Audio Research D76

A 1 watt frequency response test will tell you a lot as well. Most highly stable vacuum tube amplifiers can be very flat up to 20 kHz, but will start rolling above that, usually being down by about 1 db at anywhere from 40 kHz to 60 kHz. Response that has not started rolling off by about 75 kHz is usually a recipe for stability problems in high fidelity equipment.

Dave
 
Response that has not started rolling off by about 75 kHz is usually a recipe for stability problems in high fidelity equipment.

The fun ones are the amps that have a rising response by that point. Pretty sure bet its going to go off the rails with a less than ideal load.
 
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