I would start with the AWS A2.4 standard.
We recently concluded a six week quality assurance course with our detailers, fitters and welders which included welding symbols.
As for your question, if the 12x12 plate is centered, like a base plate, on the end of the tube, I would imagine that a weld all-around symbol would be sufficient with the fillet weld symbol on the arrow side of the reference line. As for the weld size, that is up to your own shop standard unless the EOR has directed otherwise.
Check out Figure 9(B) in the ANSI/AWS A2.4.
Hope that is of some help.
ziggy

The described symbol means weld both sides of the joint.
How long is the tubing?
Is it short enough that you could weld the inside?
If not, the detailer should have used a different symbol.
Could he have meant that the tubing was to be welded only on two sides?
You better check and find out.
Who knows, it might have to be a full penetration weld.
Tim

The people who put welding symbols on drawings very often don't know the literal interpretation. According to A2.4-98 the correct symbol for what you have described most likely should have been an arrow side fillet with either the "all-around" circle at the reference line/arrow junction, or another arrow pointing to the opposite side. The arrow side/opposite side fillet symbol really means to weld the inside and outside of wherever the arrow is pointing but not necessarily all four sides unless arrows point there. A2.4 Fig. 9 page 17 &18 help to illustrate the arrow side symbol.

Also, "The American Welder" Mar/Apr 2000 article "Blueprint Bloopers: Common Mistakes with Welding Symbols" page 18 verifies the above info.

I agree with ziggy. He is correct. Most of the time, detailer doesnt know the meaning of the weld symbol that they are assigning in their drawing.