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I have a trinity sIII 18 watter, I was just wondering if it would be safe to hook it up to a 2x12 fender bassman cab, and what impedance I should put it to. the impedance switch on the amp 4, 8, and 16 if I am not mistaken.

Well it would be nice to know it's impedance but...sure you can use that cab.
It's probably 4 ohms. You could look inside and see the wiring and speaker config.
Stew

It's ideal to match impedances but if you dont know for sure, set it at 8. If I recall, the Fenders were 4 ohms.

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Stephen
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I believe the bassman heads were 4 ohms out - you can put a multimeter testing for ohms and if it reads around 4 assume its a 4 - an 8 ohm cab will read higher like 6 or 7 usually - its been awhile since I did this.