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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|