Getting lost with analysing Class AB amplifier circuit

[bborisov567]

Was the 20mV difference measured in circuit or out of circuit?

In curcuit.

Not sure what 1.06M or 1.08M hFE is supposed to mean?

I don't know either. Usually the transistor tester shows a meaningful number. I tested some other transistors of that type and they all show 1.xxM value. Looks like a bug in the tester


Offset on the working channel i 2-3mv, difference on Vbe is also that small. Probably it will be the best to replace both transsitors with brand new.

[magic]

No harm in that if you find the same type or suitable substitutes. If you match them on β it will be possible to adjust output to zero by making R601+R602+R603 = R608 and with small mismatch it will still be doable by tweaking R601.

If Vbe is equal out of circuit but becomes different in circuit, that could be a problem elsewhere.

[bborisov567]

Recently i found another vintage amplifier and started trying to do some repair. This one has -dc supply on the output though. Output transistors are ok. Decided not to open a new topic for a quick question. What is the purpose of T505 and T506. It kinda looks like a current mirror but shouldnt T506’s base be connected to its collector? Where exatcly is the signal path for the “negative” part of the curcuit? Also there should be signal on T503’s collector, right? Schematic is here:

  https://2.bp.blogspot.com/-caBoNeF4L1M/UmSsQvmugGI/AAAAAAAAC6c/u7ErQenGXf8/s1600/660_2_2.gif

https://3.bp.blogspot.com/-aHtKx9ZJ5ss/UmSsQsr6ryI/AAAAAAAAC6Y/nRZxCKyJ8Ps/s1600/660_2_1.gif



Hope i dont get annoying with stupid questions about old amplifiers.

[dietert1]

No current mirror, but two constant current sources. Base voltages fixed by diodes (1.4 V above negative supply), current determined by emitter resistors. Signal path is from T503 to T507.

Regards, Dieter