Power horizontal deflection transistors work fine in output stages ?

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The simple answer is no.
The gain is normally too low and the DC SOA is way too small - not a problem in their original application where they work as switches.
That being said, although these are generally marked as 'BUxxx' for Euro type markings, it does not follow automatically that all BU marked transistors are for that purpose. You should check for speciffic types.
 
I used to use HV deflection transistors in commercial HV amplifiers, up to 1200V. As mentioned above, the beta can drop to one or two, and SOA is limited. They are not robust in anything other than their designed application. For the HV amps, drive current isn't a problem because any simple LV stage can come up with enough current, even if the beta dropped below one, when you consider that the needed output current is very low. Not so for audio. What the transistor gets you is the HV capability, but they aren't fast. If you got a pile of them for free, maybe you could wire a bunch as parallel darlingtons, and force a design, but performance wouldn't be very good for all the effort involved.
 
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