In a message dated 11/5/04 4:57:10 PM Central Standard Time, craxd1@ezwv.com
writes:
Dennis,
Well they probably would somewhat but I would think the currents in the
face plate would be reduced a good degree this way. If the negative from
the power supply is attached directly to the chassis pan like most are,
the the electrons will flow towards the capacitor, It the wiper lead is
attached to the pan, and the current passed it first, they should tend
to follow it more. The reason being is there's two mechanical
connections between the chassis and the cap through the faceplate
----------------------------------------------
Will,
I agree with you that more wiper contact is better, especially when there
are large amounts of circulating RF currents (High Q) in the tank.
But......
I believe that in your analysis you may be confusing the DC plate power
supply currents with the circulating RF currents in the tune and load caps.
Since a part of tuning capacitance is the internal capacitance of the tube,
the best possible scenario would be to have both the tune and load cap
frames or wipers going to a single point ground at the tube RF ground location.
Those circulating RF currents should not pass through the negative lead of
the power supply at all.
Your wiper direct idea is nice because I have seen poor connections between
the wipers and the frames, the ball bearings and the frames, and the frames
to the chassis via the mounting screws, cause all kinds of strange problems.
I even had a small fire once caused by RF currents heating the ball
bearings of an old load cap hot enough to ignite the bearing grease. (At least
it
showed me where the intermittent tuning problem was coming from, and taught me
an important lesson about the magnitude of the circulating RF currents in any
high Q tank circuit!)
At the very least, I solder short lengths of wire, (or flat braid like
solderwick if I have it handy), to the frames of both the tune and load caps,
and
then tie the wires together and solder them to the chassis between the two
caps. This does two things for me very cheaply, (the key word here being
CHEAPLY),...It eliminates the possible grounding problem between the frames
and
the chassis, and it removes that 'tingle' one sometimes gets when they
accidentally touch the set screw of their tuning knob when the tune cap is not
properly grounded.
This technique probably would not work as well at UHF frequencies, but I
have successfully applied it from 160 to 6 meters.
Regards,
Dennis O.
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