[TenTec] Ig drift on Titan 425

Gary Smith Gary at doctorgary.net
Sun Feb 8 02:09:55 EST 2009


I was just heading to bed when I posted about this and then saw your 
post was also just posted so I am responding to it before pulling the 
plug. I'll insert comments to help give a better picture. 

> 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. 

I've only used the 100 watts into the 5KW Bird dummy load. None of 
the components got warm to the touch except the dummy itself after 
prolonged experimenting and even that was just slightly warm in the 
cold basement. The reason I ran that power was it was suggested that 
at 100W drive, it should output around 2400 W and if it was not able 
to do around that, that the tubes might likely be soft and the cause 
of my problem. In on air operation I use around 45 watts drive. I 
sometimes contest with 1 watt barefoot. :) 

> 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. 

I do have a Nye Transmatch but I don't need it. All my antennas have 
been trimmed to 1:1 at the feedpoint. The amp's SWR meter doesn't 
blip and neither does the one in the exciter when under load. The MFJ 
analyzer is happy with the antenna settings both at the feed point 
and in the shack. The dummy reads flat from 1.8 through 50Mhz and 
darned close to 1:1 on 2 meters. When I adjust into the dummy load, 
it's the same results as transmitting into the antenna. Took a lot of 
work to get those antennae set up properly but it was worth it. 

SWR is fine. 

> 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. 

I recorded these values last week. I might have been able to improve 
the output some. These values are all from the metering on the 425 
which might be off. Some of the values I had to guess at because the 
needle was in-between numbers. 

35W = 1000 2.0KV .68 

50W = 1400 ~1950 .95 

75W = 1600 ~1950 1.0 

100W= 1900 1950 1.2 

It should be noted that with the 50 watt & up drive, when I would 
stop transmission and then transmit again, the Ig would redline or if 
not pin the meter, the overdrive light would light and the needle was 
way up there. Much worse as the power increased. I could never use 
100 watts on 160 even if I wanted to because every time I would key 
after listening, I would be pinning the meter if set on Ig. 

It should also be noted that when I tuned up on 14MHZ into the dummy 
that I got somewhere around 2200W out and when I would transmit the 
next time, there was no marked change in Ig and it was pretty much 
stable. My memory says it would immediately jump to maybe 35Ma and 
then drop somewhat slowly to between 25 and 30. Nothing close to 
overdriving the meter. 

So 20 meters is nicely stable with 100 watts drive. 

> One needs to 
> know or observe if the plate current is changing or the HV is changing as 
> well. 

There are no rapid fluctuations of the KV. Same goes for the plate 
current. With sustained key down like tuning on 160, the Ig 
constantly drops downward, never goes up unless I change the plate or 
load cap value as in tuning. 

On 160 while tuning the tank everything is varying as I tune. Once I 
have the output where it is and kill the exciter, when I transmit 
next it almost always goes into overdrive and usually pins the meter. 
I have to lower the drive to say 30 watts and then without touching 
anything on the amp, if I increase drive the Ig drops and continues 
to do so up to the level it was tuned to before I had shut off the 

On 160 & on 80, if the Ig does not redline and I leave the exciter 
on, the Ig will continue to drift down. When I kill the exciter again 
& transmits, the problem repeats itself as described above. 

I tried this same procedure on 20 meters and I had none of the issues 
with drifting I had on the lower bands. I haven't tried it on 10, 15 
or 40. It does have drift on 80 that mimics the 160 experience but it 
is not as pronounced as 160 shows. 

> 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. 

I'm using RG-213 that is new last fall 2008 from Davis in New 
Hampshire. I'm using the 213 for everything including from the 
exciter to the amp. None of the coax is warm to the touch after 
anything I've done. I did make it a point to check that possibility. 

> 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. 

I try to be pretty obsessive about having things right and indeed the 
connectors are tight. They're teflon connectors and soldered with 
care. I remember trying to wiggle all the coax connectors when I was 
troubleshooting last week and there was no give anywhere. I did feel 
along the entire coax between the amp and the dummy load just to be 
sure the coax wasn't warm. I did touch the connectors too. No issue 
there at all. 

I really appreciate the thoughts. 

Anything come to mind with the above info? 


> 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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> >> 
> > 
> > 
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