Originally Posted by heinrichs
I really have to remember that different tubes are so incredibly different... Compared to transistors were I can make an ok amp with almost any power transistors in the output. I dont know if this is true.... but right know it feels true, tubes are confusing
That's the difference between high gain devices like transistors and low gain devices. Even in a low gain topology, the behaviour of a transistor circuit is largely independent of the active device (BJTs mainly). Unless you need some special characteristic, like an unusually low noise figure, it doesn't matter much what type of transistor you use so long as it can process the highest frequencies of interest. I've done lots of projects with those packets of "transistors anonymous" Rat Shack used to sell.
With tubes, it's different since these are low gain devices, and circuit performance is a good deal more dependent on the characteristics. That's why "tube rolling", especially where NFB isn't being included, can get you a different sound.
You can try and listen with the gNFB disconnected. It won't stay that way for very long.
Transistors have ratings; vacuum tubes have guide lines