VINTAGE VS. NEW

The million dollar gestion is "are vintage tubes better than new tubes?"  It is generally accepted that vintage tubes which were made by the famous manufacturers are of better quality, assuming of course that they are in proper and good physical and electrical condition. 

Availability of vintage tubes, especially in instances where you want matched pairs or quads is limited and can get pretty expensive. Replacements for single tubes to be used in a set where a failure occurs can also be problematic.

Be aware of the risks involved before jumping in to this endeavor. I'd recommend owning a good tube tester and learning how to use it before investing in the vintage tube adventure.

HEAT:

Yes, tubes get hot, very hot. This is normal. If however the metal plates are glowing red, shut down the unit immediately and seek service. Replace faulty tubes as necessary.

TUBE ROLLING:

Stay with the correct tube numbers is rule number 1. Tubes are part of the circuit, so replacing with an incorrect tube is the same as installing an incorrect value of capacitor or resistor, not good.

A very common mistake is made with rectifier tubes for the MKIII, MKIV and ST-70. The factory calls for a 5AR4/GZ34 only. This tube features a slow start warm up that is critical to the amplifier. Always use the right tube and always use a good quality brand. The tube should test both strong and balanced for optimal performance.

Different brands and differing internal designs can and will present differently so you can experience some variety in musical quality from one to another.

DRIVER TUBES:

Many of the required driver tubes are no longer in production so you will have to shop for NOS or good used tubes in these instances.

 A good example is the all but extinct 7199 driver tube for the ST-70. The ST-70 revised circuit uses a 6GH8A or a 6U8A. These are no longer in production but are plentiful in NOS or good used.

The 6AN8 / 6AN8A is another example (used in the MKIII). You  will only find NOS or used driver tubes for this amplifier.

The 12DW7 / 7247 tube used in the ST-35 is still being manufactured by Electro-Harmonix and JJ Electronics.