name that tube

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.

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.

I suggest you read some more! At 11 watts Pd of a 45 tube specified at 10 watts, draw your own conclusion. Different transconductance or not, we are talking about increase in B+ resulting to Increase in plate voltage are we not?

Audiofeil does not need to explain it to you. It has something to do with Operating Points where the tubes are bias. But your statement says "5Y3 and 5V4 are always interchangeable", what we are trying to say is IT DEPENDS! You have to know where the output tube is operating before saying a blanket statement like that.

BTW, why are you concern about current levels when voltage already increases and tells you that the power did the same?

Yes, a 5Y3 can be substituted with a 5U4 because in using the latter, the filament supply was already establish that it can take 3 Amps draw. A 5Y3 only takes 2 amps. But, this is a different scenario over all and not analogous to substituting 5V4 to 5Y3. VERY BAD INFORMATION!

Ahh, I am out of here! You guys decide what is right and wrong. Consider my post as such, with no real info. I forgot that this is Audiogon after all. Sometimes I wonder why do I have to participate in threads like this. If someone can show me how to delete my previous post, please let me know.