matched power transistors......

[matthias]

I've got a question!
I have a Sunn Concert Slave head, sounded killer by the way!
Blah blah blah, long story short,
3 of the 5 power transistors have failed.
When testing them I noticed that 3 were 3055's and 2 were HEP704
the 3055's had #'s hand written on them, 86 and 87.
the HEP704 were not.
I'm wondering if these numbers represented thier original voltage drop? as in .86V or .87V.
But then on testing a known good (as in new) 3055, the voltage drop is around .5+V
one 704 and one 3055 survived.
NTE says that 3055 is a work alike for 704.
these transistors are all transformer coupled.
Do these need to be matched?
by what parameters are they matched?
Is there a method by which I can match a set my self?
here is a schem that seems to work, poweramp only: http://sunn.ampage.org/schematics/concertbass.gif
Previous schems I posted were wrong, sorry.
-Matthias

[teemuk]

I'd say those four power transistors after transformer coupling (Q2, Q3, Q4, Q5) need matching. The tolerances of transistors are not as large as with fets so using equal model should be all the matching you need - the emittter resistors handle the rest. Also, if three of them failed it's likely that the rest will fail soon also. I would change either all four (three gone bad from the group of four) or all five (the class-A driver Q1 gone bad also).

[trevize]

here you have the datasheets
http://www.alldatasheet.net/view.jsp?Searchword=2N3

These are really cheap and powerful transistors, i saw them for 1.3 euros here in italy.

BTW Your amp has a REALLY nice and simple preamp.

[matthias]

Thanks Teemuk,
I think Q1 is good, but whats another $1.50.
I figure I'll do 'em all.
What do you mean by "equal model"?

Trevize, yeah, that pre-amp does look nice and simple, but I have the Slave head which is the poweramp only. But I guess there is enough room in the chassis if I wanted to clone the preamp.

[teemuk]

Sorry, my english sucks sometimes. By "equal" model I meant similar model. In this case, with "similar model" I mean that the model can be a similar substitute to original one (i.e. 3055 and HEP704) but the model of all transistors must be the same (no combinations of different models). In my opinion, using two different models at the same time was a mismatch begging for problems. The Q1 is an exception since it doesn't belong into the symmetrical group of the four other power transistors.