[Ron Ramirez]

The 5Y3GT and 5U4GB have the same basing, so if you plug a 5U4 into a 5Y3 socket, the radio will still work -

HOWEVER -

The 5U4 draws 3 amps of filament current at 5 volts, while the 5Y3 draws 2 amps. So the 5U4 is not really a good substitute for the 5Y3. Continuous use of a 5U4 in a 5Y3 will place a heavy strain on the radio's power transformer, which could lead to its failure.

So...replace that 5U4 with a 5Y3 right away. 5Y3 tubes are not difficult to find, and they are inexpensive.

Also I must echo what Gary and Joe have already warned you: please do replace all of the paper and electrolytic capacitors if you intend to use the radio. Also check all of the resistors, and replace any found to be more than 20% off published specifications.