Guru
Joined: 11/02/2018
Location: AustraliaPosts: 3321
| Posted: 12:03pm 15 Jan 2023 |
Yes, the purpose of Q5 & Q6 is to limit the audio amplitude. They probably used B-E reverse breakdown as it clips the audio more gradually than zenners would, producing fewer harsh harmonics.
Most small signal transistors should be ok there, eg BC547 or 8 or 9.
Q7 is a thermally coupled matched pair. Haven't seen a through-hole one of those in a very long time. Surface mount dual transistors might be easier to find, though soldering legs on it would be fiddly.
As a last resort two TO92 transistors from the same batch could be glued back to back. As the leads won't align with your PCB use strip-board to make an adapter. Any low-noise audio transistors with a Vce raring of 60V or more should work.
Edited 2023-01-16 08:05 by phil99