Well, there's several confusing posts here. My books don't list a 6415. It might be in Frank's Tube Pages though.
So perhaps it may seem redundant to state this, but for anybody who may stumble across this thread in the future, here goes: A 12 AT7 or 12AX7 or 12AU7 may be run with a heater voltage of 12 volts by applying the heater supply to pins 4 and 5. One heater wire goes to 4, the other wire to 5. Pin 9 is not connected to anything. The same tube can be run on 6 volts: wire one heater wire to pin 9. The other heater wire must be wired to both Pins 4 and 5 (merely bridge them together).
Not all tubes have this dual-voltage capability. A 12FQ8 for example does not have the middle tap (got any Wurlitzer organs? Plenty of 12FQ8's) so it must always be run on 12 volts. Same with the 6$$* tubes where the $ would be a letter and the * a number, like the 6SN7, they must always be run on 6 volts.
The pure number tubes like the 7027and 7247 and others don't follow any scheme as far as heater numbers... They are whatever number was open next. The 7027 is a 6 volt big bottle power tube, the 7247 is a 9 pin miniature dual triode either 6 or 12 volts
So when somebody has a tube XYZ and they want to sub it for a ABC tube, they need to look at the number of pins, first. Obviously a 9 pin miniature tube won't fit a Compactron socket. If the number of pins are the same, look at each individual pin to see if it's the same pin out. Are pins 4,5 and 9 the heater pins? Same with plates and cathodes and grids. If they are the same, no rewiring necessary, if they aren't the same, the socket will need rewiring to even attempt to use the different tube.
So let's say the wiring is the same, or you can make it the same. Now...is the heater voltage the same? How about heater current? Not so critical in miniature tubes, but critical in power tubes. Ok, now how about plate voltage? Does the replacement tube have limits on plate voltage? Then comes the rest of the electrical characteristics. Similar gain (transconductance)? Then look at the tube charts and ideally draw load lines to see if they will be biased similarly without changing anything. It's possible the new tube will be biased into cutoff, or clip too easily if the new tube will be at a far different bias point.
I may have left something out, that was the speedy version, so if I've fat-fingered a response, don't shoot me. I'm only the piano player...oh wait, I don't play piano.
And if all this is old-hat to anybody here, I'm not trying to sound condescending either, but there are some new folks who may think "hey this new tube has 9 pins and it's a 12 something, this tube in my amp also has 9 pins and it's a 12 something also. Let's see if this new tube works!!"
