[Amps] A Meeting Ground

[Will Matney]

Dennis12Amplify at aol.com wrote:

> In a message dated 11/5/04 4:57:10 PM Central Standard Time, 
> craxd1 at 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.


Well, the DC currents and RF currents are intermixed in the chassis 
itself. The RF current circulates through the tank circuit to the load 
and back to ground. It is also across the cathode and anode of the 
tube(s). The reason for mentioning the PSU ground is the B- is at the 
cathode. But, the tank capacitors and coil charge and discharge through 
the load and then back. The power fed to the tank then is created by the 
tube(s) and PSU. By the impedances being different, more RF should go to 
ground through the wipers than through the shaft. Actually, that's how 
they're designed to operate. It's really a simple parallel resistor 
circuit but with mighty low resistances.

>  
>  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.


Exactly, and with the shortest connection possible.

>  
>  Those circulating RF currents should not pass through the negative 
> lead of the power supply at all.


No they wont, they are blocked by the plate choke from doing so, and 
filtered by the bypass capacitors. They intermix with each other through 
the chassis only at the tube(s).

>  
>  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.


Some smaller air variable caps don't even have ball bearings, the shaft 
is only riding on the brass collars I.D. as a bushing where the stem 
rotates inside the threaded collar. Enough RF going through these 
bearings eventually pits the surface or can weld the stem to the body.

>  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!)


Exactly. About like hooking the ground lead on a motors shaft and doing 
welding on the frame. Bearings sure don't like it.

>  
>  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.


Exactly, and you did what should have been done, "leads as short as 
possible". Oren Elliott makes their caps with optional nylon shaft ends 
because of getting a nip by the brass set screws in a knob. They start 
at about 1/8" away from the end of the threaded collar and extend to how 
ever long you want it. These are the kind I always purchased.

>  
>  This technique probably would not work as well at UHF frequencies, 
> but I have successfully applied it from 160 to 6 meters.


I've used them at 6 meters very successfully. But as you state, VHF in 
the order of 100+ MHz is another story.

>  
> Regards,
>  
> Dennis O.



Best & 73's

Will Matney