Question about output Transistors

[Denis.BR]

Hey people!

I have a question just for curiosity: I foud this amp, a Gradiente Model 80 in a pawn shop for a very cheap price. Its a vintage, with 30 years old but it works perfectly, with all the original components in place.

But I was watching the wiring diagram and noticed that the output transistors are two NPN RCA3055.

I'm not a genious in electronics, but I thought that the output transistors should be one NPN and one PNP.

How does a circuit like this works?

Edit: you can find the diagram at http://www.4shared.com/file/17216381/b0ac6fc5/m80.html

[Jan Dupont]

Not necessary
If you look closely you see that the signal to the Base of the negative output 3055 comes from the collector of the driver transistor (the signal to the 3055 base has been inverted)

[Denis.BR]

I see..

So the T802 amplifies what the T107 pre amplified? What is the advantage?

[Jan Dupont]

No special advantages...
Just another way to do it

[Denis.BR]

Nice! one more information for the data bank!

Thanks a lot!

[Jan Dupont]

You're welcome

[Denis.BR]

And what about these capacitors in the output? They are foi coupling, right?

What happens if I decrease or increase it's value?

[dmfraser]

The quasi-complementary output using 2 NPN transistors was used because in the mid 60s high power silicon PNP transistors were not really invented yet. They did not come out until about 1970 and we then moved to fully complementary amplifier output stages.