No slower than MJ15003’s. They are pretty much as big of a die as will go in a TO-220. Beta hold up at high current is better than 2N3055’s. If you are looking for high current (say 10 amps peak), are stuck in a TO-220 form factor, and a 3 or so MHz isn’t a deal breaker then they are the go-to devices. Second breakdown capability isn’t wonderful, but useable on +/-30 ish volt rails. Which is what all those old car amps had. And they worked down to 2 ohms.
People get the idea that faster is always better, and might be tempted to use D44/D45’s. Beta hold up is really nice too. Don’t. They are fragile. Useable on linear amps only up to about +/-18V, only suitable for switching above that. For FAST replacements of TO-220 outputs the MJE1503X series does better. They have been used with varying degrees of success. But if you try to run 4 ohms, they are more fragile than the D525/B596, which will run outside ratings but not just die on you. Others will. It’s a cliff and you just fall off it. The BD911/12 won’t fall off that cliff, you are just stuck with 3 MHz, beta falling to 20 at 10A, and the limited average dissipation of TO-220s (On realistic heat sinks).
People get the idea that faster is always better, and might be tempted to use D44/D45’s. Beta hold up is really nice too. Don’t. They are fragile. Useable on linear amps only up to about +/-18V, only suitable for switching above that. For FAST replacements of TO-220 outputs the MJE1503X series does better. They have been used with varying degrees of success. But if you try to run 4 ohms, they are more fragile than the D525/B596, which will run outside ratings but not just die on you. Others will. It’s a cliff and you just fall off it. The BD911/12 won’t fall off that cliff, you are just stuck with 3 MHz, beta falling to 20 at 10A, and the limited average dissipation of TO-220s (On realistic heat sinks).
