Leach Amp: Q13, Q14 why 350 volts?

[swede]

Hi,

As Prof. Leach states in his parts list, Q13 and Q14 in the Leach Amp is hard to get (2n3439) since it is discontinued.

http://users.ece.gatech.edu/~mleach/lowtim/part3.html

At least here in Sweden it's kind of tricky to get hold of it. It is also hard to find a replacement BVceo=350 volts... So I wonder if anyone know if it is possible to use a different approach and use a whole other kind of transistor with let's say BVceo of about 80 Volts, since the rails are no more than +/- 58 Volts max.

Or can I use just any transistor with a hfe of 40 and Ic-max of 1 amp?

Regards,
//magnus

 

[Nelson Pass]

The 2N3440 should work fine, and should be
available more easily.

 

[swede]

Thanks a lot,

Prof. Leach suggest the very same transistor on his website, and it's available at a local supplier for about $2.5 US dollars a piece.

My next question is:

Why the high voltages?
From what I've understood, the voltage should not need to be higher than the rail voltages.(?)

Or is it that transistors with that high of a voltage is more durable under stress (high currents), or what?

It might be something I've missed in the theory, but I'm eager to learn. The same goes for Q12 and Q15.

Regards,
//magnus

 

[Jocko Homo]

Low C_ob.


Jocko

 

[djk]

The transistors must be able to stand the difference between B+ and B- and an extra of 10% for high line voltage.And remember to look at the secondary breakdown voltage too.