First, obviously, check the speaker. Give the cone a gentle poke to make sure it's firm and not deteriorating. Turn the amp on and check to make sure there's no orange plate glow on the power tubes (blue glow is OK). Don't confuse this with the normal orange glow of the tube's heater filament. Orange glow indicates that the tubes are dangerously underbiased--this isn't necessarily a failure, but would have to be addressed right away, as it might burn a tube or transformer quickly. Pull all the tubes and look at the tube sockets to see if any of them look cracked or burned. You probably won't be able to do this, but it won't hurt to ask the seller if he'll pull the chassis and let you have a look around inside. The most obvious thing to check for is burned or cracked resistors. Again, this wouldn't necessarily indicate a problem with the amp, but rather that the components are simply worn out.

Check the electrolytic capacitors for "bubbles" or residue on the ends of the cans--they'll need to be replaced, if so. It's an accepted fact that electrolytics "die" after years of use and must be replaced. Ask if the amp's been played regularly, or stored away--ironically, amps that are played regularly often sound better than those that have been mothballed, because electrolytic caps need regular "exercise" to keep from drying out. Of course, the big test is in the sound! When you plug in and turn on, be sure to give enough time for the amp to warm up (I'd try to play it for at least a half-hour or so). A 60-cycle hum indicates bad filter capacitors, which will need to be replaced. Listen carefully for crackles and drop-outs which might indicate bad internal components--often these don't become apparent until the amp is really warmed up.

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