Acoustic 370 oscillation in power stage

[blargg]

I have an Acoustic 371 with oscillation issues.Gave up on it 10+ years ago, but after seeing this thread I'd like to get back on it.Thanks for all the helpful info.

[km6xz]

The 2N3055 was from an era where each manufacturer produced parts that were quite different in actual performance. If you have a good transistor of the type, use a curve tracer when trying to match modern replacements. Nowadays, when transistor fab is not such a mystery, parts are a lot more consistent.
The 2N3773 was intended to be the first power transistors with an extended secondary break region so were actually better in power amps and inductive load servo amps than the 3055 which are not as tough when in a circuit with the wide range of currents/voltage across dynamic loads.
When checking the old transistors on a curve tracer notice the step trace loop size. That is a good indication of junction capacitance. The original 2N3055 was not a HF transistor and will show significant looping in the traces. Modern higher gain-bandwidth product power transistors used in the old circuits that were compensated for the old transistors, end up being over compensated. Adjusting the values of the compensation caps might be in order.

[Steve Conner]

This is true. There was a change in transistor manufacturing sometime in the 80s. The old process that made slow, rugged transistors was abandoned (it took something like 24 hours to cook) So if you buy a 2N3055 now, it'll have a faster chip inside that's more prone to oscillation.

The easiest way to fix an Acoustic 370 is buy the old style RCA 2N3055H surplus. But here are some ideas to modify one for modern transistors.

Move the Zobel network so it connects before L301. After is wrong, why on earth did they do it like that.

Reduce R329 in the Zobel network to 4 ohms.

Put 100 ohm resistors in series with the base connections of Q305, Q310. These are equivalent to grid stoppers in a tube amp. I've found them necessary in every solid-state amp I've built.

Put a 22uF electrolytic cap across the base and emitter of Q304. Might help the grid stoppers work more effectively.

[blargg]

Does anyone know of a source for an eq inductor #7197 1.5 Henry? I have tried Doug Forbes's Acousticcontrol.com but can't get a response.

[gbono]

vishay MPN = TD5Q4TAEB1504F
caddell burns = 6950-63

good luck