watts.  This seems like a lot 
of drive for an amp of this type.  And secondly, it may not be loaded 
correctly thus there is excessive heating of the tube elements, namely the 
grid.  Also the comment about no SWR change between the exciter and the amp 
does not convey to me that the load is or is not changing.  The  SWR should 
be measured between the amp and the load or antenna tuner or antenna as the 
case may be.

Also, I don't see any evidence of replacing cables, connectors and such 
between the amp and its load.  These, while looking good, can heat and cause 
the load to change thus the tuning of the amp changes.  If this amp is 
producing 2100 watts out, I doubt that the tubes are faulty.  One needs to 
know or observe if the plate current is changing or the HV is changing as 
well.

I've repaired several of these amps.  I've never found the tuning or loading 
to drift.  They produce a lot of power with a relatively small amount of 
drive.  I can usually get about 2KW out with only 60 to 70 watts of drive. 
They produce enough power to heat coax like RG-8 and RG-213 that is not of 
premium quality.  Same true for PL-259 connectors and connectors that are 
only hand tight.  This heating can be reflected to the amp as tuning drift 
when in actuality it is the load connected to the amp that is changing.

73
Bob, K4TAX




----- Original Message ----- 
From: "Gary Smith" <Gary at doctorgary.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Saturday, February 07, 2009 1:20 PM
Subject: Re: [TenTec] Ig drift on Titan 425


> 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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