12BH7 vacuum tube is a medium mu dual triode. 12BH7 was designed as a driver in TV sets.
12BH7 Tube is capable of driving plenty of current with low distortion. This makes it ideal as a driver for power tubes in modern tube audio amplifiers.
12BH7A tube with suffix 'A' has a controlled heater warm-up for use in a series filament circuit. In modern tube amps, there is no difference between the two. You can use either 12BH7 or 12BH7A.
Since 12BH7 is a high current driver, the internal components are much bigger than other small-signal tubes like 12AX7. The larger plate boxes in 12BH7 house large cathode, which can emit more electrons.
The larger plate boxes in 12BH7 tubes are more prone to microphonics. This is not an issue when used as a driver tube, but if your application calls for low microphonics, we recommend that you add a low noise screening option.
JJ ECC99 is very similar to the 12BH7 tube. JJ originally designed the ECC99 tube to drive the venerable 300B tubes. ECC99 can be used as a substitute for 12BH7 in many cases. Please refer to your amp manufacturer's recommendation if you aren't sure.