They are not interchangeable.. they require some similar voltages, but that is all.. the 211 is higher mu and also higher plate resistance and requires a much more difficult to wind output transformer.. the 845 is more forgiving here and therefore can be built more inexpensively.. but beware, both of these tubes built into a proper amplifier should be expensive... if you find a cheap 845 or 211, you will not really get to know the 845 and, especially, the 211 "sound"..
the tubes themselves are another story.. NOS 211's and VT-4c's are relatively cheap.. $200 to $300 a pair.. NOS 845's are more like $1200.. luckily, the chinese 845 sounds quite good for it's paltry cost, like $100 a pair!
Peace,
Me
the tubes themselves are another story.. NOS 211's and VT-4c's are relatively cheap.. $200 to $300 a pair.. NOS 845's are more like $1200.. luckily, the chinese 845 sounds quite good for it's paltry cost, like $100 a pair!
Peace,
Me