Output transistors and gain

[Axtman]

I recently pulled the output transistors from a Sunn Beta Lead amp that I am servicing. One side had 2N3055 transistors. The gain (hFE) tested 8, 8, and 35. According to the datasheet, those 2N3055 transistors should test 25 to 70.

Do transistors that age or are stressed get lower in gain? Should I consider transistors lower than spec to be bad?

[Helmholtz]

hFE varies with collector current.
If you tested at low mAs, hFE will be low.
Datasheet hFE is at 4A.

[R.G.]

The answer is full of asterisks and footnotes. The current gain of bipolar devices is not constant; it varies with collector current, temperature, and phase of the moon. Testing at different currents will definitely produce different numbers on the same device.

Bipolars do age, especially with high temperatures. A gain of 8 is not by itself suspect, but a triplet of 8, 8 and 35 will have problems. I would guess that the two 8's are the paralleled output devices and the 35 is the driver.

That being said, If I had the outputs out, I would put in a new set of MJ15022 devices, and if it's a PNP bottom side, I'd put in a set of the complementary MJ15023s. The whole output stage will be more reliable.