name that tube

No, but 5V4s and 5Y3s are basically interchangable . . . the 5V4 IIRC has a bit higher current capability and a little less forward voltage drop, but the differences aren't huge.

Hmmm, pardon me but IMHO,NOT ALWAYS THE CASE! It depends on the design of the circuit. In preamps, maybe but in amps not always.

For example, you have a 45 SET amp whose output tube Power Dissipation (Pd) is operating at 10 Watts maximum rating (as specified by RCA receiving tube manual) using a 5Y3's. What do you think would happen if you use a 5V4?

5V4, tube voltage drop is 25 volts at 175 ma. max I=175 ma.
5Y3, tube voltage drop is 60 volts at 175 ma. max I=125 ma.

Good bye 45 tube!

One way to identify the tube is to post a pic showing the plate structure of the tube. Most of the time, the 5V4's in ST bottles are flat plates. Also, you can use a magnifying glass to look for any trace of the faded markings on the tube, usually for RCA, the mark is directly in line with the guide pin as you go up on the glass.

"Goodbye 45 tube"??? Give me a break.

A 10-watt tube in SE class A has . . . what, 250-300v on the plate at 35-30mA? So changing the B+ by 35v will change the plate dissapation on the output tube by about 1-1/2 watts . . . differences in transconductance across various 45s will cause at least this much varation.

Not to mention a variation in line voltage by 10% . . . again, the differences AREN'T huge. But I guess if you own/build an amplifier that runs all the components really close to the bleeding edge, then maybe EVERY tube should be hand-picked and measured first.