[Amps] A Meeting Ground

[Will Matney]

Tony King wrote:

> At 10:33 PM 11/5/2004, Will Matney wrote:
> <snip>
>
>> 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).
>
> <snip>
>
> I have to ask... with all the talk about the combined currents on the 
> chassis... call that common... signal ground... chassis ground... 
> ground or take your pick...  no one has said anything about skin 
> effect... resistance, especially on the surface (where your RF is) of 
> aluminum.


Well the skin effect does come into play here as the RF does go towards 
the outer skin of the chassis material the same as in a conductor. The 
higher the frequency, the more towards the outer skin it travels and 
less current through the center.

>   What are you doing about that? Are you treating your aluminum 
> chassis with chromate?


No chromate that I know of. There's only three materials I've seen used 
for RF chassis and is copper, mild steel, and aluminum.

>
> 73, Tony W4ZT



Best & 73's

Will Matney