Without a doubt tuning drift is caused by the fixed caps in the loading
circuit and not by the .001 blocking caps.
Was this a bad brand of caps? Bad design (too low current / voltage
ratings)? Aging of components?
If I have to get into cutting out a bad cap I'm going to go through the
trouble of replacing all the fixed caps with higher ratings..including the
DC blocking caps. I'm beginning to not trust any of the components in my
Titan. It's turning into a ground up re-build. But it will be a
better-than-new amp once I'm finished.
Anyone taking odds that now that I've replaced every cap in the PS, new
meter bulbs, tightened and lubricated band switch wafers, replaced all the
loading caps....
That the first time I hit the key the tubes will head off into the sunset?
Tom K2TA
tom@k2ta.com
----- Original Message -----
From: "Dr. Gerald N. Johnson, electrical engineer" <geraldj@isunet.net>
To: <tentec@contesting.com>
Sent: Sunday, March 16, 2003 8:30 PM
Subject: Re: [TenTec] Omni VI & Titan tuning repeatability
> The plate blocking capacitors should be the least effect at 160 meters
> because the tube plate C is far less than the tuning C. At 10 meters the
> tube plate C is a large fraction of the circuit tuning C so changes in
> the blocking capacitors may have a greater effect. On the other hand the
> tube C is maybe 20 or 30 PF and the 1000 pf series C of the blocking
> capacitors can change a lot without changing the effective tuning C. But
> the posted complaint was loss of output at 160 meters, not 10 meters.
>
> On the other hand, loading padding capacitors are only in the circuit on
> the low bands and drift in their value will change the output of the
> amplifier.
>
> Only changing 45% seems small for the Z5U dielectric. I've seen some
> loose 80% from room to soldering temperature. Suitable only for
> uncritical bypass applications when the bypass capacitor has been
> selected to have several times the value needed for stability. NOT
> suitable for any kind of tuned circuit.
>
> 858s are good, expensive, and hard to find, but good for RF circuits.
>
> 73, Jerry, K0CQ
>
> --
> Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
> Reproduction by permission only.
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