The major cause of the Titan I power output and tuning drift problem is due to
dielectric heating of the plate circuit DC blocking capacitors. Ten Tec uses
two 0.001 uF/6 kV disc ceramic capacitors with a Z5U dielectric characteristic
in parallel (C6 and C7) for this application.
Type Z5U disc ceramic capacitors exhibit poor RF current and temperature
performance (especially at the lower frequencies), and a very small change in
temperature due to dielectric heating causes a significant change in
capacitance, varying as much as +20% between the temperature range of +20C
to +35C (capacitance can actually vary as much as 45% over the temperature
range of +10C to +35C). Such a change affects the Titan I tank circuit tuning
capacitor (C26/C14) setting significantly as the temperature of the blocking
capacitor changes with operating time.
Replacing the two disc ceramics with 0.001/5kV Type 858 doorknob capacitors
having an X5V dielectric characteristic and rated for RF current handling
applications eliminates the drift problem.
I'll be happy to email temp. vs dielectric curves for the subject caps, mod
instructions, and photos to anyone requesting.
73, Dave, K1FK
Fort Kent, ME
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