"I know it works perfectly." It just might in this ONE case you site. The two tubes are not close to each other at all. 12at7 gain 60 12ax7 gain 100
Say they are run at 250 volts B+ 12ax7 plate current 1.2 ma 12at7 10ma
Plate resistance 12at7 10900 ohms 12ax7 62500 ohms. Grid voltage 12at7 -12 volts 12ax7 -2 volts. Etc... Lewm is on the money these two tubes, short of the heater current and can be run on the same b+ , are not even close.
It would be advisable to have a schematic and know the size of the transformers and how much current the power supply can handle to know if there could be a problem with the parts. There is some math to do to set the tubes where load lines are ideal.
That being said I prefer the sound of 12at7 over 12ax7 in phono stages. But would design it accordingly. If the guy who has the Mac preamp actually figured out where to run the 12 at7 properly it could even sound better.
Enjoy the ride
Tom
PS. In regards to matched tubes ,over rated in my book. Plus short of having a load line tester , tube testers are ballpark at best. Also the op could use 2 different types he has in on hand one on each side see how much difference he hears even with two different brands.
Say they are run at 250 volts B+ 12ax7 plate current 1.2 ma 12at7 10ma
Plate resistance 12at7 10900 ohms 12ax7 62500 ohms. Grid voltage 12at7 -12 volts 12ax7 -2 volts. Etc... Lewm is on the money these two tubes, short of the heater current and can be run on the same b+ , are not even close.
It would be advisable to have a schematic and know the size of the transformers and how much current the power supply can handle to know if there could be a problem with the parts. There is some math to do to set the tubes where load lines are ideal.
That being said I prefer the sound of 12at7 over 12ax7 in phono stages. But would design it accordingly. If the guy who has the Mac preamp actually figured out where to run the 12 at7 properly it could even sound better.
Enjoy the ride
Tom
PS. In regards to matched tubes ,over rated in my book. Plus short of having a load line tester , tube testers are ballpark at best. Also the op could use 2 different types he has in on hand one on each side see how much difference he hears even with two different brands.