The ornamental design for a deskfan with a tubular base, as shown and described.
FIG. 1 is a front perspective view of the desk fan;
FIG. 2 is a rear perspective view of the desk fan;
FIG. 3 is a rear elevational view of the desk fan;
FIG. 4 is a front elevational view of the desk fan;
FIG. 5 is a side elevational view of the desk fan;
FIG. 6 is a side elevational view of the opposed side of the desk fan;
FIG. 7 is a top view of the desk fan; and,
FIG. 8 is a bottom view of the desk fan.
The broken line showing of the fan blades and hub is included for the purpose of illustrating environmental structure and forms no part of the claimed design.