[TenTec] Ig drift on amps

Gary Smith Gary at doctorgary.net
Sun Feb 8 01:03:55 EST 2009

Thanks for the suggestions. 

I'm not near the amplifier at the moment and can't open the amp to 
have a look see but I did bring my manual with me and took a macro of 
the the band switching section of the schematic and since I can't 
attach it for here, it's on my website.

I couldn't see on the photo of the amp just where the padding caps 
are. Perhaps the idea of using doorknobs in the amp is not practical 
space wise. It seems as the tuner mentioned in the thread might have 
more space than the 425's tank?

If there's space in the 425, it seems it might be worthwhile to just 
replace all the padding caps with superior versions than are in there 
right now. I'd be more than happy to do that. One kind offlist email 
suggested amp ratings of at least 2.5 at 1.8Mhz. I would prefer to 
use something well suited and add in some overkill factor if that's 
reasonable, I do want stability and this amplifier is really 
excellent other than this one current issue. If I can beef up the 
components to make it more bulletproof, I'm for that.

So based on the above info & the schematic section, which caps might 
I be most likely to want to replace and what would be best to replace 
them with? I am out of doorknobs except for a couple of fixed vacuum 
caps but they're the size of a large doorknob.

Thanks again for the help, I've apparently got this issue narrowed 
down quite a bit with everyone's help.


P.S. Some folks have sent me helpful links to past discussions others 
have had regarding this same problem. Seems like a good idea would be 
to include this drift issue in the wiki and once I get the specifics 
of what to replace and with what, I'll take photos of the process to 
put on my web page and then whoever runs the wiki might like the all 
the info & photos uploaded to them so they can add it to the 425 

> Regarding the 238 tuner caps, I experienced some SWR drift with mine at high 
> power and on certain bands. I one case I could use the same antenna and 
> feedline on the lower end of 80M with no change in SWR.  However, at the 
> upper end with the same antenna and feed line on 75M there was SWR drift.  I 
> could always tune it so the SWR was low but then when the caps cooled the 
> SWR had changed.  Of course the SWR drift caused the tuning on the Centurion 
> to change.
> I used a laser temperature meter to see what was heating. It was the ceramic 
> caps located at the back of the Low Z / High Z switch.  I replaced the 220 
> pf caps units wired in parallel with some Russian made 470 pf 16KV RF caps 
> purchased on E- Bay.   There's a couple of websites that show how these can 
> fit in.  It takes drilling about 4 holes in the bottom for mounting and 
> wiring them in place.   Problem solved.  As to a source of caps, should you 
> not like Surplus Sales of Nebraska, try Fair Radio Sales.  Or as I did, 
> E-Bay.
> One other problem I found was the accumulation of paint, dirt and grime on 
> the insulation of the window/ladder line that runs along the side the house 
> and up the tower.  Yes, the fellows that painted the house actually sprayed 
> the ladder or window line section that runs under the eve to the tower. 
> Again, this was a heating issue causing "what ever Z" change to take place. 
> I cleaned the paint off of that section of line using lacquer thinner and 
> the problem was solved.
> As some of you may or may not know, I use a couple of retired BC 
> transmitters for 160M and 75M AM operation.  In running these at a legal 375 
> watts of carrier output with 125% positive modulation, I use a Tentec 238 
> tuner for the matching chores.  Trust me,  that application will separate 
> the good tuners from the poor tuners.  I don't care what the PEP rating of 
> the tuner is.  I have one of the popular 3 KW PEP rated tuners that will 
> flash over on modulation peaks and finally the "heavy duty" internal balun 
> smoked, while under the exact same conditions,  the Tentec 238 stays cool 
> and performs exactly the same tasks as needed.  Oh yes, the match values for 
> the tuners are always set using a antenna bridge.  I find that method 
> absolutely necessary when using a tuner with a T network configuration.
> 73
> Bob, K4TAX
> ----- Original Message ----- 
> From: "Jim Brown K9YC" <k9yc at audiosystemsgroup.com>
> To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
> Sent: Saturday, February 07, 2009 4:49 PM
> Subject: Re: [TenTec] Ig drift on amps
> > On Sat, 7 Feb 2009 16:09:07 -0600, Duane Calvin wrote:
> >
> >>Yes, Ron, I've experienced the same symptoms in my 238B
> >
> > Ten Tec uses N1500 caps in the 229 and 238 tuners. Some matching
> > conditions cause these caps to carry a lot of current, enough to
> > cause them to change value by a fair amount. There are also matching
> > conditions that cause them to burn up. I run 229s and 238s with my
> > Titan during contests at the 1.2 kw level. I own six of these
> > tuners, and I've fried at least one cap in all but one of them.
> >
> > On the oldest of my tuners, a 229B, I had enough heating around the
> > front end of the rotary inductor to toast the spring that holds the
> > traveling tap in contact with the inductor. I managed to repair it
> > with a replacement spring that I found in a surplus house.
> >
> > 73,
> >
> > Jim K9YC
> >
> >
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> > TenTec at contesting.com
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> > 
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